Architecture OF 8085

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Introduction to 8085 Architecture and Programming

Contents

1. Internal architecture of 8085 microprocessor
2. 8085 system bus
3. 8085 pin description.
4. 8085 functional description.
5. Programming model of 8085 microprocessor
6. Addressing modes.
7. Instruction set classification.
8. Instruction format.
9. Sample programs.

1. Internal Architecture of 8085 Microprocessor

Control Unit
Generates signals within uP to carry out the instruction, which has been decoded. In
reality causes certain connections between blocks of the uP to be opened or closed, so
that data goes where it is required, and so that ALU operations occur.

Arithmetic Logic Unit
The ALU performs the actual numerical and logic operation such as ‘add’, ‘subtract’,
‘AND’, ‘OR’, etc. Uses data from memory and from Accumulator to perform
arithmetic. Always stores result of operation in Accumulator.

Registers

The 8085/8080A-programming model includes six registers, one accumulator, and
one flag register, as shown in Figure. In addition, it has two 16-bit registers: the stack
pointer and the program counter. They are described briefly as follows.

The 8085/8080A has six general-purpose registers to store 8-bit data; these are
identified as B,C,D,E,H, and L as shown in the figure. They can be combined as
register pairs - BC, DE, and HL - to perform some 16-bit operations. The
programmer can use these registers to store or copy data into the registers by using
data copy instructions.

Accumulator

The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). This
register is used to store 8-bit data and to perform arithmetic and logical operations.
The result of an operation is stored in the accumulator. The accumulator is also
identified as register A.

Flags

The ALU includes five flip-flops, which are set or reset after an operation according
to data conditions of the result in the accumulator and other registers. They are called
Zero(Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; they are
listed in the Table and their bit positions in the flag register are shown in the Figure
below. The most commonly used flags are Zero, Carry, and Sign. The microprocessor
uses these flags to test data conditions.

For example, after an addition of two numbers, if the sum in the accumulator id larger
than eight bits, the flip-flop uses to indicate a carry -- called the Carry flag (CY) -- is
set to one. When an arithmetic operation results in zero, the flip-flop called the
Zero(Z) flag is set to one. The first Figure shows an 8-bit register, called the flag
register, adjacent to the accumulator. However, it is not used as a register; five bit
positions out of eight are used to store the outputs of the five flip-flops. The flags are
stored in the 8-bit register so that the programmer can examine these flags (data
conditions) by accessing the register through an instruction.

These flags have critical importance in the decision-making process of the micro-
processor. The conditions (set or reset) of the flags are tested through the software
instructions. For example, the instruction JC (Jump on Carry) is implemented to
change the sequence of a program when CY flag is set. The thorough understanding
of flag is essential in writing assembly language programs.

Program Counter (PC)

This 16-bit register deals with sequencing the execution of instructions. This register
is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a
16-bit register.

The microprocessor uses this register to sequence the execution of the instructions.
The function of the program counter is to point to the memory address from which the
next byte is to be fetched. When a byte (machine code) is being fetched, the program
counter is incremented by one to point to the next memory location

Stack Pointer (SP)

The stack pointer is also a 16-bit register used as a memory pointer. It points to a
memory location in R/W memory, called the stack. The beginning of the stack is
defined by loading 16-bit address in the stack pointer. The stack concept is explained
in the chapter "Stack and Subroutines."

Instruction Register/Decoder
Temporary store for the current instruction of a program. Latest instruction sent here
from memory prior to execution. Decoder then takes instruction and ‘decodes’ or
interprets the instruction. Decoded instruction then passed to next stage.

Memory Address Register
Holds address, received from PC, of next program instruction. Feeds the address bus
with addresses of location of the program under execution.

Control Generator
Generates signals within uP to carry out the instruction which has been decoded. In
reality causes certain connections between blocks of the uP to be opened or closed, so
that data goes where it is required, and so that ALU operations occur.

Register Selector
This block controls the use of the register stack in the example. Just a logic circuit
which switches between different registers in the set will receive instructions from
Control Unit.

General Purpose Registers

uP requires extra registers for versatility. Can be used to store additional data during a
program. More complex processors may have a variety of differently named registers.

Microprogramming
How does the µP knows what an instruction means, especially when it is only a
binary number? The microprogram in a uP/uC is written by the chip designer and tells
the uP/uC the meaning of each instruction uP/uC can then carry out operation.

2. 8085 System Bus
Typical system uses a number of busses, collection of wires, which transmit binary
numbers, one bit per wire. A typical microprocessor communicates with memory and
other devices (input and output) using three busses: Address Bus, Data Bus and
Control Bus.

Address Bus
One wire for each bit, therefore 16 bits = 16 wires. Binary number carried alerts
memory to ‘open’ the designated box. Data (binary) can then be put in or taken
out.The Address Bus consists of 16 wires, therefore 16 bits. Its "width" is 16 bits. A
16 bit binary number allows 216 different numbers, or 32000 different numbers, ie
0000000000000000 up to 1111111111111111. Because memory consists of boxes,
each with a unique address, the size of the address bus determines the size of memory,
which can be used. To communicate with memory the microprocessor sends an
address on the address bus, eg 0000000000000011 (3 in decimal), to the memory. The
memory the selects box number 3 for reading or writing data. Address bus is
unidirectional, ie numbers only sent from microprocessor to memory, not other way.

Question?: If you have a memory chip of size 256 kilobytes (256 x 1024 x 8 bits),
how many wires does the address bus need, in order to be able to specify an address in
this memory? Note: the memory is organized in groups of 8 bits per location,
therefore, how many locations must you be able to specify?

Data Bus
Data Bus: carries ‘data’, in binary form, between µP and other external units, such as
memory. Typical size is 8 or 16 bits. Size determined by size of boxes in memory and
µP size helps determine performance of µP. The Data Bus typically consists of 8
wires. Therefore, 28 combinations of binary digits. Data bus used to transmit "data",
ie information, results of arithmetic, etc, between memory and the microprocessor.
Bus is bi-directional. Size of the data bus determines what arithmetic can be done. If
only 8 bits wide then largest number is 11111111 (255 in decimal). Therefore, larger
number have to be broken down into chunks of 255. This slows microprocessor. Data
Bus also carries instructions from memory to the microprocessor. Size of the bus
therefore limits the number of possible instructions to 256, each specified by a
separate number.

Control Bus
Control Bus are various lines which have specific functions for coordinating and
controlling uP operations. Eg: Read/NotWrite line, single binary digit. Control
whether memory is being ‘written to’ (data stored in mem) or ‘read from’ (data taken
out of mem) 1 = Read, 0 = Write. May also include clock line(s) for
timing/synchronising, ‘interrupts’, ‘reset’ etc. Typically µP has 10 control lines.
Cannot function correctly without these vital control signals.

The Control Bus carries control signals partly unidirectional, partly bi-directional.
Control signals are things like "read or write". This tells memory that we are either
reading from a location, specified on the address bus, or writing to a location
specified. Various other signals to control and coordinate the operation of the system.
Modern day microprocessors, like 80386, 80486 have much larger busses. Typically
16 or 32 bit busses, which allow larger number of instructions, more memory
location, and faster arithmetic. Microcontrollers organized along same lines, except:
because microcontrollers have memory etc inside the chip, the busses may all be
internal. In the microprocessor the three busses are external to the chip (except for the
internal data bus). In case of external busses, the chip connects to the busses via
buffers, which are simply an electronic connection between external bus and the
internal data bus.

3. 8085 Pin description.

Properties

Single + 5V Supply
4 Vectored Interrupts (One is Non Maskable)
Serial In/Serial Out Port
Decimal, Binary, and Double Precision Arithmetic
Direct Addressing Capability to 64K bytes of memory

The Intel 8085A is a new generation, complete 8 bit parallel central processing unit
(CPU). The 8085A uses a multiplexed data bus. The address is split between the 8bit
address bus and the 8bit data bus. Figures are at the end of the document.

Pin Description

The following describes the function of each pin:

A6 - A1s (Output 3 State)

Address Bus; The most significant 8 bits of the memory address or the 8 bits of the I/0
address,3 stated during Hold and Halt modes.

AD0 - 7 (Input/Output 3state)

Multiplexed Address/Data Bus; Lower 8 bits of the memory address (or I/0 address)
appear on the bus during the first clock cycle of a machine state. It then becomes the
data bus during the second and third clock cycles. 3 stated during Hold and Halt
modes.

ALE (Output)

Address Latch Enable: It occurs during the first clock cycle of a machine state and
enables the address to get latched into the on chip latch of peripherals. The falling
edge of ALE is set to guarantee setup and hold times for the address information.
ALE can also be used to strobe the status information. ALE is never 3stated.

SO, S1 (Output)

Data Bus Status. Encoded status of the bus cycle:

S1 S0
O O HALT
0 1 WRITE
1 0 READ
1 1 FETCH

S1 can be used as an advanced R/W status.

RD (Output 3state)

READ; indicates the selected memory or 1/0 device is to be read and that the Data
Bus is available for the data transfer.

WR (Output 3state)

WRITE; indicates the data on the Data Bus is to be written into the selected memory
or 1/0 location. Data is set up at the trailing edge of WR. 3stated during Hold and Halt
modes.

READY (Input)

If Ready is high during a read or write cycle, it indicates that the memory or
peripheral is ready to send or receive data. If Ready is low, the CPU will wait for
Ready to go high before completing the read or write cycle.

HOLD (Input)

HOLD; indicates that another Master is requesting the use of the Address and Data
Buses. The CPU, upon receiving the Hold request. will relinquish the use of buses as
soon as the completion of the current machine cycle. Internal processing can continue.

The processor can regain the buses only after the Hold is removed. When the Hold is
acknowledged, the Address, Data, RD, WR, and IO/M lines are 3stated.

HLDA (Output)

HOLD ACKNOWLEDGE; indicates that the CPU has received the Hold request and
that it will relinquish the buses in the next clock cycle. HLDA goes low after the Hold
request is removed. The CPU takes the buses one half clock cycle after HLDA goes
low.

INTR (Input)

INTERRUPT REQUEST; is used as a general purpose interrupt. It is sampled only
during the next to the last clock cycle of the instruction. If it is active, the Program
Counter (PC) will be inhibited from incrementing and an INTA will be issued. During
this cycle a RESTART or CALL instruction can be inserted to jump to the interrupt
service routine. The INTR is enabled and disabled by software. It is disabled by Reset
and immediately after an interrupt is accepted.

INTA (Output)

INTERRUPT ACKNOWLEDGE; is used instead of (and has the same timing as) RD
during the Instruction cycle after an INTR is accepted. It can be used to activate the
8259 Interrupt chip or some other interrupt port.

RST 5.5
RST 6.5 - (Inputs)
RST 7.5

RESTART INTERRUPTS; These three inputs have the same timing as I NTR except
they cause an internal RESTART to be automatically inserted.
RST 7.5 ~~ Highest Priority
RST 6.5
RST 5.5 o Lowest Priority
The priority of these interrupts is ordered as shown above. These interrupts have a
higher priority than the INTR.

TRAP (Input)

Trap interrupt is a nonmaskable restart interrupt. It is recognized at the same time as
INTR. It is unaffected by any mask or Interrupt Enable. It has the highest priority of
any interrupt.

RESET IN (Input)

Reset sets the Program Counter to zero and resets the Interrupt Enable and HLDA
flipflops. None of the other flags or registers (except the instruction register) are
affected The CPU is held in the reset condition as long as Reset is applied.

RESET OUT (Output)

Indicates CPlJ is being reset. Can be used as a system RESET. The signal is
synchronized to the processor clock.

X1, X2 (Input)

Crystal or R/C network connections to set the internal clock generator X1 can also be
an external clock input instead of a crystal. The input frequency is divided by 2 to
give the internal operating frequency.

CLK (Output)

Clock Output for use as a system clock when a crystal or R/ C network is used as an
input to the CPU. The period of CLK is twice the X1, X2 input period.

IO/M (Output)

IO/M indicates whether the Read/Write is to memory or l/O Tristated during Hold and
Halt modes.

SID (Input)

Serial input data line The data on this line is loaded into accumulator bit 7 whenever a
RIM instruction is executed.

SOD (output)

Serial output data line. The output SOD is set or reset as specified by the SIM
instruction.

Vcc

+5 volt supply.

Vss
Ground Reference.

4. 8085 Functional Description
The 8085A is a complete 8 bit parallel central processor. It requires a single +5 volt
supply. Its basic clock speed is 3 MHz thus improving on the present 8080's
performance with higher system speed. Also it is designed to fit into a minimum
system of three IC's: The CPU, a RAM/ IO, and a ROM or PROM/IO chip.

The 8085A uses a multiplexed Data Bus. The address is split between the higher 8bit
Address Bus and the lower 8bit Address/Data Bus. During the first cycle the address
is sent out. The lower 8bits are latched into the peripherals by the Address Latch
Enable (ALE). During the rest of the machine cycle the Data Bus is used for memory
or l/O data.

The 8085A provides RD, WR, and lO/Memory signals for bus control. An Interrupt
Acknowledge signal (INTA) is also provided. Hold, Ready, and all Interrupts are
synchronized. The 8085A also provides serial input data (SID) and serial output data
(SOD) lines for simple serial interface.

In addition to these features, the 8085A has three maskable, restart interrupts and one
non-maskable trap interrupt. The 8085A provides RD, WR and IO/M signals for Bus
control.

Status Information

Status information is directly available from the 8085A. ALE serves as a status strobe.
The status is partially encoded, and provides the user with advanced timing of the
type of bus transfer being done. IO/M cycle status signal is provided directly also.
Decoded So, S1 Carries the following status information:

HALT, WRITE, READ, FETCH

S1 can be interpreted as R/W in all bus transfers. In the 8085A the 8 LSB of address
are multiplexed with the data instead of status. The ALE line is used as a strobe to
enter the lower half of the address into the memory or peripheral address latch. This
also frees extra pins for expanded interrupt capability.

Interrupt and Serial l/O

The8085A has5 interrupt inputs: INTR, RST5.5, RST6.5, RST 7.5, and TRAP. INTR
is identical in function to the 8080 INT. Each of the three RESTART inputs, 5.5, 6.5.
7.5, has a programmable mask. TRAP is also a RESTART interrupt except it is non-
maskable.

The three RESTART interrupts cause the internal execution of RST (saving the
program counter in the stack and branching to the RESTART address) if the interrupts

are enabled and if the interrupt mask is not set. The non-maskable TRAP causes the
internal execution of a RST independent of the state of the interrupt enable or masks.

The interrupts are arranged in a fixed priority that determines which interrupt is to be
recognized if more than one is pending as follows: TRAP highest priority, RST 7.5,
RST 6.5, RST 5.5, INTR lowest priority This priority scheme does not take into
account the priority of a routine that was started by a higher priority interrupt. RST
5.5 can interrupt a RST 7.5 routine if the interrupts were re-enabled before the end of
the RST 7.5 routine. The TRAP interrupt is useful for catastrophic errors such as
power failure or bus error. The TRAP input is recognized just as any other interrupt
but has the highest priority. It is not affected by any flag or mask. The TRAP input is
both edge and level sensitive.

Basic System Timing

The 8085A has a multiplexed Data Bus. ALE is used as a strobe to sample the lower
8bits of address on the Data Bus. Figure 2 shows an instruction fetch, memory read
and l/ O write cycle (OUT). Note that during the l/O write and read cycle that the l/O
port address is copied on both the upper and lower half of the address. As in the 8080,
the READY line is used to extend the read and write pulse lengths so that the 8085A
can be used with slow memory. Hold causes the CPU to relingkuish the bus when it is
through with it by floating the Address and Data Buses.

System Interface

8085A family includes memory components, which are directly compatible to the
8085A CPU. For example, a system consisting of the three chips, 8085A, 8156, and
8355 will have the following features:
· 2K Bytes ROM
· 256 Bytes RAM
· 1 Timer/Counter
· 4 8bit l/O Ports
· 1 6bit l/O Port
· 4 Interrupt Levels
· Serial In/Serial Out Ports
In addition to standard l/O, the memory mapped I/O offers an efficient l/O addressing
technique. With this technique, an area of memory address space is assigned for l/O
address, thereby, using the memory address for I/O manipulation. The 8085A CPU
can also interface with the standard memory that does not have the multiplexed
address/data bus.

5. The 8085 Programming Model

In the previous tutorial we described the 8085 microprocessor registers in reference to
the internal data operations. The same information is repeated here briefly to provide
the continuity and the context to the instruction set and to enable the readers who
prefer to focus initially on the programming aspect of the microprocessor.

The 8085 programming model includes six registers, one accumulator, and one flag
register, as shown in Figure. In addition, it has two 16-bit registers: the stack pointer
and the program counter. They are described briefly as follows.

ACCUMULATOR A (8)
FLAG REGISTER

B (8) C (8)

D (8) E (8)

H (8) L (8)

Stack Pointer (SP) (16)

Program Counter (PC) (16)

Data Bus Address Bus

8 Lines Bidirectional 16 Lines unidirectional

Registers

The 8085 has six general-purpose registers to store 8-bit data; these are identified as
B,C,D,E,H, and L as shown in the figure. They can be combined as register pairs -
BC, DE, and HL - to perform some 16-bit operations. The programmer can use these
registers to store or copy data into the registers by using data copy instructions.

Accumulator

The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). This
register is used to store 8-bit data and to perform arithmetic and logical operations.
The result of an operation is stored in the accumulator. The accumulator is also
identified as register A.

Flags
The ALU includes five flip-flops, which are set or reset after an operation according
to data conditions of the result in the accumulator and other registers. They are called
Zero(Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; their bit
positions in the flag register are shown in the Figure below. The most commonly used
flags are Zero, Carry, and Sign. The microprocessor uses these flags to test data
conditions.

D7 D6 D5 D4 D3 D2 D1 D0
S Z AC P CY

For example, after an addition of two numbers, if the sum in the accumulator id larger
than eight bits, the flip-flop uses to indicate a carry -- called the Carry flag (CY) -- is
set to one. When an arithmetic operation results in zero, the flip-flop called the
Zero(Z) flag is set to one. The first Figure shows an 8-bit register, called the flag
register, adjacent to the accumulator. However, it is not used as a register; five bit
positions out of eight are used to store the outputs of the five flip-flops. The flags are
stored in the 8-bit register so that the programmer can examine these flags (data
conditions) by accessing the register through an instruction.

These flags have critical importance in the decision-making process of the micro-
processor. The conditions (set or reset) of the flags are tested through the software
instructions. For example, the instruction JC (Jump on Carry) is implemented to
change the sequence of a program when CY flag is set. The thorough understanding
of flag is essential in writing assembly language programs.

Program Counter (PC)

This 16-bit register deals with sequencing the execution of instructions. This register
is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a
16-bit register.
The microprocessor uses this register to sequence the execution of the instructions.
The function of the program counter is to point to the memory address from which the
next byte is to be fetched. When a byte (machine code) is being fetched, the program
counter is incremented by one to point to the next memory location

Stack Pointer (SP)

The stack pointer is also a 16-bit register used as a memory pointer. It points to a
memory location in R/W memory, called the stack. The beginning of the stack is
defined by loading 16-bit address in the stack pointer.

This programming model will be used in subsequent tutorials to examine how these
registers are affected after the execution of an instruction.

6. The 8085 Addressing Modes
The instructions MOV B, A or MVI A, 82H are to copy data from a source into a
destination. In these instructions the source can be a register, an input port, or an 8-bit
number (00H to FFH). Similarly, a destination can be a register or an output port. The
sources and destination are operands. The various formats for specifying operands are
called the ADDRESSING MODES. For 8085, they are:

1. Immediate addressing.
2. Register addressing.
3. Direct addressing.
4. Indirect addressing.

Immediate addressing

Data is present in the instruction. Load the immediate data to the destination provided.
Example: MVI R,data

Register addressing
Data is provided through the registers.
Example: MOV Rd, Rs

Direct addressing

Used to accept data from outside devices to store in the accumulator or send the data
stored in the accumulator to the outside device. Accept the data from the port 00H and
store them into the accumulator or Send the data from the accumulator to the port
01H.
Example: IN 00H or OUT 01H

Indirect Addressing

This means that the Effective Address is calculated by the processor. And the
contents of the address (and the one following) is used to form a second address. The
second address is where the data is stored. Note that this requires several memory
accesses; two accesses to retrieve the 16-bit address and a further access (or accesses)
to retrieve the data which is to be loaded into the register.

7. Instruction Set Classification
An instruction is a binary pattern designed inside a microprocessor to perform a
specific function. The entire group of instructions, called the instruction set,
determines what functions the microprocessor can perform. These instructions can be
classified into the following five functional categories: data transfer (copy)
operations, arithmetic operations, logical operations, branching operations, and
machine-control operations.

Data Transfer (Copy) Operations

This group of instructions copy data from a location called a source to another
location called a destination, without modifying the contents of the source. In
technical manuals, the term data transfer is used for this copying function. However,
the term transfer is misleading; it creates the impression that the contents of the
source are destroyed when, in fact, the contents are retained without any modification.
The various types of data transfer (copy) are listed below together with examples of
each type:

Types Examples

1. Between Registers. 1. Copy the contents of the register B into
register D.

2. Specific data byte to a register or a 2. Load register B with the data byte 32H.
memory location.

3. Between a memory location and a 3. From a memory location 2000H to register
register. B.

4. Between an I/O device and the 4.From an input keyboard to the
accumulator. accumulator.

Arithmetic Operations

These instructions perform arithmetic operations such as addition, subtraction,
increment, and decrement.

Addition - Any 8-bit number, or the contents of a register or the contents of a
memory location can be added to the contents of the accumulator and the sum is
stored in the accumulator. No two other 8-bit registers can be added directly (e.g., the
contents of register B cannot be added directly to the contents of the register C). The
instruction DAD is an exception; it adds 16-bit data directly in register pairs.

Subtraction - Any 8-bit number, or the contents of a register, or the contents of a
memory location can be subtracted from the contents of the accumulator and the
results stored in the accumulator. The subtraction is performed in 2's compliment, and
the results if negative, are expressed in 2's complement. No two other registers can be
subtracted directly.

Increment/Decrement - The 8-bit contents of a register or a memory location can be
incremented or decrement by 1. Similarly, the 16-bit contents of a register pair (such
as BC) can be incremented or decrement by 1. These increment and decrement
operations differ from addition and subtraction in an important way; i.e., they can be
performed in any one of the registers or in a memory location.

Logical Operations

These instructions perform various logical operations with the contents of the
accumulator.

AND, OR Exclusive-OR - Any 8-bit number, or the contents of a register, or of
a memory location can be logically ANDed, Ored, or Exclusive-ORed with the
contents of the accumulator. The results are stored in the accumulator.

Rotate- Each bit in the accumulator can be shifted either left or right to the next
position.

Compare- Any 8-bit number, or the contents of a register, or a memory location can
be compared for equality, greater than, or less than, with the contents of the
accumulator.

Complement - The contents of the accumulator can be complemented. All 0s are
replaced by 1s and all 1s are replaced by 0s.

Branching Operations

This group of instructions alters the sequence of program execution either
conditionally or unconditionally.

Jump - Conditional jumps are an important aspect of the decision-making process in
the programming. These instructions test for a certain conditions (e.g., Zero or Carry
flag) and alter the program sequence when the condition is met. In addition, the
instruction set includes an instruction called unconditional jump.

Call, Return, and Restart - These instructions change the sequence of a program
either by calling a subroutine or returning from a subroutine. The conditional Call and
Return instructions also can test condition flags.

Machine Control Operations

These instructions control machine functions such as Halt, Interrupt, or do nothing.

The microprocessor operations related to data manipulation can be summarized in
four functions:
1. copying data
2. performing arithmetic operations
3. performing logical operations
4. testing for a given condition and alerting the program sequence

Some important aspects of the instruction set are noted below:

1. In data transfer, the contents of the source are not destroyed; only the contents of
the destination are changed. The data copy instructions do not affect the flags.
2. Arithmetic and Logical operations are performed with the contents of the
accumulator, and the results are stored in the accumulator (with some
expectations). The flags are affected according to the results.
3. Any register including the memory can be used for increment and decrement.
4. A program sequence can be changed either conditionally or by testing for a given
data condition.

8. Instruction Format
An instruction is a command to the microprocessor to perform a given task on a
specified data. Each instruction has two parts: one is task to be performed, called the
operation code (opcode), and the second is the data to be operated on, called the
operand. The operand (or data) can be specified in various ways. It may include 8-bit
(or 16-bit ) data, an internal register, a memory location, or 8-bit (or 16-bit) address.
In some instructions, the operand is implicit.

Instruction word size

The 8085 instruction set is classified into the following three groups according to
word size:

1. One-word or 1-byte instructions
2. Two-word or 2-byte instructions
3. Three-word or 3-byte instructions

In the 8085, "byte" and "word" are synonymous because it is an 8-bit microprocessor.
However, instructions are commonly referred to in terms of bytes rather than words.

One-Byte Instructions

A 1-byte instruction includes the opcode and operand in the same byte. Operand(s)
are internal register and are coded into the instruction.
For example:

Task Op Operand Binary Hex
code Code Code
Copy the contents of the accumulator in MOV C,A 0100 1111 4FH
the register C.
Add the contents of register B to the ADD B 1000 0000 80H
contents of the accumulator.
Invert (compliment) each bit in the CMA 0010 1111 2FH
accumulator.

These instructions are 1-byte instructions performing three different tasks. In the first
instruction, both operand registers are specified. In the second instruction, the operand

B is specified and the accumulator is assumed. Similarly, in the third instruction, the
accumulator is assumed to be the implicit operand. These instructions are stored in 8-
bit binary format in memory; each requires one memory location.

MOV rd, rs
rd <-- rs copies contents of rs into rd.
Coded as 01 ddd sss where ddd is a code for one of the 7 general registers which is
the destination of the data, sss is the code of the source register.

Example: MOV A,B
Coded as 01111000 = 78H = 170 octal (octal was used extensively in instruction
design of such processors).

ADD r
A <-- A + r

Two-Byte Instructions
In a two-byte instruction, the first byte specifies the operation code and the second
byte specifies the operand. Source operand is a data byte immediately following the
opcode. For example:

Task Opcode Operand Binary Hex Code
Code
Load an 8-bit data MVI A, Data 3E First Byte
0011 1110
byte in the
accumulator. Data Second Byte

DATA

Assume that the data byte is 32H. The assembly language instruction is written as

Mnemonics Hex code

MVI A, 32H 3E 32H

The instruction would require two memory locations to store in memory.

MVI r,data
r <-- data
Example: MVI A,30H coded as 3EH 30H as two contiguous bytes. This is an
example of immediate addressing.

ADI data
A <-- A + data

OUT port

where port is an 8-bit device address. (Port) <-- A. Since the byte is not the data but
points directly to where it is located this is called direct addressing.

Three-Byte Instructions

In a three-byte instruction, the first byte specifies the opcode, and the following two
bytes specify the 16-bit address. Note that the second byte is the low-order address
and the third byte is the high-order address.
opcode + data byte + data byte

For example:

Task Opcode Operand Binary code Hex Code
Transfer the JMP 2085H C3 First byte
program 1100 0011
sequence to 85 Second Byte
1000 0101
the memory
location 0010 0000 20 Third Byte
2085H.

This instruction would require three memory locations to store in memory.

Three byte instructions - opcode + data byte + data byte

LXI rp, data16

rp is one of the pairs of registers BC, DE, HL used as 16-bit registers. The two data
bytes are 16-bit data in L H order of significance.
rp <-- data16

Example:
LXI H,0520H coded as 21H 20H 50H in three bytes. This is also immediate
addressing.

LDA addr

A <-- (addr) Addr is a 16-bit address in L H order. Example: LDA 2134H coded as
3AH 34H 21H. This is also an example of direct addressing.

9. Sample Programs
Write an assembly program to add two numbers

Program

MVI D, 8BH
MVI C, 6FH
MOV A, C

ADD D
OUT PORT1
HLT

Write an assembly program to multiply a number by 8

Program

MVI A, 30H
RRC
RRC
RRC
OUT PORT1
HLT

Write an assembly program to find greatest between two numbers

Program

MVI B, 30H
MVI C, 40H
MOV A, B
CMP C
JZ EQU
JC GRT
OUT PORT1
HLT

EQU: MVI A, 01H
OUT PORT1
HLT

GRT: MOV A, C
OUT PORT1
HLT

Architecture OF 8085

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