Microprocessors & Microcomputers Lecture Notes

LECTURE NOTES
ON
MICROPROCESSORS & MICROCOMPUTER
COURSE CODE:BCS- 301
Prepared By
Dr. M.R.Kabat
Dr. A.K.Rath
Dr. S.Panigrahi
Department of Computer Science & Engineering and IT
VSS University of Technology, Burla, Odisha

Syllabus
BCS-301 MICROPROCESSORS & MICROCOMPUTER (3-1-0)
Module – 1(8 Lectures)
Microprocessors, Microcomputers and Assembly Language: Microprocessors, Microprocessor
Instruction Set, Computer Languages, Microcomputers.
Interfacing I/O devicesand Memory. Memory mapped I/O and I/O mapped I/o.
Module – II (8 Lectures)
The Processors: 8086/8088- Architectures, Pin Diagrams and Timing Diagrams:- Register Organisation
of 8086, Architecture, Signal Descriptions of 8086, Physical Memory Organisation, General Bus Operation,
I/O Addressing Capability, Special Processor Activities, Minimum Mode 8086 System and Timings,
Maximum Mode 8086 System and Timings.
Module – III (8 Lectures)
8086 Instruction Set and ALP:- Machine Language Instruction Formats, Addressing Modes of 8086,
Instruction Set of 8086, Assembler Directives and Operators, ALP
Module-IV (8 Lectures)
Special Architectural Features and Related Programming:- Introduction to Stack, Stack structure of
8086, Interrupts and Interrupt Service Routines, Interrupt Cycle of 8086, Non Maskable Interrupt, Maskable
Interrupt (INTR), Interrupt Programming, Passing Parameters, to Size More than 64K, MACROS, Timings
and Delays;
Module – V (8 Lectures)
General-Purpose Programmable Peripheral Devices:- The 8255A Programmable Peripheral Interface,
Illustration: Interfacing Keyboard and Seven-Segment Display, Illustration: Bidirectional Data Transfer
between Two Microcomputers, The 8259A Programmable Interrupt Controller, Direct Memory Access
(DMA) and the 8257 DMA Controller

INTRODUCTION TO MICROPROCESSOR ARCHITECTURES
A Microprocessor is a multipurpose programmable logic device which reads the binary
instructions from a storage device called ‘Memory’ accepts binary data as input and process
data according to the instructions and gives the results as output. So, you can understand the
Microprocessor as a programmable digital device, which can be used for both data processing
and control applications. In view of a computer student, it is the CPU of a Computer or heart of
the computer. A computer which is built around a microprocessor is called a microcomputer. A
microcomputer system consists of a CPU (microprocessor), memories (primary and
secondary) and I/O devices as shown in the block diagram in Fig 1. The memory and I/O
devices are linked by data and address (control) buses. The CPU communicates with only one
peripheral at a time by enabling the peripheral by the control signal. For example to send data
to the output device, the CPU places the device address on the address bus, data on the data
bus and enables the output device. The other peripherals that are not enabled remain in high
impedance state called tri-state.
Fig.1 Block diagram of a Microcomputer
Evolution of Microprocessors
The first Microprocessor (4004) was designed by Intel Corporation which was founded by
Moore and Noyce in 1968.
In the early years, Intel focused on developing semiconductor memories (DRAMs and
EPROMs) for digital computers.
In 1969, a Japanese Calculator manufacturer, Busicom approached Intel with a design for a
small calculator which need 12 custom chips. Ted Hoff, an Intel Engineer thought that a
general purpose logic device could replace the multiple components.
This idea led to the development of the first so called microprocessor. So, Microprocessors
started with a modest beginning of drivers for calculators.

With developments in integration technology Intel was able to integrate the additional chips
like 8224 clock generator and the 8228 system controller along with 8080 microprocessor with
in a single chip and released the 8 bit microprocessor 8085 in the year 1976. The 8085
microprocessor consisted of 6500 MOS transistors and could work at clock frequencies of 3-5
MHz. It works on a single +5 volts supply. The other improved 8 bit microprocessors include
Motorola MC 6809, Zilog Z-80 and RCA COSMAC.
In 1978, Intel introduced the 16 bit microprocessor 8086 and 8088 in 1979. IBM selected
the Intel 8088 for their personal computer (IBM-PC).8086 microprocessor made up of 29,000
MOS transistors and could work at a clock speed of 5-10 MHz. It has a 16-bit ALU with 16-bit
data bus and 20-bit address bus. It can address up to 1MB of address space. The pipelining
concept was used for the first time to improve the speed of the processor. It had a pre-fetch
queue of 6 instructions where in the instructions to be executed were fetched during the
execution of an instruction. It means 8086 architecture supports parallel processing. The 8088
microprocessor is similar to 8086 processor in architecture ,but the basic difference is it has
only 8-bit data bus even though the ALU is of 16-bit.It has a pre-fetch queue of 4-instructions
only.
In 1982 Intel released another 16-bit processor called 80186 designed by a team under the
leadership of Dave Stamm. This is having higher reliability and faster operational speed but at a
lower cost. It had a pre-fetch queue of 6-instructions and it is suitable for high volume
applications such as computer workstations, word-processor and personal computers. It is
made up of 134,000 MOS transistors and could work at clock rates of 4 and 6 MHz. This is also
comes under first generation of Microprocessors.
Intel released another 16 bit microprocessor 80286 having 1, 34,000 transistors in 1981. It was
used as CPU in PC-ATs in 1982. It is the second generation microprocessor, more advanced to
80186 processor. It could run at clock speeds of 6 to 12.5 MHz .It has a 16-bit data bus and 24-
bit address bus, so that it can address up to 16MB of address space and 1GB of virtual
memory. It had a pre-fetch queue of 6 instructions .Intel introduced the concept of protected
mode and virtual mode to ensure proper operation. It also had on-chip memory management
unit (MMU) .This was popularly called as Intel 286 in those days.
In 1985, Intel released the first 32 bit processor 80386, with 275,000 transistors. It has 32-
bit data bus and 32-bit address bus so that it can address up to a total of 4GB memory also a
virtual memory space of 64TB.It could process five million instructions per second and could
work with all popular operating systems including Windows. It has a pre-fetch queue of length
16-bytes with extensive memory management capabilities. It is incorporated with a concept
called paging in addition to segmentation technique. It uses a math co-processor called 80387.
Intel introduced 80486 microprocessor with a built-in maths co-processor and with 1.2
million transistors. It could run at the clock speed of 50 MHz This is also a 32 bit processor but
it is twice as fast as 80386.The additional features in 486 processor are the built-in Cache and
built-in math co-processors. The address bus here is bidirectional because of presence of
cache memory.

On 19th
October, 1992, Intel released the Pentium-I Processor with 3.1 million transistors. So,
the Pentium began as fifth generation of the Intel x86 architecture. This Pentium was a
backward compatible while offering new features. The revolutionary technology followed is that
the CPU is able to execute two instruction at the same time. This is known as super scalar
technology. The Pentium uses a 32-bit expansion bus, however the data bus is 64 bits.
The 7.5 million transistors based chip, Intel Pentium II processor was released in 1997. It
works at a clock speed of 300M.Hz. Pentium II uses the Dynamic Execution Technology which
consists of three different facilities namely, Multiple branch prediction, Data flow analysis, and
Speculative execution unit. Another important feature is a thermal sensor located on the
mother board can monitor the die temperature of the processor. For thermal management
applications.
Intel Celeron Processors were introduced in the year 1999. Pentium-III processor with 9.5
million transistors was introduced in 1999. It also uses dynamic execution micro-architecture, a
unique combination of multiple branch prediction, dataflow analysis and speculative execution.
The Pentium III has improved MMX and processor serial number feature. The improved MMX
enables advanced imaging, 3D streaming audio and video, and speech recognition for
enhanced Internet facility.
Pentium-IV with 42 million transistors and 1.5 GHz clock speed was released by Intel in
November 2000. The Pentium 4 processor has a system bus with 3.2 G-bytes per second of
bandwidth. This high bandwidth is a key reason for applications that stream data from memory.
This bandwidth is achieved with 64 –bit wide bus capable of transferring data at a rate of 400
MHz. The Pentium 4 processor enables real-time MPEG2 video encoding and near real-time
MPEG4 encoding, allowing efficient video editing and video conferencing.
Intel with partner Hewlett-Packard developed the next generation 64-bit processor architecture
called IA-64 .This first implementation was named Itanium. Itanium processor which is the first
in a family of 64 bit products was introduced in the year 2001.The Itanium processor was
specially designed to provide a very high level of parallel processing ,to enable high
performance without requiring very high clock frequencies .Key strengths of the Itanium
architecture include ,up to 6 instructions/cycle. The Itanium processor can handle up to 6
simultaneous 64 –bit instructions per clock cycle.
The Itanium II is an IA-64 microprocessor developed jointly by Hewlett-Packard (HP)
and Intel and released on July 8,2002..It is theoretically capable of performing nearly 8 times
more work per clock cycle than other CISC and RISC architectures due to its parallel
computing micro-architecture. The recent Itanium processor features a split L2 cache, adding a
dedicated 1MB L2 cache for instructions and thereby effectively growing the original 256KBL2
cache, which becomes a dedicated data cache. The first Itanium 2 processor (code named
McKinley) was more powerful than the original Itanium processor, with approximately two
times performance.
Pentium 4EE was released by Intel in the year 2003 and Pentium 4E was released in the
year 2004.

The Pentium Dual-Core brand was used for mainstream X86-architecture
microprocessors from Intel from 2006 to 2009 The 64 bit Intel Core2 was released on July
27,2006. In terms of features, price and performance at a given clock frequency, Pentium Dual-
Core processors were positioned above Celeron but below Core and Core 2 microprocessors
in Intel's product range. The Pentium Dual-Core was also a very popular choice for over
clocking, as it can deliver optimal performance (when over clocked) at a low price.
The Pentium Dual Core, which consists of 167 million transistors was released on January 21,
2007. Intel Core Duo consists of two cores on one die, a 2 MB L2 cache shared by both cores,
and an arbiter bus that controls both L2 cache and FSB access.
Core 2 Quad processors are multi-chip modules consisting of two dies similar to those used in
Core 2 Duo, forming a quad-core processor. While this allows twice the performance to a dual-
core processors at the same clock frequency in ideal conditions, this is highly workload
specific and requires applications to take advantage of the extra cores.
In September.2009, new Core i7 models based on the Lynnfield desktop quad-core processor
and the Clarksfield quad-core mobile were added, and models based on the Arrandale dual-
core mobile processor have been announced. The first six-core processor in the Core lineup is
the Gulftown, which was launched on March 16, 2010. Both the regular Core i7 and the Extreme
Edition are advertised as five stars in the Intel Processor Rating.
ASSEMBLY LANGUAGE PROGRAMMING EXAMPLES:
Addition Programs
Example 1: Addition of two 8-bit numbers whose sum is 8-bits.
Explanation: This assembly language program adds two 8-bit numbers stored in two memory
locations .The sum of the two numbers is 8-bits only.The necessary algorithm and flow charts
are given below.
ALGORITHM:
Step1. : Initialize H-L pair with memory address XX00 (say: 9000).
Step2. : Clear accumulator.
Step3. : Add contents of memory location M to accumulator.
Step4. : Increment memory pointer (i.e. XX01).
Step5. : Add the contents of memory indicated by memory pointer to accumulator.
Step6. : Store the contents of accumulator in 9002.
Step7. : Halt
PROGRAM:
Address
of the
memory
location
Hex code Label
Mnemonics
Comments
Op-code Operand

8000 21,00,90 LXI H, 9000 Initialise memory pointer to
point the first data location
9000.
8003 3E MVI A, 00 Clear accumulator
8004 00
8005 86 ADD A, M The first number is added to
accumulator [A] [A] + M
8006 23 INX H Increment the memory
pointer to next location of
the Data.
8007 86 ADD A, M The 2nd
number is added to
contents of accumulator
8008 32 STA 9002 The contents of accumulator
are stored in memory
location 9002.8009 02
800A 90
800B 76 HLT Stop the execution
Ex: Input: Ex: (i) 9000 – 29 H Ex :(ii) 9000 –49 H
9001 – 16 H 9001 –32 H
Result: Ex: (i) 9002 – 3F H Ex :( ii) 9002 – 7B
Example 2: Addition of two 8-bit numbers whose sum is 16 bits.
Explanation: The first 8-bit number is stored in one memory location (say 8500) and the second
8-bit number is stored in the next location (8501).Add these two numbers and check for carry.
Store the LSB of the sum in one memory location (8502) and the MSB (carry) in the other
location(8503).
ALGORITHM:
Step1. : Initialize H-L pair with memory address X (say: 8500).
Step2. : Clear accumulator.
Step3. : Add contents of memory location M to accumulator.
Step4. : Increment memory pointer (i.e. 8501).

Step6. : Check for Carry
Step 7 : Store the sum in 8502.
Step8 : Store the Carry in 8503 location
Step 9 : Halt
PROGRAM:
Address of
the memory
location
Hex code Label Mnemonics Comments
Op-
code
Operand
8000 21,00,85 LXI H, 8500 H Initialise memory pointer to
point the first data location
9000.
8003 3E MVI A,00 Clear accumulator
8004 00
8005 86 ADD A, M The first number is added to
accumulator [A] [A]+M
8006 0E MVI C,00 Initial value of Carry is 0
8007 00
8008 23 INX H Increment the memory
pointer to next location of the
Data.
number is added to
800A 32 JNC FWD Is Carry exists ? No,go to the
label FWD
800B 0E
800C 80
800D 0C INR C Make carry =1
800E 32 FWD STA 8502 H The sum is stored in memory
location 8502.
800F 02
8010 85
8011 79 MOV A,C
8012 32 STA 8503 H Store the carry at 8503
location

8013 03
8014 85
8015 76 HLT Stop the execution
Ex: Input: Ex : 8500 – 97 H RESULT: 8502 – 32 H
8501 – 98H 8503 -- 01 H
Example 3: Decimal addition of two 8-bit numbers whose sum is 16 bits.
Explanation: Decimal addition of two 8-bit numbers is same as that of two 8-bit numbers
program. Except that the use of DAA instruction. The first 8-bit number is stored in one
memory location (say 8500) and the second 8-bit number is stored in the next
location(8501).Add these two numbers and use the DAA instruction to get the result in
decimal. Also check for carry. Store the LSB of the sum in one memory location(8502) and the
MSB (carry) in the other location(8503).
ALGORITHM:
Step1. : Initialize H-L pair with memory address XXXX (say: 8500).
Step2. : Clear Carry register C.
Step3. : Move contents of memory location M to accumulator.
Step4. : Increment memory pointer (i.e. 8501).
Step6. : Apply the instruction DAA(Decimal adjust after addition)
Step7: Check for Carry
Step8: Store the sum in XX02.
Step9: Store the Carry in XX03 location
Step10: Halt
PROGRAM
Address of
the
memory
location
Hex
code
Label Mnemonics Comments
Op-
code
Operand
8000 21,
00,85
LXI H, 8500
H
Initialise memory pointer to
point the first data location 9000.

8003 0E MVI C, 00 Clear accumulator
8004 00
8005 7E MOV A, M The first number is added to
accumulator [A] [A]+M
8006 23 INX H Increment the memory pointer to
next location of the Data.
number is added to
8008 27 DAA
8009 D2 JNC FWD Is Carry exists? No, go to the
label FWD
0D
80
800C 0C INR C Make carry =1
800D 32 FWD STA 8502 H The contents of accumulator are
stored in memory location 8502.
800E 02
800F 85
8010 79 MOV A, C Carry is moved to accumulator
8011 32 STA 8503 H A Carry is stored in the location
8503
8012 03
8013 85
Ex: Input: Ex : 8500 – 67 D RESULT: 8502 – 52 D
8501 – 85 D 8503 – 01 (Carry)
Example 4: Addition of two 16-bit numbers whose sum is 16 bits or more
Explanation: First 16-bit number is stored in two consecutive locations (Ex 8500 &8501)
because in each location we can store only one 8-bit number. Store the second 16-bit number
in the next two consecutive locations (For Ex: 8502 &8503).Add the LSB of the first number to
the LSB of the second number and the MSB of the first number to the MSB of the second
number using the DAD instruction. Store the sum in the next two locations and the carry (if
any) in the third location

ALGORITHM:
Step1: First 16 bit number is in locations 8500 & 8501 respectively
Step2: Second 16-bit number is in locations 8502 & 8503
Step3: Add the two 16-bit numbers using DAD Instruction.
Step4: Sum is stored in locations 8504 & 8505.
Step5: Carry (if any) is stored in the location 8506.
Step6: Halt
PROGRAM:
ADDRESS HEX –
CODE
LABEL MNEMONIC COMMENTS
OPCO
DE
OPERAND
8000 2A,00,85 LHLD 8500 H First 16-bit number in H-L pair
8001 00
8002 85
8003 EB XCHG Exchange first number to D-E
Pair
8004 2A LHLD 8502 H
8005 02
8006 85
8007 0E MVI 00 MSB of the sum is initially 00
8008 00
8009 19 DAD D Add two 16 –bit numbers
800A D2 JNC FWD Is Carry? If yes go to the next
line .Else go to the 800E
LOCATION800B 0E
800C 80
800D OC INR C Increment carry
800E 22 FWD SHLD 8504 H Store the LSB of the Sum in
8504 & MSB in 8505 locations
800F 04
8010 85
8011 79 MOV A,C MSBs of the sum is in

Accumulator
8012 32 STA 8506 H Store the MSB (Carry) of the
result in 8506 location
8013 06
8014 85
8015 76 HLT Stop execution
Ex: INPUT: 8500- 12 H LSB of the Ist
Number RESULT : 8504 - 25H LSB of the Sum
8501- 13 H MSB of the Ist
Number 8505 – 25H MSB of the Sum
8502 -13 H LSB of the IInd
Number 8506 -- 00 Carry .
8503 -12H MSB of the IInd
number
Subtraction Programs:
Example 5: Subtraction of two 8-bit numbers without borrows.
Explanation: It’s a simple program similar to addition of two 8- bit numbers, except that we use the
instruction SUB instead of ADD. The first 8-bit number is stored in XX00 memory location and the
second 8-bit number is stored in the XX01 location .Use the SUB instruction and store the result in
the XX02 location.
ALGORITHM:
Step1. : Initialise H-L pair with the address of minuend.
Step2. : Move the minuend into accumulator
Step3. : Increment H-L pair
Step4. : Subtract the subtrahend in memory location M from the minuend.
Step5. : Store the result in XX02.
Step6. : Stop the execution
ADDRESS HEX
CODE
OPCOD
E
OPERAN
D

8000 21 LXI H, 8500 Initialise H-L pair and get the
First number in to 8500
location
8001 00
8002 85
8003 7E MOV A,M [A] [M]
8004 23 INX H [M+1] [M]
8005 96 SUB M A [A] – [M]
8006 23 INX H Next memory location
8007 77 MOV M,A Store the result in the
location 8502

PROGRAM:
INPUT: Ex : 8500- 59H Result: 8502 – 29H
8501- 30H
Example 6: Subtraction of two 8-bit Decimal numbers.
Explanation: In this program we can’t use the DAA instruction after SUB or SBB instruction
because it is decimal adjust after addition only. So, for decimal subtraction the number which
is to be subtracted is converted to 10’s complement and then DAA is applied.
ALGORITHM:
Step1. : Initialise H-L pair with the address of second number (XX01).
Step2. : Find its ten’s complement
Step3. : Decrement the H-L pair for the first number (XX00)
Step4. : Add the first number to the 10’s complement of second number.
Step5. : Store the result in XX02.
Step6. : Stop the execution
PROGRAM:
ADDRESS HEX
CODE
LAB
EL
MNEMONIC COMMENTS
OPCODE OPERAND
8000 21 LXI H,8500 Initialise H-L pair and get
theSecond number in to 8501
location
8001 00
8002 85
8003 3E MVI A,99 [A] 99
8004 99
8005 96 SUB M 9’s complement of second
number
8006 3C INR A 10’s complement of second
number
8007 2B DCX H Address of the first number

8008 86 ADD M Add first number to 10’s
complement of second number
8009 27 DAA
800A 32 STA 8502 Store the result in the location
8502
800B 02
800C 85
800D 76 HLT Stop the execution
Ex: Input: 8500 -76 D Result: 8502 - 41 D
8501- 35 D
Example 6: Subtraction of two 16 –bit numbers.
Explanation: It is very similar to the addition of two 16-bit numers.Here we use SUB &SBB
instructions to get the result .The first 16-bit number is stored in two consecutive locations and
the second 16-bit number is stored in the next two consecutive locations.The lsbs are
subtracted using SUB instruction and the MSBs aare subtracted using SBB instruction.The
result is stored in different locations.
ALGORITHM:
Step1. : Store the first number in the locations 8500 & 8501.
Step2. : Store the second number in the locations 8502 &8503.
Step4. : Subtract the second number from the first number with borrow.
Step5. : Store the result in locations 8504 & 8505.
Step6. : Store the borrow in location 8506
Step 7: Stop the execution
PROGRAM:

Ex: INPUT : 8500- FF H LSB of the Ist
Number RESULT: 8504 - 11H LSB
8501 - FF H MSB of the Ist
Number 8505 – 11 H MSB
8502 -EE H LSB of the IInd
Number
8503 –EE H MSB of the IInd
number
Multiplication Programs
Example 7: Multiplication of two 8-bit numbers. Product is 16-bits.
ADDRESS HEX
CODE
LABE
L
MNEMONIC COMMENTS
OPCOD
E
OPERAN
D
8000 2A,
00,85
LHLD 8500 H First 16-bit number in H-L pair
8003 EB XCHG Exchange first number to D-E Pair
8004 2A LHLD 8502 H Get the second 16-bit number in H-L
pair
8005 02
8006 85
8007 7B MOV A, E Get the lower byte of the First
number in to Accumulator
8008 95 SUB L Subtract the lower byte of the
second number
8009 6F MOV L, A Store the result in L- register
800A MOV A, D Get higher byte of the first number
800A 9C SBB H Subtract higher byte of second
number with borrow
800B 67 MOV H, A
800C 22 SHLD 8504 Store the result in memory locations
with LSB in 8504 & MSB in 8505
800D 04
80OE 85
80OF 76 HLT Stop execution

Explanation: The multiplication of two binary numbers is done by successive addition. When
multiplicand is multiplied by 1 the product is equal to the multiplicand, but when it is multiplied
by zero, the product is zero. So, each bit of the multiplier is taken one by one and checked
whether it is 1 or 0 .If the bit of the multiplier is 1 the multiplicand is added to the product and
the product is shifted to left by one bit. If the bit of the multiplier is 0 , the product is simply
shifted left by one bit. This process is done for all the 8-bits of the multiplier.
ALGORITHM:
Step 1 : Initialise H-L pair with the address of multiplicand.(say 8500)
Step 2 : Exchange the H-L pair by D-E pair. so that multiplicand is in D-E pair.
Step 3 : Load the multiplier in Accumulator.
Step 4 : Shift the multiplier left by one bit.
Step 5 : If there is carry add multiplicand to product.
Step 6 : Decrement the count.
Step 7 : If count ≠≠≠≠ 0; Go to step 4
Step 8 : Store the product i.e. result in memory location.
Step 9 : Stop the execution
PROGRAM:
ADDRESS HEX
-
COD
E
LABE
L
MNEMONIC COMMENTS
OPCOD
E
OPERAND
8000 2A,
00,8
5
LHLD H, 8500 Load the multiplicand in to H-L pair
8003 EB XCHG Exchange the multiplicand in to D-E
pair
8004 3A LDA 8502 Multiplier in Accumulator
8005 02
8006 85
8007 21 LXI H.0000 Initial value in H-L pair is 00
8008 00
8009 00
800A 0E MVI C,08 Count =08
800B 08
800C 29 LOO
P
DAD H Shift the partial product left by one
bit.

800D 17 RAL Rotate multiplier left by one bit
800E D2 JNC FWD Is Multiplier bit =1? No go to label
FWD
800F 12
8010 80
8011 19 DAD D Product =Product +Multiplicand
8012 0D FWD DCR C COUNT=COUNT-1
8013 C2 JNZ LOOP
8014 0C
8015 80
8016 22 SHLD 8503 Store the result in the locations 8503
& 8504
8017 03
8018 85
INPUT :
Addres
s
Data
8500 8AH – LSB of Multiplicand
8501 00 H – MSB of Multiplicand
8502 52 H - Multiplier
Result: 8503 34 H – LSB of Product
8504 2C H – MSB of Product
Division Programs
Example 7: Division of a 16- bit number by a 8-bit number.

Explanation: The division of a 16/8-bit number by a 8-bit number follows the successive
subtraction method. The divisor is subtracted from the MSBs of the dividend .If a borrow
occurs, the bit of the quotient is set to 1 else 0.For correct subtraction process the dividend is
shifted left by one bit before each subtraction. The dividend and quotient are in a pair of
register H-L.The vacancy arised due to shifting is occupied by the quotient .In the present
example the dividend is a 16-bit number and the divisor is a 8-bit number. The dividend is in
locations 8500 &8501.Similarly the divisor is in the location 8502.The quotient is stored at 8503
and the remainder is stored at 8504 locations.
ALGORTHM:
STEP1. : Initialise H-L pair with address of dividend.
STEP2. : Get the divisor from 8502 to register A & then to Reg.B
STEP3. : Make count C=08
STEP4. : Shift dividend and divisor left by one bit
STEP 5: Subtract divisor from dividend.
STEP6. : If carry = 1 : goto step 8 else step7.
STEP7. : Increment quotient register.
STEP8. : Decrement count in C
STEP9. : If count not equal to zero go to step 4
STEP10: Store the quotient in 8503
STEP11: Store the remainder in 8504
STEP12: Stop execution.
PROGRAM:
ADD
R-
ESS
HEX –
CODE
OPCODE OPERAND
8000 21 LHLD H, 8500 Initialize the H-L pair for
dividend
8001 00
8002 85
8003 3A LDA 8502 H Load the divisor from
location 8502 to accumulator
8004 02
8005 85
8006 47 MOV B,A Move Divisor to Reg.B from A
8007 0E MVI C,08 Count =08
8008 08
8009 29 BACK DAD H Shift dividend and quotient
left by one bit

Ex: Input & Result
800A 7C MOV A,H MSB of dividend in to
accumulator
800B 90 SUB B Subtract divisor from MSB
bits of divisor
800C DA JC FWD Is MSB part of dividend >
divisor ? No,goto label FWD
800D 11
800E 80
800F 67 MOV H,A MSB of the dividend in Reg.H
8010 2C INR L Increment quotient
8011 0D FWD DCR C Decrement count
8012 C2 JNZ BACK If count is not zero jump
to8009 location
8013 09
8014 80
8015 22 SHLD 8503H Store quotient in 8503 and
remainder in 8504 locations
8016 03
8017 85
8018 76 HLT Stop execution
Address Data
8500 64 LSB of
Dividend
8501 00 MSB of
Dividend
8502 07 Divisor
8503 0E Quotient
8504 02 Remainder

Largest & Smallest numbers in an Array
Example 8: To find the largest number in a data array
Explanation: To find the largest number in a data array of N numbers (say)first the count is
placed in memory location (8500H) and the data are stored in consecutive
locations.(8501….onwards).The first number is copied to Accumulator and it is compared with
the second number in the memory location. The larger of the two is stored in Accumulator. Now
the third number in the memory location is again compared with the accumulator. And the
largest number is kept in the accumulator. Using the count, this process is completed , until all
the numbers are compared .Finally the accumulator stores the smallest number and this
number is stored in the memory location.85XX.
ALGORTHM:
Step1: Store the count in the Memory location pointed by H-L register.
Step2: Move the I st number of the data array in to accumulator
Step3: Compare this with the second number in Memory location.
Step4: The larger in the two is placed in Accumulator
Step5: The number in Accumulator is compared with the next number in memory .
Step 6: The larger number is stored in Accumulator.
Step 7; The process is repeated until the count is zero.
Step 8: Final result is stored in memory location.
PROGRAM
ADD
R-
ESS
HEX –
CODE
OPCOD
E
OPERAN
D
8000 21,00,8
5
LXI H, 8500 INITIALISE H-L PAIR
8003 7E MOV C,M Count in the C register
8004 23 INX H First number in H-L pair
8005 4E MOV A,M Move first number in to
Accumulator
8006 0D DCR C Decrement the count
8007 91 LOOP1 INX H Get the next number
8008 BE CMP M Compare the next number
with previous number

8009 D2 JNC LOOP2 Is next number >previous
maximum?No,go to the
loop2
800A 0D
800B 80
800C 7E MOV A,M If,yes move the large
800D 0D LOOP2 DCR C Decrement the count
800E C2 JNZ LOOP1 If count not equal to
zero,repeat
800F 07
8011 80
8012 78
8013 32 STA 85XX Store the largest number in
the location 85XX
8014 XX
8015 85
Ex : Input : 8500- N(Say N=7 ) Result : 8508 - 7F
8501-05
8502-0A
8503-08
8504-14
8505 -7F
8506-25
8507-2D
Example 9 : To find the smallest number in a data array.

Explanation: To find the smallest number in a data array of N numbers (say)first the count is
placed in memory location (8500H) and the data are stored in consecutive
locations.(8501….onwards).The first number is copied to Accumulator and it is compared with
the second number in the memory location.The smaller of the two is stored in
Accumulator.Now the third number in the memory location is again compared with the
accumulator.and the smallest number is kept in the accumulator.Using the count,this process
is completed until all the numbers are compared .Finally the accumulator stores the smallest
number and this number is stored in the memory location.85XX.
ALGORTHM :
Step1: Store the count in the Memory location pointed by H-L register.
Step2: Move the I st number of the data array in to accumulator
Step3: Compare this with the second number in Memory location.
Step4: The smaller in the two is placed in Accumulator
Step5: The number in Accumulator is compared with the next number in memory .
Step 6: The smaller number is stored in Accumulator.
Step 7; The process is repeated until the count is zero.
Step 8: Final result is stored in memory location.
PROGRAM
ADD
R-
ESS
HEX –
CODE
OPCOD
E
OPERAN
D
8000 21 LXI H, 8500 Initialise the H-L pair.
8001 00
8002 85
8003 7E MOV C,M Count in the C register
8004 23 INX H First number in H-L pair
8005 4E MOV A,M Move first number in to
Accumulator
8006 0D DCR C Decrement the count
8007 91 LOOP1 INX H Get the next number
8008 BE CMP M Compare the next number
with previous number
8009 D2 JC LOOP2 Is next number <previous
smallest ?If yes go to the
loop2

800A 0D
800B 80
800C 7E MOV A,M No,move the smaller
800D 0D LOOP2 DCR C Decrement the count
800E C2 JNZ LOOP1 If count not equal to
zero,repeat
800F 07
8011 80
8012 78
8013 32 STA 85XX Store the smallest number
in the location 85XX
8014 XX
8015 85
Ex: Input : 8500 - N((Say N=7) Result : 8508 – 04
8501-09
8502-0A
8503-08
8504-14
8505 -7F
8506-04
8507-2D
Stack and Subroutines
Stack is a set of memory locations in the Read/Write memory which is used for temporary
storage of binary information during the execution of a program. It is implemented in the Last-
in-first-out (LIFO) manner. i.e., the data written first can be accessed last, One can put the data
on the top of the stack by a special operation known as PUSH. Data can be read or taken out
from the top of the stack by another special instruction known as POP.

Stack is implemented in two ways. In the first case, a set of registers is arranged in a
shift register organization. One can PUSH or POP data from the top register. The whole block of
data moves up or down as a result of push and pop operations respectively. In the second
case, a block of RAM area is allocated to the stack. A special purpose register known as stack
pointer (SP) points to the top of the stack. Whenever the stack is empty, it points to the bottom
address. If a PUSH operation is performed, the data are stored at the location pointed to by SP
and it is
decremented by one. Similarly if the POP operation is performed, the data are taken out of the
location pointed at by SP and SP is incremented by one. In this case the data do not move but
SP is incremented or decremented as a result of push or pop operations respectively.
Application of Stack: Stack provides a powerful data structure which has applications in many
situations. The main advantage of the stack is that,
We can store data (PUSH) in it with out destroying previously stored data. This is not true in the
case of other registers and memory locations.
stack operations are also very fast
The stack may also be used for storing local variables of subroutine and for the transfer of
parameter addresses to a subroutine. This facilitates the implementation of re-entrant
subroutines which is a very important software property.
The disadvantage is, as the stack has no fixed address, it is difficult to debug and document a
program that uses stack.
Stack operation: Operations on stack are performed using the two instructions namely PUSH
and POP. The contents of the stack are moved to certain memory locations after PUSH
instruction. Similarly, the contents of the memory are transferred back to registers by POP
instruction.
For example let us consider a Stack whose stack top is 4506 H. This is stored in the 16-
bit Stack pointer register as shown in Fig.29

Figure.29 The PUSH operation of the Stack
Let us consider two registers (register pair) B & C whose contents are 25 & 62.
Reg. B Reg. C
After PUSH operation the status of the Stack is as shown in Fig 3.30
Figure .30 After PUSH operation the status of the stack
Let us now consider POP operation: The Figs 31 & 32 explains before and after the POP
operation in detail
25 62

.
Figure 31 The POP operation of the Stack

Figure 32 After POP operation the status of the stack
Before the operation the data 15 and 1C are in the locations 4502 & 4503 and after the pop
operation the data is copied to B-C pair and now the SP register points to 4504 location. This is
shown in Fig.3.32
Programming Example FOR PUSH & POP
Write a program to initialize the stack pointer (SP) and store the contents of the register pair H-
L on stack by using PUSH instruction. Use the contents of the register pair for delay counter
and at the end of the delay retrieve the contents of H-L using POP.
Memory
Location
Label Mnemonics Operand Comments
8000
8003
8006
8007
.
LXI
LXI
PUSH
DELAY
SP, 4506 H
H,2565 H
H
CALL
Initialize
Stack
pointer
Push the

.
.
.
8.00A
.
.
.
POP
.
.
.
H
contents.

Subroutine: It is a set of instructions written separately from the main program to execute a
function that occurs repeatedly in the main program.
For example, let us assume that a delay is needed three times in a program. Writing delay
programs for three times in a main program is nothing but repetition. So, we can write a
subroutine program called ‘delay’ and can be called any number of times we need
Similarly, in 8085 microprocessor we do not find the instructions for multiplication and
division. For this purpose we write separate programs. So, in any main program if these
operations are needed more than once, the entire program will become lengthy and complex.
So, we write subroutine programs MUL & DIV separately from main program and use the
instruction CALL MUL (or) CALL DIV in the main program. This can be done any number of
times. At the end of every subroutine program there must be an instruction called ‘RET’. This
will take the control back to main program.
The 8085 microprocessor has two instructions to implement the subroutines. They are CALL
and RET. The CALL instruction is used in the main program to call a subroutine and RET
instruction is used at the end of the subroutine to return to the main program. When a
subroutine is called, the contents of the program counter, which is the address of the
instruction following the CALL instruction is stored on the stack and the program execution is
transferred to the subroutine address. When the RET instruction is executed at the end of the
subroutine, the memory address stored on the stack is retrieved and the sequence of execution
is resumed in the main program.
Diagrammatic representation
Let us assume that the execution of the main program started at 8000 H. It continues until a
CALL subroutine instruction at 8020 H is encountered. Then the program execution transfers to
8070 H. At the end of the subroutine 807B H. The RET instruction is present. After executing
this RET, it comes back to main program at 8021 H as shown in the following Fig. 33

Fig.33 Diagrammatic representation of subroutine program execution
The same is explained using the assembly language program example.
Program Example:
Memory
Address
Mnemonics Operand Comments
8000
|
|
|
8020
8021
8022
8023
|
|
|
802F
LXI
CALL
Next
instruction
|
|
|
HLT
SP, 8400 H
8070 H
Initialize the Stack pointer at 8400
H
Call a subroutine program stored
at the location 8070 H. (It is a three
by Instruction)
The address of the next instruction
following CALL instruction.
End of the main program
.
Memory
Address

Subroutine
Program:
Delay
programs:
In many situations it may be desired to provide some delay between the execution of two
instructions by a microprocessor. The delay can be produced by either hardware chip like 8253
or by writing a software program using registers of the processor. Here we will discuss the
software delay program. This delay program is not a part of the main program. Hence it is
called delay sub-routine program. For small delays we can use only one register. But for
longer delays one has to use two or three registers. The technique involved here is, a register
is loaded with a number and then decremented by using the instruction DCR until it becomes
zero. The time of execution of the microprocessor is equal to the delay time produced.
For example, we have constructed a display system where the LEDs receive the input from a
microprocessor. Since the microprocessor is a very fast device it sends the signal at very high
speeds there by our eye cannot recognize the display pattern. So, if you provide some delay
between two input signals, the display can be visualized clearly. Similarly to observe the
rotations of a stepper motor, a delay is needed between every two excitation signals applied to
the motor.
Delay Subroutine with one register:
Program
Address Label Machine
code
9000 MVI A, FF Get FF in register A
8070
|
|
|
|
807B
807C
807F
Instructions
RET
Next
Subroutine
RET
Beginning of the Subroutine.
End of the program
Instructions of next subroutine if
any
End of the subroutine.

9002 LOOP DCR A Decrement register A.
9003 JNZ LOOP Has the content of
register B becomes
zero? No, jump to
LOOP. Yes, proceed
ahead.
9006 RET Return to main
program
Calculation of Delay time for the above program:
In the above program register A is loaded by FFH B(255 decimal) and it is decremented in a
loop until it becomes zero. The delay produced by this program is as follows
We should know the number of times each instruction of the above program is being
executed. The number of states required for the execution of each instruction is as follows:
Instructions States
MVI A, FFH 7
(loop) DCR A 4
JNZ loop 7/10
RET 10

Total T States=3584
The time required for one T-state in INTEL 8085 microprocessor is nearly 330n.sec
Delay time is= 3584 x 333n.sec
= 3.584 x 0.333 x 10-3
seconds
= 1.18272 x 10-3
seconds
= 1. 193472 milliseconds
Delay Subroutine with two registers
Program:
Address Label Machine
Code
Mnemonic Operand Comments
8400 MVI B, 10H Get desired number in register B
8402 LOOP1 MVI C, 56H Get desired number in register
8404 LOOP2 DCR C Decrement C.
8405 JNZ LOOP2 Is [C] zero? No, go to LOOP2. Yes,
proceed further
8408 DCR B Decrement register B
8409 JNZ LOOP1 Is [B] zero? No, go to LOOP1. Yes,
proceed further
840C RET Return to main program.
Delay Subroutine using register pair
Program:
Addres
s
Label Machin
e
Code
8000 LXI D, FFFF Get FFFF in register pair D-E
LOOP DCX D Decrement count

MOV A, D Move the contents of register D to
accumulator
ORA E Check if D and E are zero.
JNZ LOOP If D-E is not zero, jump to LOOP
RET Return to main program
Delay Subroutine using three registers
Program:
Addres
s
Label Machin
e
Code
8400 MVI A, 98H Get control word
8402 OUT 03 Initialize port foe LED Display
8404 MVI B, 50H
8406 MVI C, FFH
8408 MVI D, FFH
840A DCR D Delay Subroutine with three
registers
840B JNZ LOOP3
840E DCR C
840F JNZ LOOP2
8412 DCR B
8413 JNZ LOOP1
8416 MVI A, 01
8418 OUT 01 Output for LED
8419 HLT Stop.

From the above discussion it is clear that with increase of T-states required for a delay
subroutine ,the delay time also increases.
8086 Microprocessor
– It is a 16-bit µp.
– 8086 has a 20 bit address bus can access up to 220 memory locations (1 MB).
– It can support up to 64K I/O ports.
– It provides 14, 16 -bit registers.
– It has multiplexed address and data bus AD0- AD15 and A16 – A19.
– It requires single phase clock with 33% duty cycle to provide internal timing.
– 8086 is designed to operate in two modes, Minimum and Maximum.
– It can prefetches up to 6 instruction bytes from memory and put them in instr queue in
order to speed up instruction execution.
– It requires +5V power supply.
– A 40 pin dual in line package
Architectural Diagram of 8086:
The 8086 has two parts, the Bus Interface Unit (BIU) and the Execution Unit (EU).
– The BIU fetches instructions, reads and writes data, and computes the 20-bit address.
– The EU decodes and executes the instructions using the 16-bit ALU.
– The two units functions independently.
Minimum and Maximum Modes:
– The minimum mode is selected by applying logic 1 to the MN / MXതതതത input pin. This
is a single microprocessor configuration.
– The maximum mode is selected by applying logic 0 to the MN / MXതതതത input pin. This
is a multi micro processors configuration.

BIU
Instruction
Pointer(IP)
Segment register (CS,DS,ES,SS)
Adder
Instruction Queue
ADDER
EXTRA SEGMENT (16)
CODE SEGMENT (16)
STACK SEGMENT (16)
DATA SEGMENT (16)
INSTRUCTION POINTER (16)
AH (8) AL (8)
BH (8) BL (8)
CH
(8)
CL (8)
DH
(8)
DL (8)
SP (16)
BP (16)
SI (16)
DI (16)
A
B
C
D
Segment register (CS,DS,ES,SS)
Instruction Queue
EU
GPR (AX,BX,CX,DX)
Pointer Registers(SP,BP)
Index register(SI,DI)
ALU
INSTRUCTION QUEUE (6 byte)
GPR (AX,BX,CX,DX)
Pointer Registers(SP,BP)
Index register(SI,DI)
Flag
(6 byte)

Bus Interface Unit (BIU):
– The BIU performs all bus operations such as instruction fetching, reading and writing
operands for memory and calculating the addresses of the memory operands.
– The instruction bytes are transferred to the instruction queue.
– It provides a full 16 bit bidirectional data bus and 20 bit address bus.
– The bus interface unit is responsible for performing all external bus operations.
Specifically it has the following functions:
– Instruction fetch , Instruction queuing, Operand fetch and storage, Address
calculation relocation and Bus control.
– The BIU uses a mechanism known as an instruction queue to implement a
pipeline architecture.
– This queue permits prefetch of up to six bytes of instruction code. Whenever the
queue of the BIU is not full and it has room for at least two more bytes and at
the same time EU is not requesting it to read or write operands from memory,
the BIU is free to look ahead in the program by prefetching the next sequential
instruction.
– These prefetching instructions are held in its FIFO queue. With its 16 bit data
bus, the BIU fetches two instruction bytes in a single memory cycle.
– After a byte is loaded at the input end of the queue, it automatically shifts up
through the FIFO to the empty location nearest the output.
– The EU accesses the queue from the output end. It reads one instruction byte
after the other from the output of the queue. If the queue is full and the EU is
not requesting access to operand in memory.
– These intervals of no bus activity, which may occur between bus cycles, are
known as Idle state.
– If the BIU is already in the process of fetching an instruction when the EU
request it to read or write operands from memory or I/O, the BIU first
completes the instruction fetch bus cycle before initiating the operand read /
write cycle.
– The BIU also contains a dedicated adder which is used to generate the 20bit
physical address that is output on the address bus. This address is formed by
adding an appended 16 bit segment address and a 16 bit offset address.
– For example: The physical address of the next instruction to be fetched is
formed by combining the current contents of the code segment CS register and
the current contents of the instruction pointer IP register.
EXECUTION UNIT (EU)
– The Execution unit is responsible for decoding and executing all instructions.

– The EU extracts instructions from the top of the queue in the BIU, decodes
them, generates operands if necessary, passes them to the BIU and requests it to
perform the read or write bys cycles to memory or I/O and perform the
operation specified by the instruction on the operands.
– During the execution of the instruction, the EU tests the status and control flags
and updates them based on the results of executing the instruction.
– If the queue is empty, the EU waits for the next instruction byte to be fetched
and shifted to top of the queue.
– When the EU executes a branch or jump instruction, it transfers control to a
location corresponding to another set of sequential instructions.
– Whenever this happens, the BIU automatically resets the queue and then begins
to fetch instructions from this new location to refill the queue
The BIU contains the following registers:
IP - the Instruction Pointer
CS - the Code Segment Register
DS - the Data Segment Register
SS - the Stack Segment Register
ES - the Extra Segment Register
The BIU fetches instructions using the CS and IP, written CS:IP, to contract the 20-bit
address. Data is fetched using a segment register (usually the DS) and an effective address
(EA) computed by the EU depending on the addressing mode.
Internal Registers of 8086
AX AH AL Accumulator
BX BH BL Base Register
CX CH CL Count Register
DX DH DL Data Register
SP Stack Pointer
BP Base Pointer
SI Source Index Register
DI Destination Index Register
CS Code Segment Register
DS Data Segment Register
SS Stack Segment Register
ES Extra Segment Register
FR Flag Register
IP Instruction Pointer
BIU
Registers
EU
Registers

• The 8086 has four groups of the user accessible internal registers.
• These are
Instruction pointer(IP)
Four General purpose registers(AX,BX,CX,DX)
Four pointer (SP,BP,SI,DI)
Four segment registers (CS,DS,SS,ES)
Flag Register(FR)
• The 8086 has a total of fourteen 16-bit registers including a 16 bit register called the
status register (flag register), with 9 of bits implemented for status and control flags.
• Most of the registers contain data/instruction offsets within 64 KB memory segment.
• There are four different 64 KB segments for instructions, stack, data and extra data. To
specify where in 1 MB of processor addressable memory these 4 segments are located
the processor uses four segment registers:
Segment Registers
1) Code segment (CS) is a 16-bit register containing address of 64 KB segment with
processor instructions. The processor uses CS segment for all accesses to instructions
referenced by instruction pointer (IP) register.
2) Stack segment (SS) is a 16-bit register containing address of 64KB segment with
program stack. By default, the processor assumes that all data referenced by the stack
pointer (SP) and base pointer (BP) registers is located in the stack segment. SS register
can be changed directly using POP instruction.
3) Data and Extra segment (DS and ES) is a 16-bit register containing address of 64KB
segment with program data. By default, the processor assumes that all data referenced
by general registers (AX, BX, CX, and DX) and index register (SI, DI) is located in
the data and Extra segment.
Data Registers
1) AX (Accumulator)
• It is consists of two 8-bit registers AL and AH, which can be combined
together and used as a 16-bit register AX. AL in this case contains the low-
order byte of the word, and AH contains the high-order byte. Accumulator
can be used for I/O operations and string manipulation.
2) BX (Base register)

• It is consists of two 8-bit registers BL and BH, which can be combined
together and used as a 16-bit register BX. BL in this case contains the low-
order byte of the word, and BH contains the high-order byte.
• BX register usually contains a offset for data segment.
3) CX (Count register)
• It is consists of two 8-bit registers CL and CH, which can be combined
together and used as a 16-bit register CX. When combined, CL register
contains the low-order byte of the word, and CH contains the high-order
byte.
• Count register can be used in Loop, shift/rotate instructions and as a counter
in string manipulation.
• 8086 has the LOOP instruction which is used for conuter purpose when it is
executed CX/CL is automatically decremented by 1.
EX
MOV CL, 05H
START NOP
LOOP START (here CL is automatically decremented by 1without
DCR instruction.
4) DX (Data register)
• It is consists of two 8-bit registers DL and DH, which can be combined
together and used as a 16-bit register DX. When combined, DL register
contains the low-order byte of the word, and DH contains the high-order
byte.
• DX can be used as a port number in I/O operations.
• In integer 32-bit multiply and divide instruction the DX register contains
high-order word of the initial or resulting number.
Pointer register
1. Stack Pointer (SP) is a 16-bit register is used to hold the offset address for stack
segment.
2. Base Pointer (BP) is a 16-bit register is used to hold the offset address for stack
segment.
i. BP register is usually used for based, based indexed or register indirect
addressing.
ii. The difference between SP and BP is that the SP is used internally to
store the address in case of interrupt and the CALL instrn.
3. Source Index (SI) and Destination Index (DI )
These two 16-bit register is used to hold the offset address for DS and ES in case of
string manipulation instrn.
i. SI is used for indexed, based indexed and register indirect addressing, as
well as a source data addresses in string manipulation instructions.
ii. DI is used for indexed, based indexed and register indirect addressing,
as well as a destination data addresses in string manipulation
instructions.
Instruction Pointer (IP)

It is a 16-bit register. It acts as a program counter and is used to hold the offset address
for CS.
A flag is a 16-bit register containing 9 one bit flags.
i. Overflow Flag (OF)
This flag is set if an overflow occurs. i.e. if the result of a signed operation is large
enough to be accommodated in a destination register.
ii. Direction Flag (DF) –
This is used by string manipulation instructions. If this flag bit is
‘0’, the string is processed beginning from the lowest address to the
highest address. i.e. auto-incrementing mode.
Otherwise, the string is processed from the highest address towards
the lowest address, i.e. auto-decrementing mode.
iii. Interrupt-enable Flag (IF) –
If this flag is set, the maskable interrupts are recognized by the CPU. Otherwise
they are ignored. Setting this bit enables maskable interrupts.
iv. Single-step Flag (TF) –
If this flag is set, the processor enters the single step execution mode. In other
words, a trap interrupt is generated after execution of each instruction. The
processor executes the current instruction and the control is transferred to the
Trap interrupt service routine.
v. Sign Flag (SF) –
This flag is set when the result of any computation is negative. For signed
computations, the sign flag equals the MSB of the result.
vi. Zero Flag (ZF) - set if the result is zero.
vii. Auxiliary carry Flag (AF) –
set if there was a carry from or borrow to bits 0-3 in the AL register.
viii. Parity Flag (PF) –
set if parity (the number of "1" bits) in the low-order byte of the result is even.
ix. Carry Flag (CF) –
This flag is set when there is a carry out of MSB in case of addition or a borrow
in case of subtraction. For example. When two numbers are added, a carry may

be generated out of the most significant bit position. The carry flag, in this case,
will be set to 1’. In case, no carry is generated, it will be ‘0.
Segmented Memory
Reason for Segmented Memory:
8086 has a 20-bit address bus. So it can address a maximum of 1MB of memory and
each memory location is addressed by a 20 bit address.
To hold a 20-bit address there must be a 20-bit address register available within
processor but 8086 only has 16-bit registers. So 20-bit address can’t be stored inside
the 16-bit register. To avoid this problem segmented memory is used in 8086.
Total 1MB memory can be divided into some equal size segments each of having capacity
64 KB.
So max no of segments is 16. (1mb/64 kb=16)
8086 can work with only four 64KB segments at a time within this 1MB range.
Each location in a particular segment can be expressed by two addresses.
i) Segment Address (16 bit): It refers the starting address of a segment and it is
fixed for whole of the segment.
ii) Offset or Displacement Address (16 bit): It refers the individual location in
that segment and it is varied location wise.
By using these two addresses the 20 bit physical address can be calculated as below:
Physical address (20 bit) = [Segment Address (16 bit) * 10]H + Offset Address(16 bit)
According to this formula segment address is multiplied by 10 and is added to offset.
This is equivalent to shifting of segment register content towards left 4 times so that
four zero are added to right side (MSB) of the segment address and added with the
offset address to get the physical address which is 20 bit.
Physical address (20 bit)
Segment Register (16
bit)
Offset Address (16 bit)
Adder
0000
Figure 1 Fig: Physical address calculation

EX:-
Given Segment Address=3578H,
So Physical address = [Segment Address * 10]
= [3578 * 10]
= 35780+6676
= 3BDF6H
Types of Segments
There are four types of memory segments
• Code segment(CS)
• Data segment (DS)
• Stack segment(SS)
• Extra segment(ES)
Code segment (CS): This segment is used to store code/program instructions.
Data and Extra segment (DS&ES
Stack segment (SS): This segment is used to store the stack contents.
Types of Segments Registers
To hold the upper 16-bits of the starting address for each of the segments
segment registers:
– CS (Code Segment register)
– DS (Data Segment register)
– SS (Stack Segment register)
– ES (Extra Segment register)
Given Segment Address=3578H, Offset Address =6676H
So Physical address = [Segment Address * 10]H + Offset Address
= [3578 * 10]H + 6676H
= 35780+6676
= 3BDF6H
memory segments defined in 8086:
This segment is used to store code/program instructions.
&ES): This segment is used to store data
This segment is used to store the stack contents.
Registers:
bits of the starting address for each of the segments
CS (Code Segment register)
DS (Data Segment register)
SS (Stack Segment register)
ES (Extra Segment register)
This segment is used to store code/program instructions.
data used in the program.
bits of the starting address for each of the segments, there are four

Advantage of memory Segmentation:
• Allows the memory capacity to be 1 Mbytes although the actual addresses
handled are of 16-bit size.
• Allows the placing of code, data and stack portions of the same program in different
parts (segments) of memory, for data and code protection.
• Permits a program and/or its data to be put into different areas of memory each
the program is executed. i
PIN Diagram
The following signal descriptions are common for both modes.
AD15-AD0:
These are the time multiplexed memory I/O address and data lines. Address remains
on the lines during T1 state, while the data is available on the data bus during T2, T3, Tw
and T4.
Advantage of memory Segmentation:
Allows the memory capacity to be 1 Mbytes although the actual addresses
bit size.
Allows the placing of code, data and stack portions of the same program in different
parts (segments) of memory, for data and code protection.
Permits a program and/or its data to be put into different areas of memory each
i.e., provision for relocation is done.
Allows the memory capacity to be 1 Mbytes although the actual addresses to be
Allows the placing of code, data and stack portions of the same program in different
Permits a program and/or its data to be put into different areas of memory each time

A19/S6, A18/S5, A17/S4, A16/S3:
These are the time multiplexed address and status lines. During T1 these
are the most significant address lines for memory operations. During I/O
operations, these lines are low. During memory or I/O operations, status
information is available on those lines for T2, T3, Tw and T4.
• A16/S3,A17/S4-
A16,A17 are multiplexed with segment identifier signals
S3 and S4 which combinedly indicate which segment
register is presently being used for memory accesses as in
below fig..
S4 S3 Indication
0 0 Extra segment(ES)
0 1 Stack segment(SS)
1 0 Code segment(CS)
1 1 Data segment (DS)
• A18/s5: A18 is multiplexed with status S5 of the interrupt enable flag
bit which is updated at the beginning of each clock cycle.
• A19/s6: A18 is multiplexed with status S6.
۰۶۳തതതതതത / S7: (Bus High enable)
• The bus high enable is used to indicate the transfer of data over the
higher order (D15-D8) data bus as shown in table.
• It goes low for the data transfer over D15-D8 and is used to derive chip
selects of odd address memory bank or peripherals. BHE is low during
T1 for read, write and interrupt acknowledge cycles, whenever a byte is
to be transferred on higher byte of data bus.
• The status information is available during T2, T3 and T4. The signal is
active low.
۰۶۳തതതതതത A0 Indication
0 0 Whole Word
0 1 Upper byte from or to odd address
1 0 Lower byte from or to even address
1 1 None

‫۲܀‬തതതത (Read):
• This signal on low indicates the peripheral that the processor is
performing s memory or I/O read operation. The signal is active low.
READY:
• This is the acknowledgement from the slow device or memory that they
have completed the data transfer. The signal made available by the
devices is synchronized by the 8284A clock generator to provide ready
input to the 8086. This signal is active high.
• enter into wait states and remain idle : READY = 0
• no effect on the operation of µ : READY = 1
INTR (Interrupt Request):
• This is a level triggered input and hardware interrupt pin.
• If any interrupt request is pending, the processor enters the interrupt
acknowledge cycle. This can be internally masked by resulting the
interrupt enable flag.
NMI : non-maskable interrupt
• This is a edge triggered input and hardware interrupt pin which causes
Type 2 interrupt.
‫܂܁۳܂‬തതതതതതത:
• This input is examined by a ‘WAIT’ instruction.
• If the TEST pin goes low, execution will continue, else the processor
remains in an idle state.
CLK (Clock Input):
• The clock input provides the basic timing for processor operation and
bus control activity.
VCC (power supply) : +5.0V, ±10%
RESET:
• µ : reset if RESET is high
GND(Ground) : Two pins labeled GND(0 voltage
MN/‫܆ۻ‬തതതതത:
• The logic level at this pin decides whether the processor is to operate in
either minimum or maximum mode.
• if MN/‫܆ۻ‬തതതതത= 1; Minimum Mode else Maximum Mode
Pin functions for the minimum mode operation of
8086

1. MMMM////۷‫۽‬തതത
• This is a status line logically equivalent to S2തതത in maximum
mode. When it is low, it indicates the CPU is having an I/O
operation, and when it is high, it indicates that the CPU is
having a memory operation.
2. ۷‫ۯ܂ۼ‬തതതതതതത (Interrupt Acknowledge):
• when this signal it goes low, the processor has accepted the
interrupt.
3. ALE (Address Latch Enable):
• This output signal indicates the availability of the valid
address on the address/data lines, and is connected to latch
enable input of latches.
4. DT/DT/DT/DT/‫܀‬ഥ (Data Transmit/Receive):
• This output is used to decide the direction of data flow
through the transreceivers (bidirectional buffers).
• When the processor sends out data, this signal is high and
when the processor is receiving data, this signal is low.
5. ۲۳‫ۼ‬തതതതതത Data Enable:
• This signal indicates the availability of valid data over the
address/data lines. It is used to enable the transreceivers
(bidirectional buffers) to separate the data from the multiplexed
address/data signal.
• It is active from the middle of T2 until the middle of T4.
6. HOLD, HLDA- Acknowledge:
• When the HOLD line goes high, it indicates to the processor that
another master is requesting the bus access.
• The processor, after receiving the HOLD request, issues the hold
acknowledge signal on HLDA pin, in the middle of the next
clock cycle after completing the current bus cycle.
7. ‫܀܅‬തതതതത (Write):
• When it is low the processor perform memory or Io write .
Pin functions for the maximum mode operation of
8086
1. ‫܁‬૛തതതത, ‫܁‬૚തതതത, ‫܁‬૙തതതത – Status Lines:

• These signals are connected to 8288.These are the status lines
which reflect the type of operation according to the below table,
being carried out by the processor.
‫܁‬૛തതതത ‫܁‬૚തതതത ‫܁‬૙തതതത Indication
0 0 0 Interrupt acknowledge
0 0 1 Read I/O port
0 1 0 Write I/O port
0 1 1 Halt
0 0 0 Code access
0 0 1 Read memory
0 1 0 Write memory
0 1 1 Passive State
2. ‫۹۱۽ۺ‬തതതതതതതത :
• This output pin indicates that other system bus master will be
prevented from gaining the system bus, while the LOCK signal is
low.
• The LOCK signal is activated by the ‘LOCK’ prefix instruction
and remains active until the completion of the next instruction.
When the CPU is executing a critical instruction which requires
the system bus, the LOCK prefix instruction ensures that other
processors connected in the system will not gain the control of
the bus.
3. QS1, QS0 (queue status)
• These lines give information about the status of the code-prefetch
queue. These are active during the CLK cycle after while the queue
operation is performed.
QS1 QS0 Indication
0 0 No operation
0 1 First byte of opcode from the queue
1 0 Empty Queue
1 1 Subsequent byte from queue
4. ‫܂۵/ۿ܀‬૚തതതതതതതതതതതത ,‫܂۵/ۿ܀‬૙തതതതതതതതതതതത (Request/Grant)

• These pins are used by the other local bus master in maximum mode,
to force the processor to release the local bus at the end of the
processor current bus cycle.
• Each of the pin is bidirectional with RQ/GT0 having higher priority
than RQ/GT1.
In maximum mode of operation signals like ‫܀܅‬തതതതത , ALE, ۲۳‫ۼ‬തതതതത, DT/ ‫܀‬ഥ etc
are not available directly from the processor.
These signals are available from the controller 8288.
Minimum
Mode
Maximum
Mode

Microprocessors & Microcomputers Lecture Notes

ADDRESSING MODES OF 8086
Addressing mode indicates a way of locating data or operands. Depending
upon the data types used in the instruction and the memory addressing modes,
any instruction may belong to one or more addressing modes, or some
instruction may not belong to any of the addressing modes. Thus the
addressing modes describe the types of operands and the way they are
accessed for executing an instruction. Here, we will present the addressing
modes of the instructions depending upon their types. According to the flow
of instruction execution, the instructions may be categorized as
(i) Sequential control flow instructions and
(ii) Control transfer instructions.
Sequential control flow instructions are the instructions, which after
execution, transfer control to the next instruction appearing immediately after
it (in the sequence) in the program. For example, the arithmetic, logical, data
transfer and processor control instructions are sequential control flow
instructions. The control transfer instructions, on the other hand, transfer
control to some predefined address somehow specified in the instruction after
their execution. For example, INT, CALL, RET and JUMP instructions fall
under this category.
The addressing modes for sequential control transfer instructions are
explained as follows:
1. Immediate: In this type of addressing, immediate data is a part of
instruction, and appears in the form of successive byte or bytes.
Example: MOV AX, 0005H
In the above example, 0005H is the immediate data. The immediate data may
be 8-bit or 16-bit in size.
2. Direct: In the direct addressing mode, a 16-bit memory address (offset)
is directly specified in the instruction as a part of it.
Example: MOV AX, [5000H]

Here, data resides in a memory location in the data segment, whose effective
address may be computed using 5000H as the offset address and content of
DS as segment address. The effective address, here, is 10H*DS+5000H.
3. Register: In register addressing mode, the data is stored in a register
and it is referred using the particular register. All the registers, except IP, may
be used in this mode.
Example: MOV BX, AX.
4. Register Indirect: Sometimes, the address of the memory location,
which contains data or operand, is determined in an indirect way, using the
offset registers. This mode of addressing is known as register indirect mode.
In this addressing mode, the offset address of data is in either BX or SI or DI
registers. The default segment is either DS or ES. The data is supposed to be
available at the address pointed to by the content of any of the above registers
in the default data segment.
Example: MOV AX, [BX]
Here, data is present in a memory location in DS whose offset address is in
BX. The effective address of the data is given as 10H*DS+ [BX].
5. Indexed: In this addressing mode, offset of the operand is stored in one
of the index registers. DS and ES are the default segments for index registers
SI and DI respectively. This mode is a special case of the above discussed
register indirect addressing mode.
Example: MOV AX, [SI]
Here, data is available at an offset address stored in SI in DS. The effective
address, in this case, is computed as 10H*DS+ [SI].
6. Register Relative: In this addressing mode, the data is available at an
effective address formed by adding an 8-bit or 16-bit displacement with the
content of any one of the registers BX, BP, SI and DI in the default (either DS
or ES) segment. The example given before explains this mode.
Example: MOV Ax, 50H [BX]
Here, effective address is given as 10H*DS+50H+ [BX].
7. Based Indexed: The effective address of data is formed, in this
addressing mode, by adding content of a base register (any one of BX or BP)

to the content of an index register (any one of SI or DI). The default segment
register may be ES or DS.
Example: MOV AX, [BX] [SI]
Here, BX is the base register and SI is the index register. The effective address
is computed as 10H*DS+ [BX] + [SI].
8. Relative Based Indexed: The effective address is formed by adding an
8-bit or 16-bit displacement with the sum of contents of any one of the bases
registers (BX or BP) and any one of the index registers, in a default segment.
Example: MOV AX, 50H [BX] [SI]
Here, 50H is an immediate displacement, BX is a base register and SI is an
index register. The effective address of data is computed as 160H*DS+ [BX]
+ [SI] + 50H.
For the control transfer instructions, the addressing modes depend upon
whether the destination location is within the same segment or a different one.
It also depends upon the method of passing the destination address to the
processor. Basically, there are two addressing modes for the control transfer
instructions, viz. inter-segment and intra-segment addressing modes.
If the location to which the control is to be transferred lies in a different
segment other than the current one, the mode is called inter-segment mode. If
the destination location lies in the same segment, the mode is called intra-
segment.
Modes for
control
Transfer
instructions
Inter-segment
Inter-segment
Direct
Inter-segment
Indirect
Intra-segment
Intra-segment
Direct
Intra-segment
Indirect

ADDRESSING MODES FOR CONTROL TRANSFER INSTRUCTION
1. Intra-segment direct mode: In this mode, the address to which the
control is to be transferred lies in the same segment in which the control
transfer instruction lies and appears directly in the instruction as an
immediate displacement value. In this addressing mode, the
displacement is computed relative to the content of the instruction
pointer IP.
The effective address to which the control will be transferred is given by the
sum of 8 or 16 bit displacement and current content of IP. In case of jump
instruction, if the signed displacement (d) is of 8 bits (i.e. –128<d<+128), we
term it as short jump and if it is of
16 bits (i.e. –32768<+32768), it is termed as long jump.
2. Intra-segment Indirect Mode: In this mode, the displacement to which
the control is to be transferred, is in the same segment in which the
control transfer instruction lies, but it is passed to the instruction
indirectly. Here, the branch address is found as the content of a register
or a memory location. This addressing mode may be used in
unconditional branch instructions.
3. Inter-segment Direct Mode: In this mode, the address to which the
control is to be transferred is in a different segment. This addressing
mode provides a means of branching from one code segment to another
code segment. Here, the CS and IP of the destination address are
specified directly in the instruction.
4. Inter-segment Indirect Mode: In this mode, the address to which the
control is to be transferred lies in a different segment and it is passed to
the instruction indirectly, i.e. contents of a memory block containing
four bytes, i.e. IP (LSB), IP (MSB), CS (LSB) and CS (MSB)
sequentially. The starting address of the memory block may be referred
using any of the addressing modes, except immediate mode.
8086 Instruction Set and Assembler Directives
The 8086 microprocessor supports 6 types of Instructions. They are
1. Data transfer instructions
2. Arithmetic instructions

3. Bit manipulation instructions
4. String instructions
5. Program Execution Transfer instructions (Branch & loop Instructions)
6. Processor control instructions
1. Data Transfer instructions: These instructions are used to transfer the
data from source operand to destination operand. All the store, move, load,
exchange, input and output instructions belong to this group.
General purpose byte or word transfer instructions:
MOV : Copy byte or word from specified source to specified destination
PUSH : Push the specified word to top of the stack
POP : Pop the word from top of the stack to the specified location
PUSHA : Push all registers to the stack
POPA : Pop the words from stack to all registers
XCHG : Exchange the contents of the specified source and destination
operands one of which may be a register or memory location.
XLAT : Translate a byte in AL using a table in memory
Simple input and output port transfer instructions
1. IN : Reads a byte or word from specified port to the
accumulator
2. OUT : Sends out a byte or word from accumulator to a
specified port
Special address transfer instructions
1. LEA : Load effective address of operand into specified
register
2. LDS : Load DS register and other specified register from
memory
3. LES : Load ES register and other specified register from
memory.
Flag transfer registers

1. LAHF : Load AH with the low byte of the flag register
2. SAHF : Store AH register to low byte of flag register
3. PUSHF : Copy flag register to top of the stack
4. POPF : Copy word at top of the stack to flag register
2. Arithmetic instructions : These instructions are used to perform various
mathematical operations like addition, subtraction, multiplication and division
etc….
Addition instructions
1.ADD : Add specified byte to byte or word to word
2.ADC : Add with carry
3.INC : Increment specified byte or specified word by 1
4.AAA : ASCII adjust after addition
5.DAA : Decimal (BCD) adjust after addition
Subtraction instructions
1. SUB : Subtract byte from byte or word from word
2. SBB : Subtract with borrow
3. DEC : Decrement specified byte or word by 1
4. NEG : Negate or invert each bit of a specified byte or word
and add 1(2’s complement)
5. CMP : Compare two specified byte or two specified words
6. AAS : ASCII adjust after subtraction
7. DAS : Decimal adjust after subtraction
Multiplication instructions
1. MUL : Multiply unsigned byte by byte or unsigned word or
word.
2. IMUL : Multiply signed bye by byte or signed word by word
3. AAM : ASCII adjust after multiplication
Division instructions
1. DIV : Divide unsigned word by byte or unsigned double
word by word
2. IDIV : Divide signed word by byte or signed double word by
word
3. AAD : ASCII adjust after division

4. CBW : Fill upper byte of word with copies of sign bit of
lower byte
5. CWD : Fill upper word of double word with sign bit of
lower word.
3. Bit Manipulation instructions : These instructions include logical , shift
and rotate instructions in which a bit of the data is involved.
Logical instructions
1. NOT :Invert each bit of a byte or word.
2. AND : ANDing each bit in a byte or word with the
corresponding bit in another byte or word.
3. OR : ORing each bit in a byte or word with the
4. XOR : Exclusive OR each bit in a byte or word with the
5. TEST :AND operands to update flags, but don’t change
operands.
Shift instructions
1. SHL/SAL : Shift bits of a word or byte left, put zero(S) in
LSBs.
2. SHR : Shift bits of a word or byte right, put zero(S) in
MSBs.
3. SAR : Shift bits of a word or byte right, copy old MSB
into new MSB.
Rotate instructions
1. ROL : Rotate bits of byte or word left, MSB to LSB and to
Carry Flag [CF]
2. ROR : Rotate bits of byte or word right, LSB to MSB and to
Carry Flag [CF]
3. RCR :Rotate bits of byte or word right, LSB TO CF and CF to
MSB
4. RCL :Rotate bits of byte or word left, MSB TO CF and CF to
LSB
4. String instructions
A string is a series of bytes or a series of words in sequential memory
locations. A string often consists of ASCII character codes.

1. REP : An instruction prefix. Repeat following instruction until
CX=0
2. REPE/REPZ : Repeat following instruction until CX=0 or zero
flag ZF=1
3. REPNE/REPNZ : Repeat following instruction until CX=0 or
zero flag ZF=1
4. MOVS/MOVSB/MOVSW: Move byte or word from one string
to another
5. COMS/COMPSB/COMPSW: Compare two string bytes or two
string words
6. INS/INSB/INSW: Input string byte or word from port
7. OUTS/OUTSB/OUTSW : Output string byte or word to port
8. SCAS/SCASB/SCASW: Scan a string. Compare a string byte
with a byte in AL or a string word with a word in AX
9. LODS/LODSB/LODSW: Load string byte in to AL or string
word into AX
5.Program Execution Transfer instructions
These instructions are similar to branching or looping instructions.
These instructions include conditional & unconditional jump or loop
instructions.
Unconditional transfer instructions
1. CALL : Call a procedure, save return address on stack
2. RET : Return from procedure to the main program.
3. JMP : Goto specified address to get next instruction
Conditional transfer instructions
1. JA/JNBE : Jump if above / jump if not below or equal
2. JAE/JNB : Jump if above /jump if not below
3. JBE/JNA : Jump if below or equal/ Jump if not above
4. JC : jump if carry flag CF=1
5. JE/JZ : jump if equal/jump if zero flag ZF=1
6. JG/JNLE : Jump if greater/ jump if not less than or equal
7. JGE/JNL : jump if greater than or equal/ jump if not less than
8. JL/JNGE : jump if less than/ jump if not greater than or equal
9. JLE/JNG : jump if less than or equal/ jump if not greater than
10.JNC : jump if no carry (CF=0)

11.JNE/JNZ : jump if not equal/ jump if not zero(ZF=0)
12.JNO : jump if no overflow(OF=0)
13.JNP/JPO : jump if not parity/ jump if parity odd(PF=0)
14.JNS : jump if not sign(SF=0)
15.JO : jump if overflow flag(OF=1)
16.JP/JPE : jump if parity/jump if parity even(PF=1)
17.JS : jump if sign(SF=1)
6. Iteration control instructions
These instructions are used to execute a series of instructions for certain
number of times.
1. LOOP :Loop through a sequence of instructions until CX=0
2. LOOPE/LOOPZ : Loop through a sequence of instructions while
ZF=1 and CX = 0
3. LOOPNE/LOOPNZ : Loop through a sequence of instructions
while ZF=0 and CX =0
4. JCXZ : jump to specified address if CX=0
7. Interrupt instructions
1. INT : Interrupt program execution, call service procedure
2. INTO : Interrupt program execution if OF=1
3. IRET : Return from interrupt service procedure to main program
8.High level language interface instructions
1. ENTER : enter procedure
2. LEAVE :Leave procedure
3. BOUND : Check if effective address within specified array
bounds
9.Processor control instructions
Flag set/clear instructions
1. STC : Set carry flag CF to 1
2. CLC : Clear carry flag CF to 0
3. CMC : Complement the state of the carry flag CF
4. STD : Set direction flag DF to 1 (decrement string pointers)
5. CLD : Clear direction flag DF to 0
6. STI : Set interrupt enable flag to 1(enable INTR input)
7. CLI : Clear interrupt enable Flag to 0 (disable INTR input)

10. External Hardware synchronization instructions
1. HLT : Halt (do nothing) until interrupt or reset
2. WAIT : Wait (Do nothing) until signal on the test pin is low
3. ESC : Escape to external coprocessor such as 8087 or 8089
4. LOCK : An instruction prefix. Prevents another processor from
taking the bus while the adjacent instruction executes.
11. No operation instruction
1. NOP : No action except fetch and decode
Instruction Description
AAA Instruction - ASCII Adjust after Addition
AAD Instruction - ASCII adjust before Division
AAM Instruction - ASCII adjust after Multiplication
AAS Instruction - ASCII Adjust for Subtraction
ADC Instruction - Add with carry.
ADD Instruction - ADD destination, source
AND Instruction - AND corresponding bits of two operands
Example
AAA Instruction:
AAA converts the result of the addition of two valid unpacked BCD digits to a
valid 2-digit BCD number and takes the AL register as its implicit operand.
Two operands of the addition must have its lower 4 bits contain a number in
the range from 0-9.The AAA instruction then adjust AL so that it contains a
correct BCD digit. If the addition produce carry (AF=1), the AH register is
incremented and the carry CF and auxiliary carry AF flags are set to 1. If the
addition did not produce a decimal carry, CF and AF are cleared to 0 and AH
is not altered. In both cases the higher 4 bits of AL are cleared to 0.
AAA will adjust the result of the two ASCII characters that were in the range
from 30h (“0”) to 39h(“9”).This is because the lower 4 bits of those character
fall in the range of 0-9.The result of addition is not a ASCII character but it is
a BCD digit.

Example:
MOV AH, 0 ; Clear AH for MSD
MOV AL, 6 ; BCD 6 in AL
ADD AL, 5 ; Add BCD 5 to digit in AL
AAA ; AH=1, AL=1 representing BCD 11.
AAD Instruction: ADD converts unpacked BCD digits in the AH and
AL register into a single binary number in the AX register in
preparation for a division operation.
Before executing AAD, place the Most significant BCD digit in the AH
register and Last significant in the AL register. When AAD is executed,
the two BCD digits are combined into a single binary number by setting
AL=(AH*10)+AL and clearing AH to 0.
Example:
MOV AX, 0205h ; The unpacked BCD number 25
AAD ; After AAD, AH=0 and
; AL=19h (25)
After the division AL will then contain the unpacked BCD quotient and AH
will contain the unpacked BCD remainder.
Example:
; AX=0607 unpacked BCD for 67 decimal
; CH=09H
AAD ; Adjust to binary before division
; AX=0043 = 43H =67 decimal
DIV CH ; Divide AX by unpacked BCD in CH
; AL = quotient = 07 unpacked BCD
; AH = remainder = 04 unpacked BCD

AAM Instruction - AAM converts the result of the multiplication of two
valid unpacked BCD digits into a valid 2-digit unpacked BCD number
and takes AX as an implicit operand.
To give a valid result the digits that have been multiplied must be in the range
of 0 – 9 and the result should have been placed in the AX register. Because
both operands of multiply are required to be 9 or less, the result must be less
than 81 and thus is completely contained in AL.
AAM unpacks the result by dividing AX by 10, placing the quotient (MSD) in
AH and the remainder (LSD) in AL.
Example:
MOV AL, 5
MOV BL, 7
MUL BL ; Multiply AL by BL, result in AX
AAM ; After AAM, AX =0305h (BCD 35)
AAS Instruction: AAS converts the result of the subtraction of two
valid unpacked BCD digits to a single valid BCD number and takes the
AL register as an implicit operand.
The two operands of the subtraction must have its lower 4 bit contain number
in the range from 0 to 9.The AAS instruction then adjust AL so that it contain
a correct BCD digit.
MOV AX, 0901H ; BCD 91
SUB AL, 9 ; Minus 9
AAS ; Give AX =0802 h (BCD 82)
( a )
; AL =0011 1001 =ASCII 9
; BL=0011 0101 =ASCII 5
SUB AL, BL ; (9 - 5) Result:
; AL = 00000100 = BCD 04, CF = 0
AAS ; Result:

; AL=00000100 =BCD 04
; CF = 0 NO Borrow required
( b )
; AL = 0011 0101 =ASCII 5
; BL = 0011 1001 = ASCII 9
SUB AL, BL ; ( 5 - 9 ) Result:
; AL = 1111 1100 = - 4
; in 2’s complement CF = 1
AAS ; Results:
; AL = 0000 0100 =BCD 04
; CF = 1 borrow needed.
ADD Instruction:
These instructions add a number from source to a number from some
destination and put the result in the specified destination. The add with carry
instruction ADC, also add the status of the carry flag into the result.
The source and destination must be of same type, means they must be a byte
location or a word location. If you want to add a byte to a word, you must
copy the byte to a word location and fill the upper byte of the word with
zeroes before adding.
EXAMPLE:
ADD AL, 74H ; Add immediate number 74H to content of AL
ADC CL, BL ; Add contents of BL plus
; carry status to contents of CL.
; Results in CL
ADD DX, BX ; Add contents of BX to contents ; of DX
ADD DX, [SI] ; Add word from memory at ; offset [SI] in DS to contents
of DX

; Addition of Un Signed numbers
ADD CL, BL ; CL = 01110011 =115 decimal
; + BL = 01001111 = 79 decimal
; Result in CL = 11000010 = 194 decimal
; Addition of Signed numbers
ADD CL, BL ; CL = 01110011 = + 115 decimal
; + BL = 01001111 = +79 decimal
; Result in CL = 11000010 = - 62 decimal
; Incorrect because result is too large to fit in 7 bits.
AND Instruction:
This Performs a bitwise Logical AND of two operands. The result of the
operation is stored in the op1 and used to set the flags.
AND op1, op2
To perform a bitwise AND of the two operands, each bit of the result is set to
1 if and only if the corresponding bit in both of the operands is 1, otherwise
the bit in the result I cleared to 0.
AND BH, CL ; AND byte in CL with byte in BH ; result in BH
AND BX, 00FFh ; AND word in BX with immediate ; 00FFH. Mask
upper byte, leave ; lower unchanged
AND CX, [SI] ; AND word at offset [SI] in data ; segment with word in
CX ; register. Result in CX register.
; BX = 10110011 01011110
AND BX, 00FFh ; Mask out upper 8 bits of BX
; Result BX = 00000000 01011110
; CF =0, OF = 0, PF = 0, SF = 0,
; ZF = 0

CALL Instruction
•Direct within-segment (near or intrasegment)
•Indirect within-segment (near or intrasegment)
•Direct to another segment (far or intersegment)
•Indirect to another segment (far or intersegment)
CBW Instruction - Convert signed Byte to signed word
CLC Instruction - Clear the carry flag
CLD Instruction - Clear direction flag
CLI Instruction - Clear interrupt flag
CMC Instruction - Complement the carry flag
CMP Instruction - Compare byte or word - CMP destination, source.
CMPS/CMPSB/
CMPSW Instruction - Compare string bytes or string words
CWD Instruction - Convert Signed Word to - Signed Double word
Example
CALL Instruction:
This Instruction is used to transfer execution to a subprogram or procedure.
There are two basic types of CALL’s: Near and Far.
A Near CALL is a call to a procedure which is in the same code segment as
the CALL instruction.
When 8086 executes the near CALL instruction it decrements the stack
pointer by two and copies the offset of the next instruction after the CALL on
the stack. This offset saved on the stack is referred as the return address,
because this is the address that execution will returns to after the procedure
executes. A near CALL instruction will also load the instruction pointer with
the offset of the first instruction in the procedure.
A RET instruction at the end of the procedure will return execution to the
instruction after the CALL by coping the offset saved on the stack back to IP.
A Far CALL is a call to a procedure which is in a different from that which
contains the CALL instruction. When 8086 executes the Far CALL instruction
it decrements the stack pointer by two again and copies the content of CS

register to the stack. It then decrements the stack pointer by two again and
copies the offset contents offset of the instruction after the CALL to the stack.
Finally it loads CS with segment base of the segment which contains the
procedure and IP with the offset of the first instruction of the procedure in
segment. A RET instruction at end of procedure will return to the next
instruction after the CALL by restoring the saved CS and IP from the stack.
; Direct within-segment (near or intrasegment )
CALL MULTO ; MULTO is the name of the procedure. The assembler
determines displacement of MULTO from the instruction after the CALL
and codes this displacement in as part of the instruction.
; Indirect within-segment ( near or intrasegment )
CALL BX ; BX contains the offset of the first instruction of the
procedure. Replaces contents of word of IP with contents o register BX.
CALL WORD PTR [BX] ; Offset of first instruction of procedure is in
two memory addresses in DS. Replaces contents of IP with contents of
word memory location in DS pointed to by BX.
; Direct to another segment- far or intersegment.
CALL SMART ; SMART is the name of the Procedure
SMART PROC FAR; Procedure must be declare as an far
CBW Instruction - CBW converts the signed value in the AL register
into an equivalent 16 bit signed value in the AX register by duplicating
the sign bit to the left.
This instruction copies the sign of a byte in AL to all the bits in AH. AH is
then said to be the sign extension of AL.
Example:
; AX = 00000000 10011011 = - 155 decimal
CBW ; Convert signed byte in AL to signed word in AX.
; Result in AX = 11111111 10011011
; = - 155 decimal

CLC Instruction:
CLC clear the carry flag (CF) to 0 This instruction has no affect on the
processor, registers, or other flags. It is often used to clear the CF before
returning from a procedure to indicate a successful termination. It is also use
to clear the CF during rotate operation involving the CF such as ADC, RCL,
RCR.
Example:
CLC ; Clear carry flag.
CLD Instruction:
This instruction reset the designation flag to zero. This instruction has no
effect on the registers or other flags. When the direction flag is cleared / reset
SI and DI will
automatically be incremented when one of the string instruction such as
MOVS, CMPS, SCAS, MOVSB and STOSB executes.
Example:
CLD ; Clear direction flag so that string pointers auto increment
CLI Instruction:
This instruction resets the interrupt flag to zero. No other flags are affected. If
the interrupt flag is reset, the 8086 will not respond to an interrupt signal on its
INTR input. This CLI instruction has no effect on the nonmaskable interrupt
input, NMI
CMC Instruction:
If the carry flag CF is a zero before this instruction, it will be set to a one after
the instruction. If the carry flag is one before this instruction, it will be reset to
a zero after the instruction executes. CMC has no effect on other flags.
Example:
CMC; Invert the carry flag.
CWD Instruction:
CWD converts the 16 bit signed value in the AX register into an equivalent 32
bit signed value in DX: AX register pair by duplicating the sign bit to the left.

The CWD instruction sets all the bits in the DX register to the same sign bit of
the AX register. The effect is to create a 32- bit signed result that has same
integer value as the original 16 bit operand.
Example:
Assume AX contains C435h. If the CWD instruction is executed, DX will
contain FFFFh since bit 15 (MSB) of AX was 1. Both the original value of
AX (C435h) and resulting value of DX: AX (FFFFC435h) represents the
same signed number.
Example:
; DX = 00000000 00000000
; AX = 11110000 11000111 = - 3897 decimal
CWD ; Convert signed word in AX to signed double
; word in DX:AX
; Result DX = 11111111 11111111
; AX = 11110000 11000111 = -3897 decimal.
DAA Instruction - Decimal Adjust Accumulator
DAS Instruction - Decimal Adjust after Subtraction
DEC Instruction - Decrement destination register or memory DEC
destination.
DIV Instruction - Unsigned divide-Div source
ESC Instruction
When a double word is divided by a word, the most significant word of the
double word must be in DX and the least significant word of the double word
must be in AX. After the division AX will contain the 16 –bit result (quotient)
and DX will contain a 16 bit remainder. Again, if an attempt is made to divide
by zero or quotient is too large to fit in AX (greater than FFFFH) the 8086 will
do a type of 0 interrupt.
Example:
DIV CX ; (Quotient) AX= (DX: AX)/CX
: (Reminder) DX= (DX: AX)%CX

For DIV the dividend must always be in AX or DX and AX, but the source of
the divisor can be a register or a memory location specified by one of the 24
addressing modes.
If you want to divide a byte by a byte, you must first put the dividend byte in
AL and fill AH with all 0’s. The SUB AH, AH instruction is a quick way to
do.
If you want to divide a word by a word, put the dividend word in AX and fill
DX with all 0’s. The SUB DX, DX instruction does this quickly.
Example: ; AX = 37D7H = 14, 295 decimal
; BH = 97H = 151 decimal
DIV BH ; AX / BH
; AX = Quotient = 5EH = 94 decimal
; AH = Remainder = 65H = 101 decimal
ESC Instruction - Escape instruction is used to pass instruction to a
coprocessor such as the 8087 math coprocessor which shares the
address and data bus with an 8086. Instruction for the coprocessor is
represented by a 6 bit code embedded in the escape instruction. As the
8086 fetches instruction byte, the coprocessor also catches these bytes
from data bus and puts them in its queue. The coprocessor treats all of
the 8086 instruction as an NOP. When 8086 fetches an ESC instruction,
the coprocessor decodes the instruction and carries out the action
specified by the 6 bit code. In most of the case 8086 treats ESC
instruction as an NOP.
HLT Instruction - HALT processing
IDIV Instruction - Divide by signed byte or word IDIV source
IMUL Instruction - Multiply signed number-IMUL source
IN Instruction - Copy data from a port IN accumulator, port
INC Instruction - Increment - INC destination
HALT Instruction - The HLT instruction will cause the 8086 to stop
fetching and executing instructions. The 8086 will enter a halt state. The
only way to get the processor out of the halt state are with an interrupt
signal on the INTR pin or an interrupt signal on NMI pin or a reset
signal on the RESET input.
IDIV Instruction - This instruction is used to divide a signed word by a
signed byte or to divide a signed double word by a signed word.
Example:

IDIV BL ; Signed word in AX is divided by signed byte in BL
IMUL Instruction - This instruction performs a signed multiplication.
IMUL op ; In this form the accumulator is the multiplicand and op is the
multiplier. op may be a register or a memory operand.
IMUL op1, op2 ; In this form op1 is always be a register operand and op2
may be a register or a memory operand.
Example:
IMUL BH ; Signed byte in AL times multiplied by ; signed byte in BH
and result in AX.
Example:
; 69 * 14
; AL = 01000101 = 69 decimal
; BL = 00001110 = 14 decimal
IMUL BL ; AX = 03C6H = + 966 decimal
; MSB = 0 because positive result
; - 28 * 59
; AL = 11100100 = - 28 decimal
; BL = 00001110 = 14 decimal
IMUL BL ; AX = F98Ch = - 1652 decimal
; MSB = 1 because negative result
IN Instruction: This IN instruction will copy data from a port to the AL
or AX register.
For the Fixed port IN instruction type the 8 – bit port address of a port is
specified directly in the instruction.
Example:
IN AL, 0C8H ; Input a byte from port 0C8H to AL

IN AX, 34H ; Input a word from port 34H to AX
A_TO_D EQU 4AH
IN AX, A_TO_D ; Input a word from port 4AH to AX
For a variable port IN instruction, the port address is loaded in DX register
before IN instruction. DX is 16 bit. Port address range from 0000H – FFFFH.
Example:
MOV DX, 0FF78H ; Initialize DX point to port
IN AL, DX ; Input a byte from a 8 bit port ; 0FF78H to AL
IN AX, DX ; Input a word from 16 bit port to ; 0FF78H to AX.
INC Instruction:
INC instruction adds one to the operand and sets the flag according to the
result. INC instruction is treated as an unsigned binary number.
Example:
; AX = 7FFFh
INC AX ; After this instruction AX = 8000h
INC BL ; Add 1 to the contents of BL register
INC CL ; Add 1 to the contents of CX register.
INT Instruction - Interrupt program
INTO Instruction - Interrupt on overflow.
IRET Instruction - Interrupt return
JA/JNBE Instruction - Jump if above/Jump if not below nor equal.
JAE/JNB/JNC Instructions- Jump if above or equal/ Jump if not
below/
Jump if no carry.
JA / JNBE - This instruction performs the Jump if above (or) Jump if
not below or equal operations according to the condition, if CF and ZF
= 0.
Example:
( 1 ) CMP AX, 4371H ; Compare by subtracting 4371H ; from AX

JA RUN_PRESS ; Jump to label RUN_PRESS if ; AX above 4371H
( 2 ) CMP AX, 4371H ; Compare ( AX – 4371H)
JNBE RUN_PRESS ; Jump to label RUN_PRESS if ; AX not below or
equal to 4371H
JAE / JNB / JNC - This instructions performs the Jump if above or
equal, Jump if not below, Jump if no carry operations according to the
condition, if CF = 0.
Examples:
1. CMP AX, 4371H ; Compare ( AX – 4371H)
JAE RUN ; Jump to the label RUN if AX is ; above or equal to 4371H.
2. CMP AX, 4371H ; Compare ( AX – 4371H)
JNB RUN_1 ; Jump to the label RUN_1 if AX ; is not below than 4371H
3. ADD AL, BL ; Add AL, BL. If result is with in JNC OK ; acceptable
range, continue
JB/JC/JNAE Instruction - Jump if below/Jump if carry/ Jump if not
above nor equal
JBE/JNA Instructions- Jump if below or equal / Jump if not above
JCXZ Instruction - Jump if the CX register is zero
JE/JZ Instruction - Jump if equal/Jump if zero
JG/JNLE Instruction- Jump if greater/Jump if not less than nor equal
JB/JC/JNAE Instruction - This instruction performs the Jump if below
(or) Jump if carry (or) Jump if not below/ equal operations according to
the condition, if CF = 1
Example:
1. CMP AX, 4371H ; Compare (AX – 4371H)
JB RUN_P ; Jump to label RUN_P if AX is ; below 4371H
2. ADD BX, CX ; Add two words and Jump to
JC ERROR ; label ERROR if CF = 1

JBE/JNA Instruction - This instruction performs the Jump if below or
equal (or) Jump if not above operations according to the condition, if
CF and ZF = 1
Example:
CMP AX, 4371H ; Compare (AX – 4371H )
JBA RUN ; Jump to label RUN if AX is ; below or equal to 4371H
CMP AX, 4371H ; Compare ( AX – 4371H )
JNA RUN_R ; Jump to label RUN_R if AX is ; not above than 4371H
JCXZ Instruction:
This instruction performs the Jump if CX register is zero. If CX does not
contain all zeros, execution will simply proceed to the next instruction.
Example:
JCXZ SKIP_LOOP ; If CX = 0, skip the process
NXT: SUB [BX], 07H ; Subtract 7 from data value
INC BX ; BX point to next value
LOOP NXT ; Loop until CX = 0
SKIP_LOOP ; Next instruction
JE/JZ Instruction:
This instruction performs the Jump if equal (or) Jump if zero operations
according to the condition if ZF = 1
Example:
NXT: CMP BX, DX ; Compare ( BX – DX )
JE DONE ; Jump to DONE if BX = DX,
SUB BX, AX ; Else subtract Ax
INC CX ; Increment counter
JUMP NXT ; Check again

DONE: MOV AX, CX; Copy count to AX
Example:
IN AL, 8FH ; read data from port 8FH
SUB AL, 30H ; Subtract minimum value
JZ STATR ; Jump to label if result of ; subtraction was 0
JG/JNLE Instruction:
This instruction performs the Jump if greater (or) Jump if not less than or
equal operations according to the condition if ZF =0 and SF = OF
Example:
CMP BL, 39H ; Compare by subtracting ; 39H from BL
JG NEXT1 ; Jump to label if BL is ; more positive than 39H
JNLE NEXT2 ; Jump to label if BL is not ; less than or equal 39H
JGE/JNL Instruction - Jump if greater than or equal/ Jump if not less
than
JL/JNGE Instruction - Jump if less than/Jump if not greater than or
equal
JLE/JNG Instruction - Jump if less than or equal/ Jump if not greater
JMP Instruction - Unconditional jump to - specified destination
JGE/JNL Instruction - This instruction performs the Jump if greater
than or equal / Jump if not less than operation according to the
condition if SF = OF
Example:
CMP BL, 39H ; Compare by the ; subtracting 39H from BL
JGE NEXT11 ; Jump to label if BL is ; more positive than 39H ; or equal
to 39H
JNL NEXT22 ; Jump to label if BL is not ; less than 39H

JL/JNGE Instruction - This instruction performs the Jump if less than /
Jump if not greater than or equal operation according to the condition, if
SF ≠ OF
Example:
CMP BL, 39H ; Compare by subtracting 39H ; from BL
JL AGAIN ; Jump to the label if BL is more ; negative than 39H
CMP BL, 39H ; Compare by subtracting 39H ; from BL
JNGE AGAIN1 ; Jump to the label if BL is not ; more positive than 39H
or ; not equal to 39H
JLE/JNG Instruction - This instruction performs the Jump if less than
or equal / Jump if not greater operation according to the condition, if
ZF=1 and SF ≠ OF
Example:
CMP BL, 39h ; Compare by subtracting 39h ; from BL
JLE NXT1 ; Jump to the label if BL is more ; negative than 39h or equal
to 39h
CMP BL, 39h ; Compare by subtracting 39h ; from BL
JNG AGAIN2 ; Jump to the label if BL is not ; more positive than 39h
JNA/JBE Instruction - Jump if not above/Jump if below or equal
JNAE/JB Instruction - Jump if not above or equal/ Jump if below
JNB/JNC/JAE Instruction - Jump if not below/Jump if no carry/Jump
if above or equal
JNE/JNZ Instruction - Jump if not equal/Jump if not zero
JNE/JNZ Instruction - This instruction performs the Jump if not equal /
Jump if not zero operation according to the condition, if ZF=0
Example:
NXT: IN AL, 0F8H ; Read data value from port
CMP AL, 72 ; Compare ( AL – 72 )
JNE NXT ; Jump to NXT if AL ≠ 72

IN AL, 0F9H ; Read next port when AL = 72
MOV BX, 2734H ; Load BX as counter
NXT_1: ADD AX, 0002H ; Add count factor to AX
DEC BX ; Decrement BX
JNZ NXT_1 ; Repeat until BX = 0
JNG/JLE Instruction - Jump if not greater/ Jump if less than or equal
JNGE/JL Instruction - Jump if not greater than nor equal/Jump if less
than
JNL/JGE Instruction - Jump if not less than/ Jump if greater than or
equal
JNLE/JG Instruction - Jump if not less than nor equal to /Jump if
greater than
JNO Instruction – Jump if no overflow
JNP/JPO Instruction – Jump if no parity/ Jump if parity odd
JNS Instruction - Jump if not signed (Jump if positive)
JNZ/JNE Instruction - Jump if not zero / jump if not equal
JO Instruction - Jump if overflow
JNO Instruction – This instruction performs the Jump if no overflow
operation according to the condition, if OF=0
Example:
ADD AL, BL ; Add signed bytes in AL and BL
JNO DONE ; Process done if no overflow -
MOV AL, 00H ; Else load error code in AL
DONE: OUT 24H, AL ; Send result to display
JNP/JPO Instruction – This instruction performs the Jump if not parity
/ Jump if parity odd operation according to the condition, if PF=0
Example:
IN AL, 0F8H ; Read ASCII char from UART
OR AL, AL ; Set flags

JPO ERROR1 ; If even parity executed, if not ; send error message
JNS Instruction - This instruction performs the Jump if not signed
(Jump if positive) operation according to the condition, if SF=0
Example:
DEC AL ; Decrement counter
JNS REDO ; Jump to label REDO if counter has not ; decremented to
FFH
JO Instruction - This instruction performs Jump if overflow operation
according to the condition OF = 0
Example:
ADD AL, BL ; Add signed bits in AL and BL
JO ERROR ; Jump to label if overflow occur ; in addition
MOV SUM, AL ; else put the result in memory ; location named SUM
JPE/JP Instruction - Jump if parity even/ Jump if parity
JPO/JNP Instruction - Jump if parity odd/ Jump if no parity
JS Instruction - Jump if signed (Jump if negative)
JZ/JE Instruction - Jump if zero/Jump if equal
JPE/JP Instruction - This instruction performs the Jump if parity even /
Jump if parity operation according to the condition, if PF=1
Example:
IN AL, 0F8H ; Read ASCII char from UART
OR AL, AL ; Set flags
JPE ERROR2 ; odd parity is expected, if not ; send error message
JS Instruction - This instruction performs the Jump if sign operation
according to the condition, if SF=1
Example:
ADD BL, DH ; Add signed bytes DH to BL

JS JJS_S1 ; Jump to label if result is ; negative
LAHF Instruction - Copy low byte of flag register to AH
LDS Instruction - Load register and Ds with words from memory –
LDS register, memory address of first word
LEA Instruction - Load effective address-LEA register, source
LES Instruction Load register and ES with words from memory –LES
register, memory address of first word.
LAHF Instruction: LAHF instruction copies the value of SF, ZF, AF,
PF, CF, into bits of 7, 6, 4, 2, 0 respectively of AH register. This LAHF
instruction was provided to make conversion of assembly language
programs written for 8080 and 8085 to 8086 easier.
LDS Instruction: This instruction loads a far pointer from the memory
address specified by op2 into the DS segment register and the op1 to the
register.
LDS op1, op2
Example:
LDS BX, [4326] ; copy the contents of the memory at displacement 4326H
in DS to BL, contents of the 4327H to BH. Copy contents of 4328H and
4329H in DS to DS register.
LEA Instruction - This instruction indicates the offset of the variable or
memory location named as the source and put this offset in the indicated
16 – bit register.
Example:
LEA BX, PRICE ; Load BX with offset of PRICE ; in DS
LEA BP, SS:STAK ; Load BP with offset of STACK ; in SS
LEA CX, [BX][DI] ; Load CX with EA=BX + DI
LOCK Instruction - Assert bus lock signal
LODS/LODSB/ LODSW Instruction - Load string byte into AL or
Load string word into AX.
LOOP Instruction - Loop to specified label until CX = 0
LOOPE / LOOPZ Instruction - loop while CX ≠ 0 and ZF = 1
LODS/LODSB/LODSW Instruction - This instruction copies a byte
from a string location pointed to by SI to AL or a word from a string
location pointed to by SI to AX. If DF is cleared to 0, SI will
automatically incremented to point to the next element of string.

Example:
CLD ; Clear direction flag so SI is auto incremented
MOV SI, OFFSET SOURCE_STRING ; point SI at start of the string
LODS SOUCE_STRING ; Copy byte or word from ; string to AL or AX
LOOP Instruction - This instruction is used to repeat a series of
instruction some number of times
Example:
MOV BX, OFFSET PRICE
; Point BX at first element in array
MOV CX, 40 ; Load CX with number of ; elements in array
NEXT: MOV AL, [BX] ; Get elements from array
ADD AL, 07H ; Ad correction factor
DAA ; decimal adjust result
MOV [BX], AL ; Put result back in array
LOOP NEXT ; Repeat until all elements ; adjusted.
LOOPE / LOOPZ Instruction - This instruction is used to repeat a
group of instruction some number of times until CX = 0 and ZF = 0
Example:
MOV BX, OFFSET ARRAY
; point BX at start of the array
DEC BX
MOV CX, 100 ; put number of array elements in ; CX
NEXT:INC BX ; point to next element in array
CMP [BX], 0FFH ; Compare array elements FFH
LOOP NEXT

LOOPNE/LOOPNZ Instruction - This instruction is used to repeat a
group of instruction some number of times until CX = 0 and ZF = 1
Example:
MOV BX, OFFSET ARRAY1
; point BX at start of the array
DEC BX
MOV CX, 100 ; put number of array elements in ; CX
NEXT:INC BX ; point to next elements in array
CMP [BX], 0FFH ; Compare array elements 0DH
LOOPNE NEXT
MOV Instruction - MOV destination, source
MOVS/MOVSB/ MOVSW Instruction - Move string byte or string
word-MOVS destination, source
MUL Instruction - Multiply unsigned bytes or words-MUL source
NEG Instruction - From 2’s complement – NEG destination
NOP Instruction - Performs no operation.
MOV Instruction - The MOV instruction copies a word or a byte of
data from a specified source to a specified destination.
MOV op1, op2
Example:
MOV CX, 037AH ; MOV 037AH into the CX.
MOV AX, BX ; Copy the contents of register BX ; to AX
MOV DL, [BX] ; Copy byte from memory at BX ; to DL, BX contains the
offset of byte in DS.
MUL Instruction:
This instruction multiplies an unsigned multiplication of the accumulator by
the operand specified by op. The size of op may be a register or memory
operand.
MUL op

Example: ; AL = 21h (33 decimal)
; BL = A1h(161 decimal )
MUL BL ; AX =14C1h (5313 decimal) since AH≠0, ; CF and OF will set
to 1.
MUL BH ; AL times BH, result in AX
MUL CX ; AX times CX, result high word in DX, ; low word in AX.
NEG Instruction - NEG performs the two’s complement subtraction of
the operand from zero and sets the flags according to the result. ; AX =
2CBh
NEG AX ; after executing NEG result AX =FD35h.
Example:
NEG AL ; Replace number in AL with its 2’s complement
NEG BX ; Replace word in BX with its 2’s complement
NEG BYTE PTR[BX]; Replace byte at offset BX in
; DS with its 2’s complement
NOP Instruction:
This instruction simply uses up the three clock cycles and increments the
instruction pointer to point to the next instruction. NOP does not change the
status of any flag. The NOP instruction is used to increase the delay of a delay
loop.
NOT Instruction - Invert each bit of operand –NOT destination.
OR Instruction - Logically OR corresponding of two operands- OR
destination, source.
OUT Instruction - Output a byte or word to a port – OUT port,
accumulator AL or AX.
POP Instruction - POP destination
NOT Instruction - NOT perform the bitwise complement of op and
stores the result back into op.
NOT op

Example:
NOT BX ; Complement contents of BX register.
; DX =F038h
NOT DX ; after the instruction DX = 0FC7h
OR Instruction - OR instruction perform the bit wise logical OR of two
operands.Each bit of the result is cleared to 0 if and only if both
corresponding bits in each operand are 0, other wise the bit in the result
is set to 1.
OR op1, op2
Examples:
OR AH, CL ; CL ORed with AH, result in AH.
; CX = 00111110 10100101
OR CX, FF00h ; OR CX with immediate FF00h
; result in CX = 11111111 10100101
; Upper byte are all 1’s lower bytes ; are unchanged.
OUT Instruction - The OUT instruction copies a byte from AL or a
word from AX or a double from the accumulator to I/O port specified
by op. Two forms of OUT instruction are available: (1) Port number is
specified by an immediate byte constant, ( 0 - 255 ).It is also called as
fixed port form. (2) Port number is provided in the DX register ( 0 –
65535 )
Example: (1)
OUT 3BH, AL ; Copy the contents of the AL to port 3Bh
OUT 2CH, AX ; Copy the contents of the AX to port 2Ch
(2) MOV DX, 0FFF8H ; Load desired port address in DX
OUT DX, AL ; Copy the contents of AL to ; FFF8h
OUT DX, AX ; Copy content of AX to port ; FFF8H
POP Instruction:

POP instruction copies the word at the current top of the stack to the operand
specified by op then increments the stack pointer to point to the next stack.
Example:
POP DX ; Copy a word from top of the stack to
; DX and increments SP by 2.
POP DS ; Copy a word from top of the stack to
; DS and increments SP by 2.
POP TABLE [BX]
; Copy a word from top of stack to memory in DS with
; EA = TABLE + [BX].
POPF Instruction - Pop word from top of stack to flag - register.
PUSH Instruction - PUSH source
PUSHF Instruction - Push flag register on the stack
RCL Instruction - Rotate operand around to the left through CF – RCL
destination, source.
RCR Instruction - Rotate operand around to the right through CF- RCR
destination, count
POPF Instruction - This instruction copies a word from the two
memory location at the top of the stack to flag register and increments
the stack pointer by 2.
PUSH Instruction: PUSH instruction decrements the stack pointer by 2
and copies a word from a specified source to the location in the stack
segment where the stack pointer pointes.
Example:
PUSH BX ; Decrement SP by 2 and copy BX to stack
PUSH DS ; Decrement SP by 2 and copy DS to stack
PUSH TABLE[BX] ; Decrement SP by 2 and copy word ; from memory
in DS at
; EA = TABLE + [BX] to stack.
PUSHF Instruction:

This instruction decrements the SP by 2 and copies the word in flag register to
the memory location pointed to by SP.
RCL Instruction:
RCL instruction rotates the bits in the operand specified by op1 towards left
by the count specified in op2.The operation is circular, the MSB of operand is
rotated into a carry flag and the bit in the CF is rotated around into the LSB of
operand.
RCR op1, op2
Example:
CLC ; put 0 in CF
RCL AX, 1 ; save higher-order bit of AX in CF
RCL DX, 1 ; save higher-order bit of DX in CF
ADC AX, 0 ; set lower order bit if needed.
Example:
RCL DX, 1 ; Word in DX of 1 bit is moved to left, and ; MSB of word is
given to CF and
; CF to LSB.
; CF=0, BH = 10110011
RCL BH, 1 ; Result: BH =01100110
; CF = 1, OF = 1 because MSB changed
; CF =1, AX =00011111 10101001
MOV CL, 2 ; Load CL for rotating 2 bit position
RCL AX, CL ; Result: CF =0, OF undefined
; AX = 01111110 10100110
RCR Instruction - RCR instruction rotates the bits in the operand
specified by op1 towards right by the count specified in op2. RCR op1,
op2

Example: ( 1) RCR BX, 1 ; Word in BX is rotated by 1 bit towards
; right and CF will contain MSB bit and
; LSB contain CF bit.
( 2) ; CF = 1, BL = 00111000
RCR BL, 1 ; Result: BL = 10011100, CF =0
; OF = 1 because MSB is changed to 1.
REP/REPE/REPZ/
REPNE/REPNZ - (Prefix) Repeat String instruction until specified condition
exist
RET Instruction – Return execution from procedure to calling program.
ROL Instruction - Rotate all bits of operand left, MSB to LSB ROL
destination, count.
ROL Instruction - ROL instruction rotates the bits in the operand
specified by op1 towards left by the count specified in op2. ROL moves
each bit in the operand to next higher bit position. The higher order bit
is moved to lower order position. Last bit rotated is copied into carry
flag.
ROL op1, op2
Example: ( 1 )
ROL AX, 1 ; Word in AX is moved to left by 1 bit
; and MSB bit is to LSB, and CF
; CF =0, BH =10101110
ROL BH, 1 ; Result: CF, Of =1, BH = 01011101
Example: ( 2 )
; BX = 01011100 11010011
; CL = 8 bits to rotate
ROL BH, CL ; Rotate BX 8 bits towards left
; CF =0, BX =11010011 01011100

ROR Instruction - Rotate all bits of operand right, LSB to MSB – ROR
destination, count
SAHF Instruction – Copy AH register to low byte of flag register
ROR Instruction - ROR instruction rotates the bits in the operand op1
to wards right by count specified in op2. The last bit rotated is copied
into CF.
ROR op1, op2
Example:
( 1 ) ROR BL, 1 ; Rotate all bits in BL towards right by 1 ; bit position,
LSB bit is moved to MSB
; and CF has last rotated bit.
( 2 ); CF =0, BX = 00111011 01110101
ROR BX, 1 ; Rotate all bits of BX of 1 bit position ; towards right and CF
=1,
BX = 10011101 10111010
Example ( 3 )
; CF = 0, AL = 10110011,
MOVE CL, 04H ; Load CL
ROR AL, CL ; Rotate all bits of AL towards right ; by 4 bits, CF = 0, AL
= 00111011
SAHF Instruction: SAHF copies the value of bits 7, 6, 4, 2, 0 of the
AH register into the SF, ZF, AF, PF, and CF respectively. This
instruction was provided to make easier conversion of assembly
language program written for 8080 and 8085 to 8086.
SAL/SHL Instruction - Shift operand bits left, put zero in LSB(s)
SAL/AHL destination, count
SAR Instruction - Shift operand bits right, new MAB = old MSB
SAR destination, count.
SBB Instruction - Subtract with borrow SBB destination, source
SAL / SHL Instruction - SAL instruction shifts the bits in the
operand specified by op1 to its left by the count specified in op2. As
a bit is shifted out of LSB position a 0 is kept in LSB position. CF
will contain MSB bit.

SAL op1, op2
Example:
; CF = 0, BX = 11100101 11010011
SAL BX, 1 ; Shift BX register contents by 1 bit ; position towards left
; CF = 1, BX = 11001011 1010011
SAR Instruction - SAR instruction shifts the bits in the operand
specified by op1 towards right by count specified in op2.As bit is
shifted out a copy of old MSB is taken in MSB
MSB position and LSB is shifted to CF.
SAR op1, op2
Example: ( 1 )
; AL = 00011101 = +29 decimal, CF = 0
SAR AL, 1 ; Shift signed byte in AL towards right
; ( divide by 2 )
; AL = 00001110 = + 14 decimal, CF = 1
( 2 ) ; BH = 11110011 = - 13 decimal, CF = 1
SAR BH, 1 ; Shifted signed byte in BH to right
; BH = 11111001 = - 7 decimal, CF = 1
SBB Instruction - SUBB instruction subtracts op2 from op1, then
subtracts 1 from op1 is CF flag is set and result is stored in op1 and it is
used to set the flag.
Example:
SUB CX, BX ; CX – BX. Result in CX
SUBB CH, AL ; Subtract contents of AL and ; contents CF from contents
of CH. ; Result in CH
SUBB AX, 3427H ; Subtract immediate number ; from AX

Example:
•Subtracting unsigned number
; CL = 10011100 = 156 decimal
; BH = 00110111 = 55 decimal
SUB CL, BH ; CL = 01100101 = 101 decimal
; CF, AF, SF, ZF = 0, OF, PF = 1
•Subtracting signed number
; CL = 00101110 = + 46 decimal
; BH = 01001010= + 74 decimal
SUB CL, BH ; CL = 11100100 = - 28 decimal
; CF = 1, AF, ZF =0,
; SF = 1 result negative
STD Instruction - Set the direction flag to 1
STI Instruction - Set interrupt flag ( IF)
STOS/STOSB/ STOSW Instruction - Store byte or word in string.
SCAS/SCASB/ - Scan string byte or a
SCASW Instruction string word.
SHR Instruction - Shift operand bits right, put zero in MSB
STC Instruction - Set the carry flag to 1
SHR Instruction - SHR instruction shifts the bits in op1 to right by the
number of times specified by op2.
Example:
( 1 )SHR BP, 1 ; Shift word in BP by 1 bit position to right ; and 0 is kept
to MSB
( 2 ) MOV CL, 03H ; Load desired number of shifts into CL
SHR BYTE PYR[BX] ; Shift bytes in DS at offset BX and
; rotate 3 bits to right and keep 3 0’s in MSB
( 3 )

; SI = 10010011 10101101, CF = 0
SHR SI, 1 ; Result: SI = 01001001 11010110
; CF = 1, OF = 1, SF = 0, ZF = 0
• TEST Instruction – AND operand to update flags
• WAIT Instruction - Wait for test signal or interrupt signal
• XCHG Instruction - Exchange XCHG destination, source
• XLAT/ XLATB Instruction - Translate a byte in AL
• XOR Instruction - Exclusive OR corresponding bits of two operands –
XOR destination, source
• TEST Instruction - This instruction ANDs the contents of a source byte
or word with the contents of specified destination word. Flags are
updated but neither operand is changed. TEST instruction is often used
to set flags before a condition jump instruction
Examples:
TEST AL, BH ; AND BH with AL. no result is ; stored. Update PF, SF,
ZF
TEST CX, 0001H ; AND CX with immediate ; number
; no result is stored, Update PF, ; SF
Example:
; AL = 01010001
TEST Al, 80H ; AND immediate 80H with AL to ; test f MSB of AL is 1
or 0
; ZF = 1 if MSB of AL = 0
; AL = 01010001 (unchanged)
; PF = 0, SF = 0
; ZF = 1 because ANDing produced is 00
• WAIT Instruction - When this WAIT instruction executes, the 8086
enters an idle condition. This will stay in this state until a signal is
asserted on TEST input pin or a valid interrupt signal is received on the
INTR or NMI pin.

FSTSW STATUS ; copy 8087 status word to memory
FWAIT ; wait for 8087 to finish before- ; doing next 8086 instruction
MOV AX, STATUS ; copy status word to AX to ; check bits
In this code we are adding up of FWAIT instruction so that it will stop the
execution of the command until the above instruction is finishes it’s work.so
that you are not loosing data and after that you will allow to continue the
execution of instructions.
• XCHG Instruction - The Exchange instruction exchanges the contents
of the register with the contents of another register (or) the contents of
the register with the contents of the memory location. Direct memory to
memory exchange are not supported.
XCHG op1, op2
The both operands must be the same size and one of the operand must always
be a register.
Example:
XCHG AX, DX ; Exchange word in AX with word in DX
XCHG BL, CH ; Exchange byte in BL with byte in CH
XCHG AL, Money [BX] ; Exchange byte in AL with byte ; in memory at
EA.
• XOR Instruction - XOR performs a bit wise logical XOR of the
operands specified by op1 and op2. The result of the operand is stored
in op1 and is used to set the flag.
XOR op1, op2
Example: ( Numerical )
; BX = 00111101 01101001
; CX = 00000000 11111111
XOR BX, CX ; Exclusive OR CX with BX
; Result BX = 00111101 10010110

ASSEMBLER DIRECTIVES :
Assembler directives are the directions to the assembler which indicate
how an operand or section of the program is to be processed. These are also
called pseudo operations which are not executable by the microprocessor. The
various directives are explained below.
1. ASSUME : The ASSUME directive is used to inform the assembler the
name of the logical segment it should use for a specified segment.
Ex: ASSUME DS: DATA tells the assembler that for any program
instruction which refers to the data segment ,it should use the logical segment
called DATA.
2.DB -Define byte. It is used to declare a byte variable or set aside one or
more storage locations of type byte in memory.
For example, CURRENT_VALUE DB 36H tells the assembler to reserve 1
byte of memory for a variable named CURRENT_ VALUE and to put the
value 36 H in that memory location when the program is loaded into RAM .
3. DW -Define word. It tells the assembler to define a variable of type word
or to reserve storage locations of type word in memory.
4. DD(define double word) :This directive is used to declare a variable of
type double word or restore memory locations which can be accessed as type
double word.
5.DQ (define quadword) :This directive is used to tell the assembler to
declare a variable 4 words in length or to reserve 4 words of storage in
memory .
6.DT (define ten bytes):It is used to inform the assembler to define a variable
which is 10 bytes in length or to reserve 10 bytes of storage in memory.
7. EQU –Equate It is used to give a name to some value or symbol. Every
time the assembler finds the given name in the program, it will replace the
name with the value or symbol we have equated with that name
8.ORG -Originate : The ORG statement changes the starting offset address
of the data.

It allows to set the location counter to a desired value at any point in the
program.For example the statement ORG 3000H tells the assembler to set
the location counter to 3000H.
9 .PROC- Procedure: It is used to identify the start of a procedure or
subroutine.
10. END- End program .This directive indicates the assembler that this is the
end of the program module.The assembler ignores any statements after an
END directive.
11. ENDP- End procedure: It indicates the end of the procedure (subroutine)
to the assembler.
12.ENDS-End Segment: This directive is used with the name of the segment
to indicate the end of that logical segment.
Ex: CODE SEGMENT : Start of logical segment containing
code
CODE ENDS : End of the segment named CODE.

Basic Peripherals and Their Interfacing with 8086
Interfacing with RAM And ROM
The figure 2.1
array. In this, the 16
8-bit banks on the upper halves of the data bus selected by BHE, and
AO.
a) ROM and EPROM
ROMS and EPROMs are the simplest memory chips to interface to the 8086.
Since ROMs and EPROMs are read
required to be part of the chip enable/select decoding. The 8086 address lines
must be connected to the ROM/EPROM chip
higher to all the address lines of the ROM/EPROM chips. The 8086 unused
address lines can be used as chip enable/select decoding. To interface the
ROMs/RAMs directly to the 8086
enable signals. The figure
Interfacing with RAM And ROM
shows a general block diagram of an 8086 memory
array. In this, the 16-bit word memory is partitioned into high and low
bit banks on the upper halves of the data bus selected by BHE, and
FIGURE 2.1– 8086 MEMORY ARRAY
a) ROM and EPROM
Since ROMs and EPROMs are read-only devices, A0 and BHE are not
must be connected to the ROM/EPROM chip chips starting with A1 and
ROMs/RAMs directly to the 8086-multiplexed bus, they must have output
figure 3.5.2 shows the 8086 interfaced to two 2716s.
shows a general block diagram of an 8086 memory
bit word memory is partitioned into high and low
bit banks on the upper halves of the data bus selected by BHE, and
only devices, A0 and BHE are not
chips starting with A1 and
multiplexed bus, they must have output
shows the 8086 interfaced to two 2716s.

Byte accesses are obtained by reading the full 16
the 8086 discarding the unwanted byte and accepting the desired byte.
b) Static RAMS
Since static RAMs are read/write memories, both A0 and BHE must be
included in the chip select/enable decoding of the devices and write timing
must be considered in the compatibility analysis.
For each static RAM, the memory data lines must be connected to eithe
the upper half AD15-AD0 or lower half AD7
For static RAMs without output enable pins, read and write lines must
be used as enables for chip select generation to avoid bus contention. If read
and write lines are not used to a
common input/output data pins such as 2114 will face extreme bus
contentions between chip selects and write active. The 8086 A0 and BHE
pins must be used to enable the chip the chip selects. Note that Intel 8205
three enables E1, E2, and E3, three inputs A0 and A2, and eight outputs O0
O7.
For devices with output enables such as 2142, one way to generate chip
selects for the static RAMs is by gating the 8086 WR signal with BHE and A0
to provide upper and lower bank write strobes. A possible configuration is
shown in the figure 3.5.4
two chips for each bank w
required. Note that DATA is read from the 2142 when the output disable OD
is low, WE is HIGH, and DATA is written into 2142. If multiple chip selects
Byte accesses are obtained by reading the full 16-bit word onto the bus with
FIGURE 3.5.2
tatic RAMs are read/write memories, both A0 and BHE must be
must be considered in the compatibility analysis.
For each static RAM, the memory data lines must be connected to eithe
AD0 or lower half AD7-AD0 of the 8086 data lines.
and write lines are not used to activate the chip selects, static RAMs with
pins must be used to enable the chip the chip selects. Note that Intel 8205
three enables E1, E2, and E3, three inputs A0 and A2, and eight outputs O0
figure 3.5.4. Since the Intel 2142 is a 1024 * 4 bit static RAM,
two chips for each bank with a total of 4 chips for 2K * 8 static RAM is
bit word onto the bus with
tatic RAMs are read/write memories, both A0 and BHE must be
For each static RAM, the memory data lines must be connected to either
AD0 of the 8086 data lines.
ctivate the chip selects, static RAMs with
pins must be used to enable the chip the chip selects. Note that Intel 8205 has
three enables E1, E2, and E3, three inputs A0 and A2, and eight outputs O0-
. Since the Intel 2142 is a 1024 * 4 bit static RAM,
ith a total of 4 chips for 2K * 8 static RAM is

are available with the static RAM, BHE and A0 may be use
chip selects. A possible configuration for 2K * 8 array is shown in the
3.5.5.
c) Dynamic RAM
Dynamic RAM store information as charges in capacitors. Since
capacitors can hold charges for a few milliseconds, refresh circuitry i
necessary in dynamic RAMs for retaining these charges. Therefore, dynamic
RAMs are complex devices to design a system. To relieve the designer of
most of these complicated interfacing tasks, Intel provides the 8202 dynamic
RAM controller as part of the
can be interfaced with the 8086 to build a dynamic memory system.
are available with the static RAM, BHE and A0 may be use
chip selects. A possible configuration for 2K * 8 array is shown in the
capacitors can hold charges for a few milliseconds, refresh circuitry i
RAM controller as part of the 8086 families of peripheral devices. The 8202
are available with the static RAM, BHE and A0 may be used directly as the
chip selects. A possible configuration for 2K * 8 array is shown in the figure
capacitors can hold charges for a few milliseconds, refresh circuitry is
8086 families of peripheral devices. The 8202

FIGURE 3.5.5 – 2K * 8 STATIC ARRAY WITH A0 and BHE AS DIRECT CHIP SELECT2K * 8 STATIC ARRAY WITH A0 and BHE AS DIRECT CHIP SELECT2K * 8 STATIC ARRAY WITH A0 and BHE AS DIRECT CHIP SELECT INPUTS

PIO 8255
• The parallel input-output port chip 8255 is also called as
programmable peripheral input-output port. The Intel’s 8255 is
designed for use with Intel’s 8- bit, 16-bit and higher capability
microprocessors. It has 24 input/output lines which may be
individually programmed in two groups of twelve lines each, or
three groups of eight lines.
• The two groups of I/O pins are named as Group A and Group B.
Each of these two groups contains a subgroup of eight I/O lines
called as 8-bit port and another subgroup of four lines or a 4-bit port.
Thus Group A contains an 8-bit port A along with a 4-bit port C
upper.
• The port A lines are identified by symbols PA0-PA7 while the port C
lines are identified as PC4-PC7. Similarly, Group B contains an 8-bit
port B, containing lines PB0-PB7 and a 4-bit port C with lower bits
PC0- PC3. The port C upper and port C lower can be used in
combination as an 8-bit port C.
• Both the port C are assigned the same address. Thus one may have
either three 8- bit I/O ports or two 8-bit and two 4-bit ports from
8255. All of these ports can
function independently either as input or as output ports. This can be
achieved by programming the bits of an internal register of 8255
called as control word register (CWR).
• The internal block diagram and the pin configuration of 8255 are
shown in fig.
• The 8-bit data bus buffer is controlled by the read/write control logic.
The read/write control logic manages all of the internal and external
transfers of both data and control words.
• RD , WR , A1, A0 and RESET are the inputs provided by the
microprocessor to the READ/ WRITE control logic of 8255. The 8-
bit, 3-state bidirectional buffer is used to interface the 8255 internal
data bus with the external system data bus.
• This buffer receives or transmits data upon the execution of input or
output instructions by the microprocessor. The control words or status
information is also transferred through the buffer.
• The signal description of 8255 are briefly presented as follows :
• PA7-PA0: These are eight port A lines that acts as either latched output
or buffered input lines depending upon the control word loaded into the
control word register.
• PC7-PC4 : Upper nibble of port C lines. They may act as either
output latches or input buffers lines.This port also can be used for
generation of handshake lines in mode 1 or mode 2.
• PC3-PC0 : These are the lower port C lines, other details are the same
as PC7-PC4 lines.

0 1 0 0 0 Port A to Data bus
0 1 0 0 1 Port B to Data bus
0 1 0 1 0 Port C to Data bus
0 1 0 1 1 CWR to Data bus
1 0 0 0 0 Data bus to Port A
1 0 0 0 1 Data bus to Port B
1 0 0 1 0 Data bus to Port C
1 0 0 1 1 Data bus to CWR
X X 1 X X Data bus tristated
1 1 0 X X Data bus tristated
• PB0-PB7 : These are the eight port B lines which are used as latched
output lines or buffered input lines in the same way as port A.
• RD : This is the input line driven by the microprocessor and
should be low to indicate read operation to 8255.
• WR : This is an input line driven by the microprocessor. A low
on this line indicates write operation.
• CS : This is a chip select line. If this line goes low, it enables the 8255
to respond to RD and WR signals, otherwise RD and WR signal are
neglected.
• A1-A0 : These are the address input lines and are driven by the
microprocessor.
These lines A1-A0 with RD , WR and CS from the following
operations for 8255. These address lines are used for addressing any
one of the four registers, i.e.three ports and a control word register as
given in table below.
• In case of 8086 systems, if the 8255 is to be interfaced with lower
order data bus, the A0 and A1 pins of 8255 are connected with A1
and A2 respectively.
• D0-D7 : These are the data bus lines those carry data or control word
to/from the microprocessor.
• RESET : A logic high on this line clears the control word register of
8255. All ports are set as input ports by default after reset.
RD WR CS A1 A0 Input (Read) cycle
RD WR CS A1 A0 Output (Write) cycle
RD WR CS A1 A0 Function

Control Word Register
Block Diagram of 8255 (Architecture)
• It has a 40 pins of 4 groups.
1. Data bus buffer
2. Read Write control logic
3. Group A and Group B controls
4. Port A, B and C
• Data bus buffer: This is a tristate bidirectional buffer used to interface
the 8255 to system databus. Data is transmitted or received by the
buffer on execution of
input or output instruction by the CPU.
• Control word and status information are also transferred through this
unit.
• Read/Write control logic: This unit accepts control signals ( RD ,
WR ) and also inputs from address bus and issues commands to
individual group of control blocks (Group A, Group B).
• It has the following pins.
a) CS – Chipselect : A low on this PIN enables the communication
between CPU and 8255.
b) RD (Read) – A low on this pin enables the CPU to read the data in
the ports or the status word through data bus buffer.
c) WR (Write) : A low on this pin, the CPU can write data on to the
ports or on to the control register through the data bus buffer.
d) RESET: A high on this pin clears the control register and all ports are
set to the input mode.
e) A0 and A1 (Address pins): These pins in conjunction with RD
and WR pins control the selection of one of the 3 ports.
• Group A and Group B controls : These block receive control from the
CPU and issues commands to their respective ports.
• Group A - PA and PCU (PC7 –PC4)
• Group B - PCL (PC3 – PC0)
• Control word register can only be written into no read operation of the
CW register is allowed.
• a) Port A: This has an 8 bit latched/buffered O/P and 8 bit input
latch. It can be programmed in 3 modes – mode 0, mode 1, mode 2.
b) Port B: This has an 8 bit latched / buffered O/P and 8 bit input
latch. It can be programmed in mode 0, mode1.
c) Port C : This has an 8 bit latched input buffer and 8 bit output
latched/buffer. This port can be divided into two 4 bit ports and can be used
as control signals for port A and port B. it can be programmed in mode 0.

Modes of Operation of 8255
• These are two basic modes of operation of 8255. I/O mode and
Bit Set-Reset mode (BSR).
• In I/O mode, the 8255 ports work as programmable I/O ports, while in
BSR mode only port C (PC0-PC7) can be used to set or reset its
individual port bits.
• Under the I/O mode of operation, further there are three modes of
operation of 8255, so as to support different types of applications, mode
0, mode 1 and mode 2.
• BSR Mode: In this mode any of the 8-bits of port C can be set or reset
depending on D0 of the control word. The bit to be set or reset is
selected by bit select flags D3, D2 and D1 of the CWR as given in
table.
• I/O Modes :
a) Mode 0 (Basic I/O mode): This mode is also called as basic
input/output mode. This mode provides simple input and output capabilities
using each of the three ports. Data can be simply read from and written to the
input and output ports respectively, after appropriate initialization.
D3 D2 D1 Selectedbits of port C
0 0 0 D0
0 0 1 D1
0 1 0 D2
0 1 1 D3
1 0 0 D4
1 0 1 D5
1 1 0 D6
1 1 1 D7

BSR Mode : CWR Format
PA PA6 – PA7 PA PA
8 PCU PC4 – PC7 8
2 2
PCU
5 PCL
5
PC0-PC3 PC
5 PCL
5
PB PB0 – PB7 PB PB0 – PB7
All Output Port A and Port C acting as
O/P. Port B acting as I/P
Mode 0
• The salient features of this mode are as listed below:
1. Two 8-bit ports (port A and port B)and two 4-bit ports (port C upper
and lower)are available. The two 4-bit ports can be combinedly used as a
third 8-bit port.
2. Any port can be used as an input or output port.
3. Output ports are latched. Input ports are not latched.
4. A maximum of four ports are available so that overall 16 I/O
configuration are possible.
• All these modes can be selected by programming a register
internal to 8255 known as CWR.
• The control word register has two formats. The first format is valid for
I/O modes of operation, i.e. modes 0, mode 1 and mode 2 while the
second format is valid for bit set/reset (BSR) mode of operation.
These formats are shown in following fig.

1 40
2 39
3 38
4 37
5 36
6 35
7 34
8 33
9 32
10
8255A
31
11 30
12 29
13 28
14 27
15 26
16 25
17 24
18 23
19 22
20 21
1 X X X
0-for BSR mode Bit select flags
D3, D2, D1 are from 000 to 111 for bits PC0 TO PC7
0- Reset
1- Set
PA3
PA2
PA1
PA0
RD
CS
GND
A1
A0
PC7
PC6
PC5
PC4
PC0
PC1
PC2
PC3
PB0
PB1
PB2
I/O Mode Control Word Register Format and
BSR Mode Control Word Register Format
PA4
PA5
PA6
PA7
WR
Reset
D0
D1
D2
D3
D4
D5
D6
D7
Vcc
PB7
PB6
PB5
PB4
PB3

8255A Pin Configuration
D0-D7
PA0-PA7
CS
RESET
A0
8255A
PC4-PC7
PC0-PC3
A1 PB0-PB7
RD
Vcc
WR
GND
Signals of 8255
3 4 PA0-PA7
Group A
control
Group A
Port A(8)
D0-D7
1
Data
bus
8 bit int data bus
Group A
Port C
upper(4)
PC7-PC4
2
RD
WR READ/
Group B
Port C
Lower(4)
PC0-PC3
WRITE
A0 Control Group B Group B PB7-PB0

Control Word Format of 8255
b) Mode 1: (Strobed input/output mode) In this mode the handshaking
control the input and output action of the specified port. Port C lines PC0-
PC2, provide strobe or handshake lines for port B. This group which
includes port B and PC0-PC2 is called as group B for Strobed data
input/output. Port C lines PC3-PC5 provide strobe lines for port A.
This group including port A and PC3-PC5 from group A. Thus port C is
utilized for generating handshake signals. The salient features of mode 1
are listed as follows:
1. Two groups – group A and group B are available for strobed data
transfer.
2. Each group contains one 8-bit data I/O port and one 4-bit control/data
port.
3. The 8-bit data port can be either used as input and output port. The
inputs and outputs both are latched.
4. Out of 8-bit port C, PC0-PC2 are used to generate control signals for
port B and PC3-PC5 are used to generate control signals for port A.
the lines PC6, PC7 may be used as independent data lines.
• The control signals for both the groups in input and output
modes are explained as follows:
Input control signal definitions (mode 1):
• STB(Strobe input) – If this lines falls to logic low level, the data
available at 8-bit input port is loaded into input latches.
• IBF (Input buffer full) – If this signal rises to logic 1, it indicates that
data has been loaded into latches, i.e. it works as an acknowledgement.
IBF is set by a low on STB and is reset by the rising edge of RD input.
• INTR (Interrupt request) – This active high output signal can be used
to interrupt the CPU whenever an input device requests the service. INTR
is set by a high.STB pin and a high at IBF pin. INTE is an internal flag that
can be controlled by the bit set/reset mode of either PC4(INTEA) or
PC2(INTEB) as shown in fig.
• INTR is reset by a falling edge of RD input. Thus an external input
device can be request the service of the processor by putting the data
on the bus and sending the strobe signal.
Output control signal definitions (mode 1) :
• OBF (Output buffer full) – This status signal, whenever falls to low,
indicates that
CPU has written data to the specified output port. The OBF flip-flop
will be set by a rising edge of WR signal and reset by a low going
edge at the ACK input.
• ACK (Acknowledge input) – ACK signal acts as an

1 0 1 0 1/0 X X X 1 X X X X 1 1 X
acknowledgement to be given by an output device. ACK signal,
whenever low, informs the CPU that the
data transferred by the CPU to the output device through the port is
received by
the output device.
• INTR (Interrupt request) – Thus an output signal that can be used to
interrupt the CPU when an output device acknowledges the data
received from the CPU. INTR is set when ACK, OBF and INTE are 1.
It is reset by a falling edge on WR input. The INTEA and INTEB flags
are controlled by the bit set-reset mode of PC6 and PC2 respectively.
Input control signal definitions in Mode
1
D7 D6 D5 D4 D3 D2 D1 D0 D7 D6 D5 D4 D3 D2 D1 D0
1 - Input
0 - Output
For PC6 –
PC7
INTEA
PA0 –
PA7
PC4
STB
INTEB
PB0 – PB7
PC2 STBB
PC5 IBFA
PC1 IBFB
PC3 INTRA
RD PC6 – PC7 I/O
RD
PC0 INTR
A
Mode 1 Control Word Group
A I/P
Mode 1 Control Word Group
B I/P

Programmable Interval Timer 8253
• The Intel 8253 is a programmable counter / timer chip designed for use as
an Intel microcomputer peripheral. It uses nMOS technology with a single
+5V supply and is packaged in a 24-pin plastic DIP.
• It is organized as 3 independent 16-bit counters, each with a counter rate up
to 2 MHz. All modes of operation are software programmable.
• The 82C54 is pin compatible with the HMOS 8254, and is a superset of the
8253.
• Six programmable timer modes allow the 82C54 / 8253 to be used as an
event counter, elapsed time indicator, programmable one-shot, and in many
other applications.
Block diagram
Fig: Block diagram of an 8253 programmable interval timer
The block labeled data bus buffer contains the logic to buffer the data bus to / from
the microprocessor, and to the internal registers. The block labeled read / write
logic controls the reading and the writing of the counter registers. The final block,
the control word register, contains the programmed information that is sent to the
device from the microprocessor. In effect this register defines how the 8253
logically works. The timer has three independent, programmable counters and they
are all identical

Each counter in the block diagram has 3 logical lines connected to it. Two of these
lines, clock and gate, are inputs. The third, labeled OUT is an output. The function
of these lines changes and depends on how the device is initialized or programmed.
PIN configuration
The following picture shows the pin configuration of the 8253 and a general
definition of the lines follows:
• Clock This is the clock input for the counter. The counter is 16 bits. The
maximum clock frequency is 1 / 380 nanoseconds or 2.6 megahertz. The
minimum clock frequency is DC or static operation.
• Out This single output line is the signal that is the final programmed output
of the device. Actual operation of the outline depends on how the device has
been programmed.
• Gate This input can act as a gate for the clock input line, or it can act as a
start pulse, depending on the programmed mode of the counter.
Internal 8253 register
Here is a list of the internal 8253 registers that will program the internal counters
of the 8253:

Counter #0, #1, #2 :Each counter is identical, and each consists of a 16-bit, pre-
settable, down counter. Each is fully independent and can be easily read by the
CPU. When the counter is read, the data within the counter will not be disturbed.
This allows the system or your own program to monitor the counter's value at any
time, without disrupting the overall function of the 8253.
Control Word Register This internal register is used to write information to, prior
to using the device. This register is addressed when A0 and A1 inputs are logical
1's. The data in the register controls the operation mode and the selection of either
binary or BCD ( binary coded decimal ) counting format. The register can only be
written to. You can't read information from the register.
Control Word Register
All of the operating modes for the counters are selected by writing bytes to the
control register. This is the control word format.

Bits D7 and D6 are labeled SC1 and SC0. These bits select the counter to be
programmed, it is necessary to define, using the control bits D7 and D6, which
counter is being set up.
Once a counter is set up, it will remain that way until it is changed by another
control word.
Bits D5 and D4 (RL1 / RL0) of the control word shown above are defined as the
read / load mode for the register that is selected by bits D7 and D6. Bits D5 and D4
define how the particular counter is to have data read from or written to it by the
CPU.
These bits are defined as:
The 1st value, $00, is the counter latch mode. If this mode is specified, the current
counter value is latched into an internal register at the time of the I/O write
operation to the control register. When a read of the counter occurs, it is this
latched value that is read.

If the latch mode is not used, then it is possible that the data read back may be in
the process of changing while the read is occurring. This could result in invalid
data being input by the CPU. To read the counter value while the counter is still in
the process of counting, one must first issue a latch control word, and then issue
another control word that indicates the order of the bytes to be read.
An alternative method of obtaining a stable count from the timer is to externally
inhibit counting while the register is being read. To this, an external logic to the
8253 controlled by the Z80 to inhibit count during an input read operation is to
connect.
Each technique has certain disadvantages. The first, the latching method, may give
the CPU a reading that is "old" by several cycles, depending on the speed of the
count and which byte of the counter is being read.
The second method, the external inhibiting function, requires additional hardware.
In addition, it may change the overall system operation. The counters 1 and 2 of
the MZ-700 are not designed with this additional hardware function. :-( but the
counter 0. You can use this method for your own purposes even an amplifier is
connected to the output pin of this counter.
The input to counter 0 is 1.1088MHz.
The next 3 bits of the control word are D3, D2, and D1. These bits determine the
basic mode of operation for the selected counter. The mode descriptions are as
follows:

The final bit D0 of the control register determines how the register will count: The
maximum values for the count in each count mode are 104
(10,000 decimal) in
BCD and 216
(65,536 decimal) in binary.
Modes
The following text describes all possible modes. The modes used in the MZ-700
and set by the monitor's startup are mode 0, mode 2, and mode 3.
Mode 0 (Interrupt on Terminal Count )
The counter will be programmed to an initial value and afterwards counts down at
a rate equal to the input clock frequency. When the count is equal to 0, the OUT
pin will be a logical 1. The output will stay a logical 1 until the counter is reloaded
with a new value or the same value or until a mode word is written to the device.
Once the counter starts counting down, the GATE input can disable the internal
counting by setting the GATE to a logical 0.
Mode 1 (Programmable One-Shot)
In mode 1, the device can be setup to give an output pulse that is an integer number
of clock pulses. The one-shot is triggered on the rising edge of the GATE input. If
the trigger occurs during the pulse output, the 8253 will be retriggered again.
Mode 2 (Rate Generator )
The counter that is programmed for mode 2 becomes a "divide by n" counter. The
OUT pin of the counter goes to low for one input clock period. The time between
the pulses of going low is dependent on the present count in the counter's register. I
mean the time of the logical 1 pulse.
For example, suppose to get an output frequency of 1,000 Hz ( Hertz ), the period
would be 1 / 1,000 s = 1 ms ( millisecond ) or 1,000 µs ( microseconds ). If an
input clock of 1 MHz ( Mega-Hertz ) were applied to the clock input of the counter
#0, then the counter #0 would need to be programmed to 1000 µs. This could be
done in decimal or in BCD. ( The period of an input clock of 1 MHz is 1 /
1,000,000 = 1 µs. )
The formula is: n=fi divided by fout.
fi = input clock frequency, fout = output frequency, n = value to be loaded.
My example: fi = 1 MHz = 1 x 106
Hz, fout = 1 kHz = 1 x 103
Hz.
n = 1 x 106
Hz / 1 x 103
Hz = 1 x 103
= 1,000. This is the decimal value to be
loaded or the hexadecimal value $03E8. The following program example uses the
decimal load count.

B000 3E35 LD A,$35 ; load control word
; for counter 0 mode 2
B002 3207E0 LD ($E007),A ; into port $E007
; for BCD count
B005 2104E0 LD HL,$E004 ; address to the port
; of counter 0
B008 3E00 LD A,$00
B00A 77 LD (HL),A ; load least significant
; byte of 1000 first
B00B 3E10 LD A,$10
B00D 77 LD (HL),A ; load most significant
; byte of 1000 last
B00E 3E01 LD A,1
B010 3208E0 LD ($E008),A ; start counter 0 is only
; Necessary for the MZ-700.
; Not necessary for
; counter #1 and #2
; The counter is now initialized and the output frequency
; will be 1000 Hz if the input frequency is 1 MHz.
If the count is loaded between output pulses, the present period will not be
affected. A new period will occur during the next count sequence.
Mode 3 (Square Wave Generator )
Mode 3 is similar to the mode 2 except that the output will be high for half the
period and low for half. If the count is odd, the output will be high for ( n + 1 ) / 2
and low for ( n - 1 ) / 2 counts.
For example, I'll setup counter #0 for a square wave frequency of 10 kHz ( kilo-
Hertz ), assuming the input frequency is 1 MHz.
Please refer to the formula described at mode 2.
1 x 106
/ 10 x 103
= 100. This is the decimal value to be loaded or the hexadecimal
value $0064. The following program example uses the binary load count.
B000 3E35 LD A,$36 ; load control word
; for counter 0 mode 3
B002 3207E0 LD ($E007),A ; into port $E007
; for binary count
B005 2104E0 LD HL,$E004 ; address to the port
; of counter 0
B008 3E00 LD A,$64 ; equals to
; 100 microseconds

; for 10,000 Hz
B00A 77 LD (HL),A ; load least significant
; byte of $0064 first
B00B 3E10 LD A,$00
B00D 77 LD (HL),A ; load most significant
; nyte of $0064 last
B00E 3E01 LD A,1
B010 3208E0 LD ($E008),A ; start counter 0 is only
; necessary for the MZ-700.
; Not necessary for counter
; #1 and #2
; The counter is now initialized and the output frequency
; will be 10 kHz if the input frequency is 1 MHz.
Mode 4 (Software Triggered Strobe )
In this mode the programmer can set up the counter to give an output timeout
starting when the register is loaded. On the terminal count, when the counter equals
to 0, the output will go to a logical 0 for one clock period and then returns to a
logical 1. First the mode is set, the output will be a logical 1.
Mode 5 (Hardware Triggered Strobe )
In this mode the rising edge of the trigger input will start the counting of the
counter. The output goes low for one clock at the terminal count. The counter is
retriggerable, thus meaning that if the trigger input is taken low and then high
during a count sequence, the sequence will start over.
When the external trigger input goes to a logical 1, the timer will start to time out.
If the external trigger occurs again, prior to the time completing a full timeout, the
timer will retrigger.
Programmable Interrupt Controller 8259A
• If we are working with an 8086, we have a problem here because the 8086 has
only two interrupt inputs, NMI and INTR.
• If we save NMI for a power failure interrupt, this leaves only one interrupt for
all the other applications. For applications where we have interrupts from
multiple source, we use an external device called a priority interrupt
controller (PIC) to the interrupt signals into a single interrupt input on the
processor.

Architecture and Signal Descriptions of 8259A
• The architectural block diagram of 8259A is shown in fig1. The functional
explication of each block is given in the following text in brief.
• Interrupt Request Register (RR): The interrupts at IRQ input lines are
handled by Interrupt Request internally. IRR stores all the interrupt request
in it in order to serve them one by one on the priority basis.
• In-Service Register (ISR): This stores all the interrupt requests those are
being served, i.e. ISR keeps a track of the requests being served.
• Priority Resolver : This unit determines the priorities of the interrupt
requests appearing simultaneously. The highest priority is selected and
stored into the corresponding bit of ISR during INTA pulse. The IR0 has the
highest priority while the IR7 has the lowest one, normally in fixed priority
mode. The priorities however may be altered by programming the 8259A in
rotating priority mode.
• Interrupt Mask Register (IMR) : This register stores the bits required to
mask the interrupt inputs. IMR operates on IRR at the direction of the
Priority Resolver.
• Interrupt Control Logic: This block manages the interrupt and interrupt
acknowledge signals to be sent to the CPU for serving one of the eight
interrupt requests. This also accepts the interrupt acknowledge (INTA)
signal from CPU that causes the 8259A to release vector address on to the
data bus.

• Data Bus Buffer : This tristate bidirectional buffer interfaces internal
8259A bus to the microprocessor system data bus. Control words, status and
vector information pass through data buffer during read or write operations.
• Read/Write Control Logic: This circuit accepts and decodes commands
from the CPU. This block also allows the status of the 8259A to be
transferred on to the data bus.
• Cascade Buffer/Comparator: This block stores and compares the ID’s all
the 8259A used in system. The three I/O pins CASO-2 are outputs when the
8259A is used as a master. The same pins act as inputs when the 8259A is in
slave mode. The 8259A in master mode sends the ID of the interrupting
slave device on these lines. The slave thus selected, will send its
preprogrammed vector address on the data bus during the next INTA pulse.
• CS: This is an active-low chip select signal for enabling RD and WR
operations of 8259A. INTA function is independent of CS.
• WR : This pin is an active-low write enable input to 8259A. This enables it
to accept command words from CPU.
• RD : This is an active-low read enable input to 8259A. A low on this line
enables 8259A to release status onto the data bus of CPU.
• D0-D7 : These pins from a bidirectional data bus that carries 8-bit data
either to control word or from status word registers. This also carries
interrupt vector information.
• CAS0 – CAS2 Cascade Lines : A signal 8259A provides eight vectored
interrupts. If more interrupts are required, the 8259A is used in cascade
mode. In cascade mode, a master 8259A along with eight slaves 8259A can
provide upto 64 vectored interrupt lines. These three lines act as select lines
for addressing the slave 8259A.
• PS/EN : This pin is a dual purpose pin. When the chip is used in buffered
mode, it can be used as buffered enable to control buffer transreceivers. If
this is not used in buffered mode then the pin is used as input to designate
whether the chip is used as a master (SP =1) or slave (EN = 0).
• INT : This pin goes high whenever a valid interrupt request is asserted. This
is used to interrupt the CPU and is connected to the interrupt input of CPU.
• IR0 – IR7 (Interrupt requests) :These pins act as inputs to accept interrupt
request to the CPU. In edge triggered mode, an interrupt service is requested
by raising an IR pin from a low to a high state and holding it high until it is
acknowledged, and just by latching it to high level, if used in level triggered
mode.

Pin Diagram
INTA (Interrupt acknowledge): This pin is an input used to strobe-in 8259A
interrupt vector data on to the data bus. In conjunction with CS, WR and RD
pins, this selects the different operations like, writing command words,
reading status word, etc.
• The device 8259A can be interfaced with any CPU using either polling or
interrupt. In polling, the CPU keeps on checking each peripheral device in
sequence to ascertain if it requires any service from the CPU. If any such
service request is noticed, the CPU serves the request and then goes on to
the next device in sequence.
• After the entire peripheral device are scanned as above the CPU again starts
from first device.
• This type of system operation results in the reduction of processing speed
because most of the CPU time is consumed in polling the peripheral devices.
• In the interrupt driven method, the CPU performs the main processing task
till it is interrupted by a service requesting peripheral device.
• The net processing speed of these type of systems is high because the CPU
serves the peripheral only if it receives the interrupt request
• If more than one interrupt requests are received at a time, all the requesting
peripherals are served one by one on priority basis.
• This method of interfacing may require additional hardware if number of
peripherals to be interfaced is more than the interrupt pins available with the
CPU.

Interrupt Sequence in an 8086 system
• The Interrupt sequence in an 8086-8259A system is described as follows:
1. One or more IR lines are raised high that set corresponding IRR bits.
2. 8259A resolves priority and sends an INT signal to CPU.
3. The CPU acknowledge with INTA pulse.
4. Upon receiving an INTA signal from the CPU, the highest priority ISR bit is
set and the corresponding IRR bit is reset. The 8259A does not drive data
during this period.
5. The 8086 will initiate a second INTA pulse. During this period 8259A
releases an 8-bit pointer on to a data bus from where it is read by the CPU.
6. This completes the interrupt cycle. The ISR bit is reset at the end of the
second INTA pulse if automatic end of interrupt (AEOI) mode is
programmed. Otherwise ISR bit remains set until an appropriate EOI
command is issued at the end of interrupt subroutine.
Command Words of 8259A
• The command words of 8259A are classified in two groups
1. Initialization command words (ICW) and
2. Operation command words (OCW).
• Initialization Command Words (ICW): Before it starts functioning, the
8259A must be initialized by writing two to four command words into
the respective command word registers. These are called as initialized
command words.
• If A0 = 0 and D4 = 1, the control word is recognized as ICW1. It
contains the control bits for edge/level triggered mode, single/cascade
mode, call address interval and whether ICW4 is required or not.
• If A0=1, the control word is recognized as ICW2. The ICW2 stores
details regarding interrupt vector addresses. The initialisation sequence
of 8259A is described in form of a flow chart in fig 3 below.
• The bit functions of the ICW1 and ICW2 are self explanatory as shown
in fig below.

Operating Modes of 8259
• The different modes of operation of 8259A can be programmed by setting or
resting the appropriate bits of the ICW or OCW as discussed previously.
The different modes of operation of 8259A are explained in the following.
• Fully Nested Mode : This is the default mode of operation of 8259A. IR0
has the highest priority and IR7 has the lowest one. When interrupt request
are noticed, the highest priority request amongst them is determined and the
vector is placed on the data bus. The corresponding bit of ISR is set and
remains set till the microprocessor issues an EOI command just before
returning from the service routine or the AEOI bit is set.
• If the ISR (in service) bit is set, all the same or lower priority interrupts are
inhibited but higher levels will generate an interrupt, that will be
acknowledge only if the microprocessor interrupt enable flag IF is set. The
priorities can afterwards be changed by programming the rotating priority
modes.
• End of Interrupt (EOI) : The ISR bit can be reset either with AEOI bit of
ICW1 or by EOI command, issued before returning from the interrupt
service routine. There are two types of EOI commands specific and non-
specific. When 8259A is operated in the modes that preserve fully nested
structure, it can determine which ISR bit is to be reset on EOI.
• When non-specific EOI command is issued to 8259A it will be
automatically reset the highest ISR bit out of those already set.
• When a mode that may disturb the fully nested structure is used, the 8259A
is no longer able to determine the last level acknowledged. In this case a
specific EOI command is issued to reset a particular ISR bit. An ISR bit that
Fig:

is masked by the corresponding IMR bit, will not be cleared by non-specific
EOI of 8259A, if it is in special mask mode.
• Automatic Rotation : This is used in the applications where all the
interrupting devices are of equal priority.
• In this mode, an interrupt request IR level receives priority after it is served
while the next device to be served gets the highest priority in sequence.
Once all the device are served like this, the first device again receives
highest priority.
• Automatic EOI Mode : Till AEOI=1 in ICW4, the 8259A operates in
AEOI mode. In this mode, the 8259A performs a non-specific EOI operation
at the trailing edge of the last INTA pulse automatically. This mode should
be used only when a nested multilevel interrupt structure is not required
with a single 8259A.
• Specific Rotation : In this mode a bottom priority level can be selected,
using L2, L1 and L0 in OCW2 and R=1, SL=1, EOI=0.
• The selected bottom priority fixes other priorities. If IR5 is selected as a
bottom priority, then IR5 will have least priority and IR4 will have a next
higher priority. Thus IR6 will have the highest priority.
• These priorities can be changed during an EOI command by programming
the rotate on specific EOI command in OCW2.
• Specific Mask Mode: In specific mask mode, when a mask bit is set in
OCW1, it inhibits further interrupts at that level and enables interrupt from
other levels, which are not masked.
• Edge and Level Triggered Mode : This mode decides whether the interrupt
should be edge triggered or level triggered. If bit LTIM of ICW1 =0 they are
edge triggered, otherwise the interrupts are level triggered.
• Reading 8259 Status : The status of the internal registers of 8259A can be
read using this mode. The OCW3 is used to read IRR and ISR while OCW1
is used to read IMR. Reading is possible only in no polled mode.
• Poll Command : In polled mode of operation, the INT output of 8259A is
neglected, though it functions normally, by not connecting INT output or by
masking INT input of the microprocessor. The poll mode is entered by
setting P=1 in OCW3.
• The 8259A is polled by using software execution by microprocessor instead
of the requests on INT input. The 8259A treats the next RD pulse to the
8259A as an interrupt acknowledge. An appropriate ISR bit is set, if there is
a request. The priority level is read and a data word is placed on to data bus,
after RD is activated. A poll command may give more than 64 priority

levels.
• Special Fully Nested Mode : This mode is used in more complicated system,
where cascading is used and the priority has to be programmed in the master
using ICW4. this is somewhat similar to the normal nested mode.
• In this mode, when an interrupt request from a certain slave is in service, this
slave can further send request to the master, if the requesting device connected
to the slave has higher priority than the one being currently served. In this
mode, the master interrupt the CPU only when the interrupting device has a
higher or the same priority than the one current being served. In normal mode,
other requests than the one being served are masked out.
• When entering the interrupt service routine the software has to check whether
this is the only request from the slave. This is done by sending a non-specific
EOI can be sent to the master, otherwise no EOI should be sent. This mode is
important, since in the absence of this mode, the slave would interrupt the
master only once and hence the priorities of the slave inputs would have been
disturbed.
• Buffered Mode: When the 83259A is used in the systems where bus driving
buffers are used on data buses. The problem of enabling the buffers exists. The
8259A sends buffer enable signal on SP/ EN pin, whenever data is placed on
the bus.
• Cascade Mode: The 8259A can be connected in a system containing one
master and eight slaves (maximum) to handle up to 64 priority levels. The
master controls the slaves using CAS0-CAS2 which act as chip select inputs
(encoded) for slaves.
• In this mode, the slave INT outputs are connected with master IR inputs. When
a slave request line is activated and acknowledged, the master will enable the
slave to release the vector address during second pulse of INTA sequence.
• The cascade lines are normally low and contain slave address codes from the
trailing edge of the first INTA pulse to the trailing edge of the second INTA
pulse. Each 8259A in the system must be separately initialized and
programmed to work in different modes. The EOI command must be issued
twice, one for master and the other for the slave.
• A separate address decoder is used to activate the chip select line of each
8259A.

• Following Fig shows the details of the circuit connections of 8259A in cascade
scheme.
Keyboard/Display Controller 8279
• While studying 8255, we have explained the use of 8255 in interfacing
keyboards and displays with 8086. The disadvantages of this method of
interfacing keyboard and display with 8086 is that the processor has to
refresh the display and check the status of the keyboard periodically using
polling technique. Thus a considerable amount of CPU time is wasted,
reducing the system operating speed.
• Intel’s 8279 is a general purpose keyboard display controller that
simultaneously drives the display of a system and interfaces a keyboard with
the CPU, leaving it free for its routine task. Architecture and Signal.
Descriptions of 8279
• The keyboard display controller chip 8279 provides:
a) a set of four scan lines and eight return lines for interfacing keyboards
b) A set of eight output lines for interfacing display.
• Fig shows the functional block diagram of 8279 followed by its brief
description.

• I/O Control and Data Buffers : The I/O control section controls the flow
of data to/from the 8279. The data buffers interface the external bus of the
system with internal bus of 8279.
• The I/O section is enabled only if CS is low. The pins A0, RD and WR
select the command, status or data read/write operations carried out by the
CPU with 8279.
• Control and Timing Register and Timing Control : These registers store
the keyboard and display modes and other operating conditions programmed
by CPU. The registers are written with A0=1 and WR=0. The Timing and
control unit controls the basic timings for the operation of the circuit. Scan
counter divide down the operating frequency of 8279 to derive scan
keyboard and scan display frequencies.
• Scan Counter : The scan counter has two modes to scan the key matrix and
refresh the display. In the encoded mode, the counter provides binary
count that is to be externally decoded to provide the scan lines for
keyboard and display (Four mode, the counter internally decodes the least
significant 2 bits and provides a decoded 1 out of 4 scan on SL0-SL3(Four
internally decoded scan lines may drive upto 4 displays). The keyboard and
display both are in the same mode at a time.
• Return Buffers and Keyboard Debounce and Control: This section for a
key closure row wise. If a key closer is detected, the keyboard debounce unit
debounces the key entry (i.e. wait for 10 ms). After the debounce period, if
the key continues to be detected. The code of key is directly transferred to
the sensor RAM along with SHIFT and CONTROL key status.
• FIFO/Sensor RAM and Status Logic: In keyboard or strobed input mode,
this block acts as 8-byte first-in-first-out (FIFO) RAM. Each key code of the
pressed key is entered in the order of the entry and in the mean time read by
the CPU, till the RAM become empty.
• The status logic generates an interrupt after each FIFO read operation till the
FIFO is empty. In scanned sensor matrix mode, this unit acts as sensor
RAM. Each row of the sensor RAM is loaded with the status of the
corresponding row of sensors in the matrix. If a sensor changes its state, the
IRQ line goes high to interrupt the CPU.
• Display Address Registers and Display RAM : The display address
register holds the address of the word currently being written or read by the
CPU to or from the display RAM. The contents of the registers are
automatically updated by 8279 to accept the next data entry by CPU.

Pin Diagram
The signal description of each of the pins of 8279 as follows :

• DB0-DB7 : These are bidirectional data bus lines. The data and command
words to and from the CPU are transferred on these lines.
• CLK : This is a clock input used to generate internal timing required by 8279.
• RESET : This pin is used to reset 8279. A high on this line reset 8279. After
resetting 8279, its in sixteen 8-bit display, left entry encoded scan, 2-key
lock out mode. The clock prescaler is set to 31.
• CS : Chip Select – A low on this line enables 8279 for normal read or write
operations. Other wise, this pin should remain high.
• A0 : A high on this line indicates the transfer of a command or status
information. A low on this line indicates the transfer of data. This is used to
select one of the internal registers of 8279.
• RD, WR (Input/Output) READ/WRITE – These input pins enable the data
buffers to receive or send data over the data bus.
• IRQ : This interrupt output lines goes high when there is a data in the FIFO
sensor RAM. The interrupt lines goes low with each FIFO RAM read
operation but if the FIFO RAM further contains any key-code entry to be
read by the CPU, this pin again goes high to generate an interrupt to the
CPU.
• Vss, Vcc : These are the ground and power supply lines for the circuit.
• SL0-SL3-Scan Lines : These lines are used to scan the key board matrix and
display digits. These lines can be programmed as encoded or decoded, using
the mode control register.
• RL0 - RL7 - Return Lines : These are the input lines which are connected
to one terminal of keys, while the other terminal of the keys are connected to
the decoded scan lines. These are normally high, but pulled low when a key
is pressed.
• SHIFT : The status of the shift input lines is stored along with each key code
in FIFO, in scanned keyboard mode. It is pulled up internally to keep it high,
till it is pulled low with a key closure.
• BD – Blank Display : This output pin is used to blank the display during digit
switching or by a blanking closure.
• OUT A0 – OUT A3 and OUT B0 – OUT B3 – These are the output ports for
two 16*4 or 16*8 internal display refresh registers. The data from these
lines is synchronized with the scan lines to scan the display and keyboard.
The two 4-bit ports may also as one 8-bit port.
• CNTL/STB- CONTROL/STROBED I/P Mode : In keyboard mode, this
lines is used as a control input and stored in FIFO on a key closure. The line
is a strobed lines that enters the data into FIFO RAM, in strobed input mode.
It has an interrupt pull up. The lines is pulled down with a key closer.

Modes of Operation of 8279
• The modes of operation of 8279 are as follows :
1. Input (Keyboard) modes.
2. Output (Display) modes.
• Input (Keyboard) Modes : 8279 provides three input modes. These modes
are as follows:
1. Scanned Keyboard Mode : This mode allows a key matrix to be interfaced
using either encoded or decoded scans. In encoded scan, an 8*8 keyboard or
in decoded scan, a 4*8 keyboard can be interfaced. The code of key pressed
with SHIFT and CONTROL status is stored into the FIFO RAM.
2. Scanned Sensor Matrix : In this mode, a sensor array can be interfaced with
8279 using either encoded or decoded scans. With encoded scan 8*8 sensor
matrix or with decoded scan 4*8 sensor matrix can be interfaced. The sensor
codes are stored in the CPU addressable sensor RAM.
3. Strobed input: In this mode, if the control lines goes low, the data on return
lines, is stored in the FIFO byte by byte.
• Output (Display) Modes : 8279 provides two output modes for selecting the
display options. These are discussed briefly.
1. Display Scan : In this mode 8279 provides 8 or 16 character multiplexed
displays those can be organized as dual 4- bit or single 8-bit display units.
2. Display Entry : (right entry or left entry mode) 8279 allows options for data
entry on the displays. The display data is entered for display either from the
right side or from the left side.
Command Words of 8279
• All the command words or status words are written or read with A0 = 1 and
CS = 0 to or from 8279. This section describes the various command
available in 8279.
a) Keyboard Display Mode Set – The format of the command word to select
different modes of operation of 8279 is given below with its bit definitions.
D7 D6 D5 D4 D3 D2 D1 D0 A0
0 0 D D D K K K 1

b) Programmable clock : The clock for operation of 8279 is obtained by
dividing the external clock input signal by a programmable constant called
prescaler.
• PPPPP is a 5-bit binary constant. The input frequency is divided by a
decimal constant ranging from 2 to 31, decided by the bits of an internal
prescaler, PPPPP.
D7 D6 D5 D4 D3 D2 D1 D0 A0
0 0 1 P P P P P 1
c) Read FIFO / Sensor RAM : The format of this command is given below.
• This word is written to set up 8279 for reading FIFO/ sensor RAM. In
scanned keyboard mode, AI and AAA bits are of no use. The 8279 will
automatically drive data bus for each subsequent read, in the same sequence,
in which the data was entered.
• In sensor matrix mode, the bits AAA select one of the 8 rows of RAM. If AI
flag is set, each successive read will be from the subsequent RAM location.
D7 D6 D5 D4 D3 D2 D1 D0 A0
0 1 0 AI X A A A 1
• X – don’t care

• AI – Auto Increment Flag
• AAA – Address pointer to 8 bit FIFO RAM
d) Read Display RAM: This command enables a programmer to read the
display RAM data. The CPU writes this command word to 8279 to prepare
it for display RAM read operation. AI is auto increment flag and AAAA, the
4-bit address points to the 16-byte display RAM that is to be read. If AI=1,
the address will be automatically, incremented after each read or write to the
Display RAM. The same address counter is used for reading and writing.
D7 D6 D5 D4 D3 D2 D1 D0 A0
0 1 1 AI A A A A 1
e) Write Display RAM:
AI – Auto increment Flag.
AAAA – 4 bit address for 16-bit display RAM to be written.
D7 D6 D5 D4 D3 D2 D1 D0 A0
1 0 0 AI A A A A 1
f) Display Write Inhibit/Blanking : The IW (inhibit write flag) bits are used
to mask the individual nibble as shown in the below command word. The
output lines are divided into two nibbles (OUTA0 – OUTA3) and (OUTB0
– OUTB3), those can be masked by setting the corresponding IW bit to 1.
• Once a nibble is masked by setting the corresponding IW bit to 1, the entry to
display RAM does not affect the nibble even though it may change the
unmasked nibble. The blank display bit flags (BL) are used for blanking A
and B nibbles.
• Here D0, D2 corresponds to OUTB0 – OUTB3 while D1 and D3
corresponds to OUTA0-OUTA3 for blanking and masking.
• If the user wants to clear the display, blank (BL) bits are available for
each nibble as shown in format. Both BL bits will have to be cleared for
blanking both the nibbles.
D7 D6 D5 D4 D3 D2 D1 D0 A0
1 0 1 X IW IW BL BL 1
g) Clear Display RAM : The CD2, CD1, CD0 is a selectable blanking code to
clear all the rows of the display RAM as given below. The characters A and
B represents the output nibbles.
• CD2 must be 1 for enabling the clear display command. If CD2 = 0, the
clear display command is invoked by setting CA=1 and maintaining

CD1, CD0 bits exactly same as above. If CF=1, FIFO status is cleared
and IRQ line is pulled down.
• Also the sensor RAM pointer is set to row 0. if CA=1, this combines the
effect of CD and CF bits. Here, CA represents Clear All and CF as Clear
FIFO RAM.
D7 D6 D5 D4 D3 D2 D1 D0 A0
1 1 0 CD2 CD1 CD0 CF CA 1
CD2 CD1 CD0
1 0 X
1 1 0
1 1 1
All zeros (x don’t care) AB=00
A3-A0 =2 (0010) and B3-B0=00 (0000)
All ones (AB =FF), i.e. clear RAM
h) End Interrupt / Error mode Set : For the sensor matrix mode, this
command lowers the IRQ line and enables further writing into the RAM.
Otherwise, if a change in sensor value is detected, IRQ goes high that
inhibits writing in the sensor RAM.
• For N-Key roll over mode, if the E bit is programmed to be ‘1’, the 8279
operates in special Error mode. Details of this mode are described in
scanned keyboard special error mode. X- don’t care.
D7 D6 D5 D4 D3 D2 D1 D0 A0
1 1 1 E X X X X 1
Programmable Communication Interface 8251 USART
8251 is a USART (Universal Synchronous Asynchronous Receiver Transmitter)
for serial data communication. As a peripheral device of a microcomputer system,
the 8251 receives parallel data from the CPU and transmits serial data after
conversion. This device also receives serial data from the outside and transmits
parallel data to the CPU after conversion.

Block diagram of the 8251 USART (Universal Synchronous Asynchronous
Receiver Transmitter)
The 8251 functional configuration is programed by software. Operation between
the 8251 and a CPU is executed by program control. Table 1 shows the operation
between a CPU and the device.
Table 1 Operation between a CPU and 8251

Control Words
There are two types of control word.
1. Mode instruction (setting of function)
2. Command (setting of operation)
1) Mode Instruction
Mode instruction is used for setting the function of the 8251. Mode instruction will
be in "wait for write" at either internal reset or external reset. That is, the writing of
a control word after resetting will be recognized as a "mode instruction."
Items set by mode instruction are as follows:
• Synchronous/asynchronous mode
• Stop bit length (asynchronous mode)
• Character length
• Parity bit
• Baud rate factor (asynchronous mode)
• Internal/external synchronization (synchronous mode)
• Number of synchronous characters (Synchronous mode)
The bit configuration of mode instruction is shown in Figures 2 and 3. In the case
of synchronous mode, it is necessary to write one-or two byte sync characters. If
sync characters were written, a function will be set because the writing of sync
characters constitutes part of mode instruction.

2) Command
Command is used for setting the operation of the 8251. It is possible to write a
command whenever necessary after writing a mode instruction and sync
characters.
Items to be set by command are as follows:
• Transmit Enable/Disable
• Receive Enable/Disable
• DTR, RTS Output of data.
• Resetting of error flag.

• Sending to break characters
• Internal resetting
• Hunt mode (synchronous mode)

Status Word
It is possible to see the internal status of the 8251 by reading a status word. The bit
configuration of status word is shown in Fig. 5.
Pin Description
D 0 to D 7 (l/O terminal)
This is bidirectional data bus which receive control words and transmits data from
the CPU and sends status words and received data to CPU.
RESET (Input terminal)

A "High" on this input forces the 8251 into "reset status." The device waits for the
writing of "mode instruction." The min. reset width is six clock inputs during the
operating status of CLK.
CLK (Input terminal)
CLK signal is used to generate internal device timing. CLK signal is independent
of RXC or TXC. However, the frequency of CLK must be greater than 30 times
the RXC and TXC at Synchronous mode and Asynchronous "x1" mode, and must
be greater than 5 times at Asynchronous "x16" and "x64" mode.
WR (Input terminal)
This is the "active low" input terminal which receives a signal for writing transmit
data and control words from the CPU into the 8251.
RD (Input terminal)
This is the "active low" input terminal which receives a signal for reading receive
data and status words from the 8251.
C/D (Input terminal)
This is an input terminal which receives a signal for selecting data or command
words and status words when the 8251 is accessed by the CPU. If C/D = low, data
will be accessed. If C/D = high, command word or status word will be accessed.
CS (Input terminal)
This is the "active low" input terminal which selects the 8251 at low level when the
CPU accesses. Note: The device won’t be in "standby status"; only setting CS =
High.
TXD (output terminal)
This is an output terminal for transmitting data from which serial-converted data is
sent out. The device is in "mark status" (high level) after resetting or during a
status when transmit is disabled. It is also possible to set the device in "break
status" (low level) by a command.
TXRDY (output terminal)
This is an output terminal which indicates that the 8251is ready to accept a
transmitted data character. But the terminal is always at low level if CTS = high or
the device was set in "TX disable status" by a command. Note: TXRDY status
word indicates that transmit data character is receivable, regardless of CTS or

command. If the CPU writes a data character, TXRDY will be reset by the leading
edge or WR signal.
TXEMPTY (Output terminal)
This is an output terminal which indicates that the 8251 has transmitted all the
characters and had no data character. In "synchronous mode," the terminal is at
high level, if transmit data characters are no longer remaining and sync characters
are automatically transmitted. If the CPU writes a data character, TXEMPTY will
be reset by the leading edge of WR signal. Note : As the transmitter is disabled by
setting CTS "High" or command, data written before disable will be sent out. Then
TXD and TXEMPTY will be "High". Even if a data is written after disable, that
data is not sent out and TXE will be "High".After the transmitter is enabled, it sent
out. (Refer to Timing Chart of Transmitter Control and Flag Timing)
TXC (Input terminal)
This is a clock input signal which determines the transfer speed of transmitted data.
In "synchronous mode," the baud rate will be the same as the frequency of TXC. In
"asynchronous mode", it is possible to select the baud rate factor by mode
instruction. It can be 1, 1/16 or 1/64 the TXC. The falling edge of TXC sifts the
serial data out of the 8251.
RXD (input terminal)
This is a terminal which receives serial data.
RXRDY (Output terminal)
This is a terminal which indicates that the 8251 contains a character that is ready to
READ. If the CPU reads a data character, RXRDY will be reset by the leading
edge of RD signal. Unless the CPU reads a data character before the next one is
received completely, the preceding data will be lost. In such a case, an overrun
error flag status word will be set.
RXC (Input terminal)
This is a clock input signal which determines the transfer speed of received data. In
"synchronous mode," the baud rate is the same as the frequency of RXC. In
"asynchronous mode," it is possible to select the baud rate factor by mode
instruction. It can be 1, 1/16, 1/64 the RXC.

SYNDET/BD (Input or output terminal)
This is a terminal whose function changes according to mode. In "internal
synchronous mode." this terminal is at high level, if sync characters are received
and synchronized. If a status word is read, the terminal will be reset. In "external
synchronous mode, "this is an input terminal. A "High" on this input forces the
8251 to start receiving data characters.
In "asynchronous mode," this is an output terminal which generates "high
level"output upon the detection of a "break" character if receiver data contains a
"low-level" space between the stop bits of two continuous characters. The terminal
will be reset, if RXD is at high level. After Reset is active, the terminal will be
output at low level.
DSR (Input terminal)
This is an input port for MODEM interface. The input status of the terminal can be
recognized by the CPU reading status words.
DTR (Output terminal)
This is an output port for MODEM interface. It is possible to set the status of DTR
by a command.
CTS (Input terminal)
This is an input terminal for MODEM interface which is used for controlling a
transmit circuit. The terminal controls data transmission if the device is set in "TX
Enable" status by a command. Data is transmitable if the terminal is at low level.
RTS (Output terminal)
This is an output port for MODEM interface. It is possible to set the status RTS by
a command.
8527 DMA Controller
The i8527 controller has four independent channels each of which contains an
address register and a counter. The counter decrements as each byte transfer occur,
and forces termination of the DMA operation after the last transfer. The controller
increments the address registers after each operation, so that successive data
transfers are made at contiguous ascending addresses. The arbiter resolves conflicts
among the channels for access to memory. Two methods have been used in this

chip to make the chip useful in a variety of different applications. In one mode the
channels have a fixed priority and conflicts are resolved according to the priority,
for example, Channel 0 has highest priority and Channel 3 lowest. The second
mode is a rotating priority scheme in which priority rankings are the four cycle
shifts of 0-1-2-3, when a channel is granted access to the bus the priority ranking
shifts cyclically to place the channel in the lowest priority position for the next
arbitration cycle.
Figure 5-4: Structure of the i8527 DMA controller
The chip has four signals associated with the READ and WRITE operation. MEM
READ L and MEM WRITE L are signals produced by DMA controller to exercise
memory. The two signals I/O READ L and I/O WRITE L are bidirectional, they
are inputs from the microprocessor when the microprocessor sends commands to
the 8257 and reads back the 8257 status. During the I/O operation these signals are
output from the 8257 and are functionally opposite to the memory signals. The
8257 takes control of the bus by exercising HALT (HRQ) and receives back the
"go-ahead" signal on HALT ACKNOWLEDGE (HLDA).
Two signals produced by the DMA controller can be used by the I/O port to assist
in controlling the transfer process. One signal TC--terminal count--is asserted
during the last cycle of a DMA block. This can be used to describe a DMA mode
on an I/O port or to reset the port's internal state to indicate the end of a transfer.
The second--MARK--is inserted when the remaining count on a channel became a
multiple of 128--providing a convenient timing signal for an external device.

Interrupts 8086
The meaning of ‘interrupts’ is to break the sequence of operation. While the
CPU is executing a program, an ‘interrupt’ breaks the normal sequence of
execution of instructions, diverts its execution to some other program called
Interrupt Service Routine (TSR). After executing TSR, the control is
transferred back again to the main program which was being executed at the
time of interruption.
Whenever a number of devices interrupt a CPU at a lime, and if the processor is
able to handle them properly, it is said to have multiple interrupt
processing capability.
Need for Interrupt: Interrupts are particularly useful when interfacing I/O
devices that provide or require data at relatively low data transfer rate.
Sources of Interrupts in 8086: There are two pins for Interrupts in 8086. These
are:
• Hardware Interrupts (External Interrupts) – INTR, NMI
• Software Interrupts (Internal Interrupts and Instructions) – INT n
instructions
(i) Hardware Interrupts (External Interrupts)-
The Intel microprocessors 8086 support hardware interrupts through:
• Two pins that allow interrupt requests, -INTR and NMI
• However one pin that acknowledges, INTR is INTA.
INTR and NMI

1. INTR is a maskable hardware interrupt. The interrupt can be
enabled/disabled using STI/CLI instructions or using more complicated
method of updating the Interrupt Flag (IF).
2. The 1NTR, further, is of 256 types. The INTR types may be from 00 to
FFH (or 00 to 255). If more than one type of INTR interrupt occurs at a
time, then an external chip called programmable interrupt controller is
required to handle them. The same is the case for INTR interrupt input of
8085.
• When an interrupt occurs, the processor stores FLAGS register into
stack, disables further interrupts, fetches from the bus one byte
representing interrupt type, and jumps to interrupt processing
routine address of which is stored in location
4 * <interrupt type>
Interrupt processing routine should return with the IRET instruction.
3. NMI is a non-maskable interrupt which means that any interrupt request
at NMI input cannot be masked or disabled by any means.
• This Interrupt is processed in the same way as the INTR interrupt.
• Interrupt type of the NMI is 2, i.e. the address of the NMI
processing routine is stored in location 0008h.
• This interrupt has higher priority than the maskable interrupt.
Ex: NMI, INTR.
(ii) Software Interrupts (Internal Interrupts and
Instructions)-
Software interrupts can be caused by:
• INT instruction - breakpoint interrupt. This is a type 3 interrupt.
• INT <interrupt number> instruction - any one interrupt from available
256 interrupts.
• INTO instruction - interrupt on overflow
• Single-step interrupt - generated if the TF flag is set. This is a type 1
interrupt. When the CPU processes this interrupt it clears TF flag before
calling the interrupt processing routine.
• Processor exceptions: Divide Error (Type 0), Unused Opcode (type 6)
and Escape opcode (type 7).
• Software interrupt processing is the same as for the hardware interrupts.
• - Ex: INT n (Software Instructions)
• Control is provided through:
o IF and TF flag bits
o IRET and IRETD

Action taken when Interrupts
7. It
decrements SP by 2 and pushes the
8. Disables INTR by clearing the IF.
9. It resets the TF in the flag Register.
10.It decrements SP by 2 and pushes CS on the stack.
11.It decrements SP by 2 and pushes IP on the stack.
12.Fetch the ISR address from the interrupt vector table.
13.After executing ISR (i.
Flag register content is popped so SP
gradually.
Interrupt Vector Table
Action taken when Interrupts ocurres
decrements SP by 2 and pushes the flag register on the stack.
Disables INTR by clearing the IF.
It resets the TF in the flag Register.
It decrements SP by 2 and pushes CS on the stack.
It decrements SP by 2 and pushes IP on the stack.
Fetch the ISR address from the interrupt vector table.
After executing ISR (i.e., when IRET invoked) then IP, CS a
Flag register content is popped so SP will be decremented
gradually.
Interrupt Vector Table
flag register on the stack.
Fetch the ISR address from the interrupt vector table.
., when IRET invoked) then IP, CS and
will be decremented

• Every external and internal interrupt is assigned with a type (N), that is
either implicit (in case of NMI, TRAP and divide by zero) or specified in
the instruction INT N (in case of internal interrupts).
• In case of external interrupts, the type is passed to the processor by an
external hardware like programmable interrupt controller.
• The 8086 supports a total of 256 types of the interrupts. i.e. from 00 to
FFH. Each interrupt requires 4 bytes. i.e. two bytes each for IP and CS of
its TSR. Thus a total of 1024 bytes are required for 256 interrupt types,
hence the interrupt vector table starts at location 0000:0000 and ends at
0000:03FFH.
• The interrupt vector table contains the IP and CS of all the interrupt types
stored sequentially from address 0000:0000 to 0000:03FF H.
• The interrupt type N is multiplied by 4 and the hexadecimal
multiplication obtained gives the offset address in the zeroth
code segment
at which the IP and CS addresses of the interrupt service routine (ISR) are
stored.

IP = (4
CS = (4
• The execution automatically starts from the new CS:
Interrupt Type
Type 0 – Type 4 → Intel predefined
Type 5 – Type 31 →Reserved
Type 32 – Type 255 →
Functions associated with
Type 0 (divide error)
• It is invoked by the microprocessor whenever there is an attempt to divide
a number by zero.
• ISR is responsible for displaying the message “Divide Error” on the
screen.
• IP:00000, CS:00002
Type 1 (Trap or Single step
For single stepping the trap flag must be 1.
• After execution of each instruction, 8086 automatically jumps to 00004H
to fetch 4 bytes for CS: IP of the ISR.
• The job of ISR is to dump the registers on to the
Type 2 (Non maskable Interrupt
IP = (4 × n) H
= (4 × n) H + 2 ; where n-type of interrupt
The execution automatically starts from the new CS: IP.
Type
Intel predefined
→Reserved
→ User defined Maskable Interrupt
Functions associated with Type 0 – Type 4 → Intel predefined
Type 0 (divide error)
It is invoked by the microprocessor whenever there is an attempt to divide
ISR is responsible for displaying the message “Divide Error” on the
IP:00000, CS:00002
Trap or Single step)
For single stepping the trap flag must be 1.
After execution of each instruction, 8086 automatically jumps to 00004H
to fetch 4 bytes for CS: IP of the ISR.
The job of ISR is to dump the registers on to the screen
Non maskable Interrupt)
type of interrupt
Intel predefined
It is invoked by the microprocessor whenever there is an attempt to divide
ISR is responsible for displaying the message “Divide Error” on the
After execution of each instruction, 8086 automatically jumps to 00004H

• Whenever NMI pin of the 8086 is activated by a high signal (5v), the
CPU Jumps to physical memory location 00008 to fetch CS: IP of the
ISR associated with NMI.
Type 3 (Break point)
• A break point is used to examine the CPU and memory after the
execution of a group of Instructions.
• It is one byte instruction whereas other instructions of the form “INT 3”
are 2 byte instructions.
Type 4 (Signed number overflow)
• There is an instruction associated with this INT 0 (interrupt on overflow).
• If INT 0 is placed after a signed number arithmetic as IMUL or ADD the
CPU will activate Type 4 if 0F = 1.
• In case where 0F = 0 , the INT 0 is not executed but is bypassed and acts
as a NOP
Performance of Hardware Interrupts
• NMI : Non maskable interrupts - TYPE 2 Interrupt
• INTR : Interrupt request - Between 20H and FFH
Interrupt Priority Structure
Level triggered

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