8085 Architecture

8085 INTRODUCTION
The features of INTEL 8085 are :
• It is an 8 bit processor.
• It is a single chip N-MOS device with 40 pins.
• It has multiplexed address and data bus.(AD0-AD7).
• It works on 5 Volt dc power supply.
• The maximum clock frequency is 3 MHz while
minimum frequency is 500kHz.
• It provides 74 instructions with 5 different addressing
modes.

8085 INTRODUCTION
It provides 16 address lines so it can access 2^16 =64K bytes of
memory.
It provides 5 hardware interrupts: TRAP, RST 5.5, RST 6.5, RST
7.5,INTR.
It provides Acc ,one flag register ,6 general purpose registers and
two special purpose registers(SP,PC).
It provides serial lines SID ,SOD. So serial peripherals can be
interfaced with 8085 directly.

8085 PIN DESCRIPTION
Some important pins are :
AD0-AD7: Multiplexed Address and data lines.
A8-A15: higher order address lines.
ALE: Address latch enable is an output signal. It goes high when
operation is started by processor .
S0,S1: These are the status signals used to indicate type of
operation.
RD¯: Read is active low input signal used to read data from I/O
device or memory.
WR¯:Write is an active low output signal used write data on
memory or an I/O device.

 READY: This an output signal used to check the status of
output device. If it is low, µP will WAIT until it is high.
 TRAP: It is an Edge triggered highest priority , non mask
able interrupt. After TRAP, restart occurs and execution
starts from address 0024H.
 RST5.5,6.5,7.5:These are maskable interrupts and have low
priority than TRAP.
 INTR¯&INTA:INTR is a interrupt request signal after which
µP generates INTA or interrupt acknowledge signal.
 IO/M¯:This is output pin or signal used to indicate whether
8085 is working in I/O mode(IO/M¯=1) or Memory
mode(IO/M¯=0 ).

 HOLD&HLDA:HOLD is an input signal .When µP receives HOLD
signal it completes current machine cycle and stops executing
next instruction. In response to HOLD µP generates HLDA that
is HOLD Acknowledge signal.
 RESET IN¯:This is input signal. When RESET IN¯ is low µp
restarts and starts executing from location 0000H.
 SID: Serial input data is input pin used to accept serial 1 bit
data .
 X1X2 :These are clock input signals and are connected to
external LC,or RC circuit. These are divide by two so if 6 MHz is
connected to X1X2,the operating frequency becomes 3 MHz.
 VCC & VSS : Power supply VCC=+ -5Volt& VSS=-GND reference.

Arithmetic and Logical group
Accumulator: It is 8 bit general purpose register.
It is connected to ALU.
So most of the operations are done in Acc.
Temporary register: It is not available for user
All the arithmetic and logical operations are done in the
temporary register but user can’t access it.
Flag: It is a group of 5 flip flops used to know status of various
operations done.
The Flag Register along with Accumulator is called PSW
or Program Status Word.

Arithmetic and Logical group
Flag Register is given by:
S:Sign flag is set when result of an operation is negative.
Z:Zero flag is set when result of an operation is 0.
Ac:Auxiliary carry flag is set when there is a carry out of lower nibble or
lower four bits of the operation.
CY:Carry flag is set when there is carry generated by an operation.
P:Parity flag is set when result contains even number of 1’s.
Rest are don’t care flip flops.
S Z X AC X P X CY

Register Group
Temporary registers (W,Z):These are not available for user.
These are loaded only when there is an operation being
performed.
General purpose: There are six general purpose registers in
8085 namely B,C,D,E,H & L. These are used for various data
manipulations.
Special purpose :There are two special purpose registers in
8085:
1. SP :Stack Pointer.
2. PC : Program Counter.

Register Group
Stack Pointer: This is a temporary storage memory 16 bit register.
Since there are only 6 general purpose registers, there is a need to
reuse them .
Whenever stack is to be used previous values are PUSHED on stack
and then after the program is over these values are POPED back.
Program Counter: It is 16 bit register used to point the location
from which the next instruction is to be fetched.
When a single byte instruction is executed PC is automatically
incremented by 1.
Upon reset PC contents are set to 0000H and next instruction is
fetched onwards.

INSTRUCTION REGISTER,DECODER &
CONTROL
Instruction register: When an instruction is fetched , it is
executed in instruction register. This register takes the Opcode
value only.
Instruction decoder: It decodes the instruction from
instruction register and then to control block.
Timing and control: This is the control section of µP.It accepts
clock input .

INTERRUPT CONTROL
It accepts different interrupts like TRAP INT5.5,6.5,7.5and INTR.
SERIAL IO CONTROL GROUP
• It is used to accept the serial 1 bit data by
using SID and SOD signals and it can be
performed by using SIM & RIM
instructions.

ADDRESSING MODES OF 8085
Immediate addressing:
Immediate data is transferred to address or register.
Example:
MVI A,20H. Transfer immediate data 20H to accumulator.
Number of bytes:
Either 2 or 3 bytes long.
1st byte is Opcode.
2nd byte 8 bit data .
3rd byte higher byte data of 16 bytes.

Register addressing:
Data is transferred from one register to other.
Example:
MOV A, C :Transfer data from C register to accumulator.
Number of bytes:
Only 1 byte long.
One byte is Opcode.

Direct addressing:
Data is transferred from direct address to other register or vice-versa.
Example:
LDA C200H .Transfer contents from C200H to Acc.
Number of bytes:
These are 3 bytes long.
1st byte is Opcode.
2nd byte lower address.
3rd byte higher address.

Indirect addressing:
 Data is transferred from address pointed by the data in a
register to other register or vice-versa.
Example:
MOV A, M: Move contents from address pointed by M to Acc.
Number of bytes:
These are 3 bytes long.
1st byte is Opcode.
2nd byte lower address.
3rd byte higher address.

Implied addressing:
These doesn’t require any operand. The data is specified in
Opcode itself.
Example: RAL: Rotate left with carry.
No. of Bytes:
These are single byte instruction or Opcode only.

INSTRUCTIONS SET OF 8085
DATA TRANSFER GROUP
MOV Rd, Rs.(Move data from Rs to Rd).
Example:
MOV C,B. Move the content of register B to C.
Initially After execution
B=10H. B=10H.
C=20H. C=10H.
Flags Affected :No flags affected.
Addressing mode: Register.

DATA TRANSFER GROUP
MOV Rd, M (Move data from Memory to Rd).
Example:
MOV C,M. Move the content of Memory i.e. “H or L” to C.
Suppose the Data at memory pointed By HL pair at C200H is 10H.
H=C2,L=00,C=30H H=C2,L=00,C=10H.
Addressing mode: Indirect.

DATA TRANSFER GROUP
MVI R, Data.(Move Immediate data to Register).
Example:
MVI B, 30H. (Move the data 30 H to Register B)
B=40H B=30H
Addressing mode: Immediate.

DATA TRANSFER GROUP
LXI Rp,16 bit .(Load 16 bit data to Register pair Immediate).
Example:
LXI SP, C200H. (Load Stack pointer with C200H).
SP=C800H SP=C200H.

DATA TRANSFER GROUP
STA address.(Store Acc data to address).
Example:
STA C200H. (Move the data from Acc to C200H).
Suppose in Acc the data is 10H.
A=10H, C200=20H C200=10H , A=10H
Addressing mode: Direct.

DATA TRANSFER GROUP
LHLD address.(Load HL pair with data from address).
Example:
LHLD C200H. (Move the data from C200 to HL pair).
Suppose at C200 the data is 20H,30H .
H=10H,L=20H H=20H,L=30H.
C2=20H,00=30H C2=20H,00=30H

DATA TRANSFER GROUP
XCHG (Exchange the data from HL pair to DE pair)
Example : XCHG
H=20H,L=30H, H=40H,L=70H.
D=40H,E=70H. D=20H,E=30H.

DATA TRANSFER GROUP
IN 8 bit address (Move the data from address to Acc)
Example: IN 80H
Move the data from 80H port address to Accumulator.
Suppose data at 80H is 39H.
A=20H. A=39H

DATA TRANSFER GROUP
OUT 8 bit address (Move the data from Acc to address)
Example: OUT 80H
Move the data from Acc to port address 80H.
Suppose data at Acc is 39H.
A=39H. 80=10H. A=39H,80=39H.

DATA TRANSFER GROUP
Example: Write a program to exchange contents of memory
location D000H to D001H
LDA D000H Load Acc with data from D000
MOV B,A Move the data to B
LDA D0001H Load Acc with data from D001
STA 2000H Store Acc data at D000
MOV A,B Move B’s data to A
STA 2001H Store data from D000 to D0001
RST1 Stop.

ARITHMETIC GROUP
ADD R (ADD register content with Acc and result in A ).
Example:
ADD C. (ADD the content of C with A).
Suppose the Data at C register is 10H.
. C= 10H ,A=10H A=20H,C=10H.
Flags Affected :All flags are modified.
Addressing mode: Register

ARITHMEIC GROUP
ADD M(ADD H or L Reg content with Acc and result in A ).
Example:
ADD M. (ADD the content of HL with A).
Suppose the Data at memory pointed by HL register 1020H is 10H.
. H= 10H ,L=20H . H=10H,L=20H.
A=20H,C=10H. A=30H.
Addressing mode: Register Indirect.

ARITHMETIC GROUP
ADI Data(ADD immediate data with Acc and result in A ).
Example:
ADI 30H. (ADD 30H with A).
A=20H, A=50H.

ARITHMETIC GROUP
ADC R (ADD register content with Acc and carry and result in A ).
Example:
ADC C. (ADD the content of C with A with carry).
Suppose the Data at C register is 10H and carry is 01H.
. C= 10H ,A=10H A=21H,C=10H.

ARITHMETIC GROUP
Example: Write a program to perform 16 bit addition of 1234H& 4321H.
Store answer at H & L registers.
MVI B,21H B=21H
MVI A,34H A=34H
MVI C,43H C=43H
MVI D,12H D=12H
ADD B A=34+21H
MOV L,A L=55H
MOV A,C A=43H
ADC D A=43+12H
MOV H,A H=55H
RST1 STOP.

ARITHMETIC GROUP
SUB R (Subtract register content from Acc and result in A ).
Example:
SUB B. (Subtract the content of B from A ).
Suppose the Data at B register is 10H .
. B= 10H ,A=20H A=10H,B=10H.

ARITHMETIC GROUP
SBB R (Subtract register content from Acc with borrow and result in A ).
Example:
SBB B. (Subtract the content of B from A with borrow).
Suppose the Data at B register is 10H and borrow is 01H .
. B= 0FH ,A=20H A=10H,B=0FH.

ARITHMETIC GROUP
SUI Data(Subtract immediate data from Acc and result in A ).
Example:
SUI 30H. (Subtract 30H from A).
A=80H, A=50H.
Addressing mode: Immediate

ARITHMETIC GROUP
Example: Subtract data of C800 H from C200H.Store the result at
2C00.
LDA C800H
MOV B,A
LDA C200H
SUB B
STA 2C00H
RST1

ARITHMETIC GROUP
DAD Rp (Add specified register pair with HL pair)
Example: DAD D.(Add the content of E with L and that of D with H
register and result in HL pair)
Suppose the content of HL pair is H=20H ,L=40H and DE pair is D=30H,
E=10H.
H=20H ,L=40H H=50H ,L=50H
D=30H, E=10H D=30H, E=10H
Flags Affected :Only carry flag is modified.

ARITHMETIC GROUP
DAA (Decimal adjust accumulator)
Example:
MVI A,12H
ADI 39H
DAA .
 This instruction is used to store result in BCD form. If lower nibble is
greater than 9 ,6 is added while if upper nibble is greater than 9,6 is added
to it to get BCD result.
12+39=4B 12+39=51 in BCD form.

ARITHMETIC GROUP
INR R (Increment register content by 1 ).
Example:
INR C. (Increment the content of C by 1).
C= 10H C=11H.
Flags Affected :All flags are modified except carry flag.

ARITHMETIC GROUP
DCR R (Decrement register content by 1 ).
Example:
DCR C. (Decrement the content of C by 1).
C= 10H C=0FH.
Flags Affected :All flags are modified except carry flag.

ARITHMETIC GROUP
INX Rp (Increment register pair content by 1 ).
Example:
INX SP (Increment the content of Stack pointer pair by 1).
INX B. (Increment the content of BC pair by 1).
Suppose the Data at BC register is 1010H and SP is C200H
BC= 1010H BC=1011H.
SP=C200H SP=C201H.
Flags Affected :No flags are modified.

LOGICAL GROUP
ANA R (Logically AND register content with Acc and result in A ).
Example:
ANA C (AND the content of C with A).
C= 10H ,A=10H A=10H,C=10H.
Flags Affected :S,Z,P are modified Cy= reset, AC=set.

LOGICAL GROUP
ANI Data (Logically AND immediate data with Acc and result in A ).
Example:
ANI 10H (AND 10H with A).
A=10H A=10H
Flags Affected :S,Z,P are modified Cy=reset,AC=set.

LOGICAL GROUP
ORA R (Logically OR register content with Acc and result in A5 ).
Example:
ORA C (OR the content of C with A).
C= 17H ,A=10H A=17H,C=17H.
Flags Affected :S,Z,P are modified Cy=reset,AC=reset.

LOGICAL GROUP
ORI Data (Logically OR immediate data with Acc and result in A ).
Example:
ORI 10H (OR 10H with A).
A=30H A=30H
Flags Affected :S,Z,P are modified Cy=reset,AC=set.

LOGICAL GROUP
XRA R (Logically XOR register content with Acc and result in A ).
Example:
XRA C (XOR the content of C with A).
C= 17H ,A=10H A=07H,C=17H.
Flags Affected :S,Z,P are modified Cy=reset,AC=reset.

LOGICAL GROUP
CMP R (Compare register content with Acc and result in A ).
Example:
CMP C (Compare the content of C with A).
C= 10H ,A=17H A=17H,C=17H.
Flags Affected :S=0,Z=0,P=0, Cy=reset,AC=reset.

LOGICAL GROUP
CPI Data (Compare immediate data with Acc ).
Example:
CPI 10H (Compare the content of C with A).
A=17H A=17H.
Flags Affected :S=0,Z=0,P=0, Cy=reset,AC=reset.

LOGICAL GROUP
RLC (Rotate accumulator left ).
Example:
MOV A,03H.
RLC (Rotate accumulator left).
A=03H A=06H.
Flags Affected :Only carry flag is affected.
Addressing mode: Implied.

LOGICAL GROUP
RAL (Rotate accumulator left with carry ).
Example:
MOV A,03H.
RAL (Rotate accumulator left with carry).
A=03H , carry =01H A=07H.

LOGICAL GROUP
RRC (Rotate accumulator right ).
Example:
MOV A,03H.
RRC (Rotate accumulator right).
A=03H , A=81H.

LOGICAL GROUP
Write a program to reset last 4 bits of the number 32H
Store result at C200H.
MVI A, 32H A=32H
ANI F0H 00110010 AND 11110000
=00110000=30H
STA C200H. C200=30H
RST1 Stop

BRANCH GROUP
JMP address(Unconditional jump to address)
Example:
JMP C200H.
After this instruction the Program Counter is loaded with this
location and starts executing and the contents of PC are loaded
on Stack.
Flags Affected :No Flags are affected.

CALL address(Unconditional CALL from
address)
Example:
CALL C200H.
After this instruction the Program Counter is loaded with this
location and starts executing and the contents of PC are loaded
on Stack.
Addressing mode: Immediate

BRANCH GROUP
Conditional Jump Instructions.
JC (Jump if Carry flag is set)
JNC (Jump if Carry flag is reset)
JZ (Jump if zero flag set)
JNZ (Jump if zero flag is reset)
JPE (Jump if parity flag is set)
JPO (Jump if parity odd or P flag is reset )
JP (Jump if sign flag reset )
JM (Jump if sign flag is set or minus)

BRANCH GROUP
Conditional Call Instructions.
CC (Call if Carry flag is set)
CNC (Call if Carry flag is reset)
CZ (Call if zero flag set)
CNZ (Call if zero flag is reset)
CPE (Call if parity flag is set)
CPO (Call if parity odd or P flag is reset )
CP (Call if sign flag reset )
CM (Call if sign flag is set or minus)

BRANCH GROUP
RET (Return from subroutine)
Example:
MOV A,C
RET
After this instruction the Program Counter POPS PUSHED
contents from stack and starts executing from that address .
Addressing mode: Register indirect .

BRANCH GROUP
RST (Restart instruction)
Example:
MOV A,C
RST 1.
After this instruction the Program Counter goes to address 0008H
and starts executing from that address .
Addressing mode: Register indirect.

BRANCH GROUP
The addresses of the respective RST commands are:
Instruction Address
RST 0 0000H
RST 1 0008H
RST 2 0010H
RST 3 0018H
RST 4 0020H
RST 5 0028H
RST 6 0030H
RST 7 0038H

STACK AND MACHINE CONTROL
PUSH Rp.(Push register pair contents on stack).
Example:LXI SP FFFFH.
PUSH H. (Move the content of HL pair on Stack).
Suppose at HL pair the data is H= 20H,L= 30H & SP is initialized at FFFFH
H=20H,L=30H H=20H,L=30H.
SP=FFFF H FFFD=30H,FFFE=20H

POP Rp.(Pop register pair contents from stack).
Example: POP D(POP the content of DE pair from Stack).
Suppose at DE pair the data is H= 20H,L= 30H SP was initialized at
FFFFH
D=20H,E=30H D=10H,E=80H.
FFFD=80H,FFFE=10H
Addressing mode: Register indirect

XTHL (Exchange HL register pair contents with top of stack).
Example:XTHL(Exchange top with HL pair).
Suppose at HL pair the data is H= 20H,L= 30H & SP =FFFFH
& at locations FFFF=10H and at FFFE= 80H.
H=20H,L=30H H=10H,L=80H.
SP=FFFF =10H,FFFE=80H FFFD=20H,FFFE=30H

PROGRAM
Write a program to transfer a block of data from C550H to C55FH. Store
the data from C570H to C57FH .
LXI H ,C550H
LXI B ,C570H
MVI D,0FH
UP MOV A,M
STAX B
INX H
INX B
DCR D
JNZ UP
RST1

PROGRAM
Find out errors in the following :
MVI B,D =Immediate addressing doesn’t have register as
operand .Therefore, MVI B,80H.
INX L=Increment operator always acts on the higher memory
address in register pair .Thus ,INX H.
JP 80H = Conditional jump instructions doesn’t have any
immediate operand .Thus, JP UP.
If Flag contents are AB H, what is flag status
If flag contains AB H then it’s values from D7 to D0 are 10101011.
By comparing it with flag register we get S=1,Z=0,AC=0,P=0,Cy=1.

PROGRAM
11. What are the instructions for the following actions?
Load the PC with second and third byte of instruction.
LXI H, C200H
PCHL Load PC with HL content
Thus PC= L,PC +1=H.
No change in normal execution except increment the PC.
NOP (No operation)
This instruction has no effect on code only used to cause delay .

TIMING AND STATE DIAGRAM
The µP operates with reference to clock signal. The rise and fall
of the pulse of the clock gives one clock cycle.
Each clock cycle is called a T state and a collection of several T
states gives a machine cycle.
Important machine cycles are :
1. Op-code fetch.
2. Memory read.
3. Memory write.
4. I/Op-read.
5. I/O write.

Op-code Fetch: It basically requires 4 T states from T1-T4
The ALE pin goes high at first T state always.
AD0-AD7 are used to fetch OP-code and store the lower byte
of Program Counter.
A8-A15 store the higher byte of the Program Counter while
IO/M¯ will be low since it is memory related operation.
RD¯ will only be low at the Op-code fetching time.
WR¯ will be at HIGH level since no write operation is done.
S0=1,S1=1 for Op-code fetch cycle.

Op-code fetch cycle :

Memory Read Cycle: It basically requires 3T states from T1-T3 .
AD0-AD7 are used to fetch data from memory and store the
lower byte of address.
A8-A15 store the higher byte of the address while IO/M¯ will be
low since it is memory related operation.
RD¯ will only be low at the data fetching time.
WR¯ will be at HIGH level since no write operation is done.
S0=0,S1=1 for Memory read cycle.

Memory write Cycle: It basically requires 3T states from T1-T3 .
AD0-AD7 are used to fetch data from CPU and store the lower
byte of address.
A8-A15 store the higher byte of the address while IO/M¯ will
be low since it is memory related operation.
RD¯ will be HIGH since no read operation is done.
WR¯ will be at LOW level only when data fetching is done.
S0=1,S1=0 for Memory write cycle.

SUBROUTINE
Calculation of Delay using 8 bit counter:
Consider following example:
MVI C, count(8 bit) H 7 T states
UP DCR C 4 T states
JNZ UP 10/7 T
RET 10T
Here loop UP is executed (N-1) times.
Thus delay is Td=M+[(count)x N) -3.
Where M= no. of T states outside loop.
N=no.of T states inside loop.

SUBROUTINE
Here value of M= 17, N= 14.
The maximum delay will occur if count is 255 or FF H.
Thus Td max =17+[255x14]-3= 3584 T states.
For 0.5 µsec delay for a T state, we get
Td max=0.5 µsec x 3584= 1792 µsec or 1.792 m sec.

8085 Memory Interfacing
• Generally µP 8085 can address 64 kB of memory .
• Generally EPROMS are used as program memory and RAM as
data memory.
• We can interface Multiple RAMs and EPROMS to single µP .
• Memory interfacing includes 3 steps :
1. Select the chip.
2. Identify register.
3. Enable appropriate buffer.

Example: Interface 2Kbytes of Memory to 8085 with starting
address 8000H.
Initially we realize that 2K memory requires 11 address lines
(2^11=2048). So we use A0-A10 .
Write down A15 –A0
A15
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1
1
0
0
0
0
0
0
0
0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
ADD
8000H
87FFH

Address lines A0-A10 are used to interface memory while
A11,A12,A13,A14,A15 are given to 3:8 Decoder to provide an output
signal used to select the memory chip 𝐶𝑆 or Chip select input.
𝑀𝐸𝑀𝑅 and 𝑀𝐸𝑀𝑊 are given to 𝑅𝐷 and 𝑊𝑅 pins of Memory
chip.
Data lines D0-D7 are given to D0-D7 pins of the memory chip.
In this way memory interfacing can be achieved.

The diagram of 2k interfacing is shown below:
A15-A8
Latch
AD7-AD0
D7- D0
A7- A0
8085
ALE
IO/MRDWR
2K Byte
Memory
Chip
WRRD
CS
A10- A0
A15- A11
3:8DECODER

•In this example we saw that some address lines are used for
interfacing while others are for decoding.
•It is called absolute decoding.
•We sometimes don’t requires that many address lines. So we
ignore them. But this may lead to shadowing or multiple address.
•This type of decoding is called linear decoding or partial
decoding.
•In partial decoding wastage of address takes place but it requires
less hardware and cost is also less as compared with absolute one.

8255 BLOCK DIAGRAM
• Data Bus Buffer: It is an 8 bit data buffer used to interface 8255
with 8085. It is connected to D0-D7 bits of 8255.
• Read/write control logic: It consists of inputs 𝑅𝐷, 𝑊𝑅, 𝐴0, 𝐴1, 𝐶𝑆
• RD¯,WR¯ are used for reading and writing on to 8255 and are
connected to MEMR¯,MEMW¯ of 8085 respectively.
• A0,A1 are Port select signals used to select the particular port .
• CS ¯ is used to select the 8255 device .
• It is controlled by the output of the 3:8 decoder used to decode
the address lines of 8085.

8255 BLOCK DIAGRAM
A1 A0 Selected port
0 0 Port A
0 1 Port B
1 0 Port C
1 1 Control Register
A0,A1 decide the port to be used in 8255.

8255 BLOCK DIAGRAM
 Group A and Group B Control:
 Group A control consists of Port A and Port C upper.
 Group B control consists of Port A and Port C lower.
 Each group is controlled through software.
 They receive commands from the RD¯, WR¯ pins to allow access
to bit pattern of 8085.
 The bit pattern consists of :
1. Information about which group is operated.
2. Information about mode of Operation.

8255 BLOCK DIAGRAM
PORT A,B: These are bi-directional 8 bit ports each and are used to
interface 8255 with CPU or peripherals.
Port A is controlled by Group A while Port B is controlled by Group
B Control.
PORT C: This is a bi-directional 8 bit port controlled partially by
Group A control and partially by Group B control .
It is divided into two parts Port C upper and Port C lower each of a
nibble.
It is used mainly for control signals and interfacing with
peripherals.

8255 MODES
Mode 0 : Simple I/O
• Any of A, B, CL and CH can be programmed as input or output
Mode 1: I/O with Handshake
• A and B can be used for I/O
• C provides the handshake signals
Mode 2: Bi-directional with handshake
• A is bi-directional with C providing handshake signals
• B is simple I/O (mode-0) or handshake I/O (mode-1)
BSR (Bit Set Reset) Mode
• Only C is available for bit mode access.
• Allows single bit manipulation for control applications

INTERFACING 8085 & 8255
Here 8255 is interfaced in Memory Mapped I/O mode.
Initially we write down the addresses and then interface it .
A15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Port
1 0 0 0 0 X X X X X X X X X 0 0 A
1 0 0 0 0 X X X X X X X X X 0 1 B
1 0 0 0 0 X X X X X X X X X 1 0 C
1 0 0 0 0 X X X X X X X X X 1 1 CW

Thus we get addresses ,considering don’t cares to be zero as
Port A =8000H
Port B =8001H
Port C =8002H
CWR =8003H
Then, we give A11,A12,A13 pins to A,B,C inputs of Decoder to enable
8255 or Chip Select.
A15 is logic 1 so it is given to active HIGH G1 pin& A14 ,IO/M ¯ are
given to active low G2B ¯,G2A ¯ pins.
Output from Latch is given as A0,A1 pins to 8255 while D0-D7 are
given as data inputs.

82558085 3:8 decoder
74373
(AD0-AD7)
D7-D0
A0-A7
/CS
A0
A1
O0
O1
O7
A13
A12
A11
ALE
RD ¯
WR ¯
RD¯
WR¯
G2A G2B G1
A15
A14
IO/M
A
B
C
PA
PB
PC

Example: Take data from 8255 port B. Add FF H .Output result to
port A.
MVI A,82H Initialize 8255.
OUT 83H
LDA 81H Take data from port B
ADI FFH Add FF H to data
OUT 80H. OUT Result to port A.
RST1. STOP.

INTERFACING STEPPER MOTOR
with 8255

SERIAL COMMUNICATION
Serial Communications systems are of three types:
Simplex: This is a one way communication.
Only one party can speak.
The other party only hears to the first one but cant communicate.
System A System B
unidirectional
Transmi
tter
Receiver

System A System B
OR
Transmi
tter/Rec
eiver
Receiver
/Transm
itter
Half Duplex: It is a two way communication between two ports
provided that only party can communicate at a time.
•When one party stops transmitting the other starts transmitting.
•The first party now acts as a receiver.

OR/AND.
Full Duplex: It is a two way communication between two ports and
both parties can communicate at same time.
Thus here efficient communication can be established.
Transmi
tter/Rec
eiver
Receiver
/Transm
itter.

TRANSMISSION FORMATS
Asynchronous Synchronous
1. It transfers one character at a
time.
1. It transfers group of
characters at a time.
2. Used for transfer data rates
<20KBPS
2. Used for transfer data rates
>20KBPS
3. Start and stop bit for each
character which forms a frame.
3. No start and stop bit for
each character.
4. Two Clocks are used for Tx
and Rx
4. Single clock is used for both
Tx and Rx.

INTERRUPTS IN 8085
Interrupt is a process where an external device can get the
attention of the microprocessor.
The process starts from the I/O device
The process is asynchronous.
Classification of Interrupts
Interrupts can be classified into two types:
◦ Maskable Interrupts (Can be delayed or Rejected)
◦ Non-Maskable Interrupts (Can not be delayed or Rejected)

INTERRUPTS IN 8085
Interrupts can also be classified into:
◦ Vectored (the address of the service routine is hard-wired)
◦ Non-vectored (the address of the service routine needs to be supplied externally
by the device)
An interrupt is considered to be an emergency signal that may be
serviced.
• The Microprocessor may respond to it as soon as possible.

INTERRUPTS IN 8085
The 8085 has 5 interrupt inputs.
The INTR input.
The INTR input is the only non-vectored interrupt.
INTR is mask-able using the EI/DI instruction pair.
RST 5.5, RST 6.5, RST 7.5 are all automatically vectored.
◦ RST 5.5, RST 6.5, and RST 7.5 are all mask-able.
TRAP is the only non-mask-able interrupt in the 8085
◦ TRAP is also automatically vectored.

INTERRUPTS IN 8085
Non vectored interrupts:
The 8085 recognizes 8 RESTART instructions: RST0 - RST7 .
Each of these would send the execution to a predetermined hard-wired
memory location:
Restart
Instruction
Equivalent to
RST0 CALL 0000H
RST1 CALL 0008H
RST2 CALL 0010H
RST3 CALL 0018H
RST4 CALL 0020H
RST5 CALL 0028H
RST6 CALL 0030H
RST7 CALL 0038H

INTERRUPT PRIORITY
Interrupt name Mask-able Vectored
TRAP No Yes
RST 7.5 Yes Yes
RST 6.5 Yes Yes
RST 5.5 Yes Yes
INTR YES NO

SIM INSTRUCTION
100
SOD
SDE
XXX
R7.5
MSE
M7.5
M6.5
M5.5
01234567
RST5.5 Mask
RST6.5 Mask
RST7.5 Mask
}0 - Available
1 - Masked
Mask Set Enable
0 - Ignore bits 0-2
1 - Set the masks according
to bits 0-2
Force RST7.5 Flip Flop to resetNot Used
Enable Serial Data
0 - Ignore bit 7
1 - Send bit 7 to SOD pin
Serial Out Data
•SIM Instruction helps activate a particular interrupt.
•It can also mask a maskable interrupt.

SIM INSTRUCTION
Example: Set the interrupt masks so that RST5.5 is enabled, RST6.5 is masked,
and RST7.5 is enabled.
First, determine the contents of the accumulator.
- Enable 5.5 bit 0 = 0
- Disable 6.5 bit 1 = 1
- Enable 7.5 bit 2 = 0
- Allow setting the masks bit 3 = 1
- Don’t reset the flip flop bit 4 = 0
- Bit 5 is not used bit 5 = 0
- Don’t use serial data bit 6 = 0
- Serial data is ignored bit 7 = 0
SDO
SDE
XXX
R7.5
MSE
M7.5
M6.5
M5.5
0 1 00000 1
EI ; Enable interrupts including INTR
MVI A, 0A ; Prepare the mask to enable RST 7.5, and 5.5, disable 6.5
SIM ; Apply the settings RST masks

RIM INSTRUCTION
Serial Data In
RST5.5 Interrupt Pending
0 - Available
1 - Masked
Interrupt Enable
Value of the Interrupt Enable
Flip Flop
SDI
P7.5
P6.5
P5.5
IE
M7.5
M6.5
M5.5
01234567
RST5.5 Mask
RST6.5 Mask
RST7.5 Mask
}
•Since the 8085 has five interrupt lines, interrupts may occur during an
ISR and remain pending.
•Using the RIM instruction, it is possible to can read the status of the
interrupt lines and find if there are any pending interrupts.

8253 Features
Three independent 16 bit counters.
24 pin Dual in line Package.
Counting facility in Both BCD and Binary modes.
Dc to 2 MHz operating Frequency.
Can be used as a clock generator.

CONTROL WORD
SC1 SC0 RL1 RL0 M2 M1 M0 BCD
D0D7
0 0 Counter0
0 1 Counter1
1 0 Counter2
1 1 ILLEGAL
SC1 SC0 Select counter
0 0 Counter latching
0 1 Read/load LSB
1 0 Read/load MSB
1 1 R/L MSB 1st
then LSB.
RL1 RL0 Read/Load

CONTROL WORD
0 0 0 Mode 0
0 0 1 Mode 1
X 1 0 Mode 2
X 1 1 Mode 3
1 0 0 Mode 4
1 0 1 Mode 5
M2 M1 M0
BCD =0 Binary counter
BCD =1 BCD counter

8253 SQUARE WAVE
Example: Use 8253 as a square wave generator with 1ms period if
the input frequency is 1MHz.
We use counter 0 as a square wave generator and address of
counter 0 =10H and control register =13H.
I/P frequency is 1MHz.So time is 1µsec.
Count value = Required period /Input period = 1ms/1 µsec
=1000(Decimal).
Thus we use 8253 as a decimal counter.

8253 SQUARE WAVE
Program:
MVI A,37H Initialize counter 0 mode 3
OUT 13H 16 bit count BCD
MVI A,00H Load LSB count to counter 0
OUT 10H
MVI A,10H Load MSB count to counter 0
OUT 10H.
Thus, the output will be a square wave.

8257 DMA
It is a 4 Channel DMA containing 4 individual I/P ,O/P Channels.
CH0,CH1,CH2,CH3
It is compatible with Intel processors.
The maximum frequency is 3 MHz.
It executes 3 cycles:
1.DMA read
2.DMA write.
3.DMA verify.
The external device can terminate DMA Operation

OPERATING MODES OF 8257
1. Rotating priority mode: Each channel has equal priority.
Priority is shifted from one channel to other.
2. Fixed priority mode: Each channel has a fixed priority & if
higher priority channels are busy then smaller priority will get
to serve.
3. Extended write mode: This mode is used to interface slower
devices to the system.
4. TC stop mode: If this bit is set the channel whose terminal count
is reached is disabled.
5. Auto reload mode: If this bit is set data is transferred by channel
2 only. All other channels are not used.

8085 Architecture

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8085 Architecture