8085 Architecture &
8085 Architecture &
Its Assembly language programming
Dr A Sahu
Dept of Computer Science &
Engineering
Engineering
IIT Guwahati

Outline
• 8085 Era and Features
• 8085
– Block diagram (Data Path)
– Bus Structure
– Register Structure g • Instruction Set of 8085
• Sample program of 8085
Sample program of 8085
• Simulator & Kit for 8085

8085 Microprocessor
8085 Microprocessor
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8 Bit CPU
3‐6Mhz
Simpler design: Single Cycle CPU p g g y
ISA = Pre x86 design (Semi CISC)
40 Pin Dual line Package
40 Pin Dual line Package
16 bit address
6 registers: B, C, D, E, H,L
Accumulator 8 bit

8085 Microprocessor Architecture
ReSeT6.5
INTR INTA RST5.5 RST7.5 TRAP

SID

Interrupt Control

SOD

Serial I/O Control

Bus 8 Bit
ACC

MUX

tmp R

IR

ALU

Timing and Control

I Decode
I Decode
&
M/C
Encodin
g

Z

B

Flag

W

C

D

E

H

L
SP
PC

Inc/Dec. ter
/
Add latch

Add Buff

Data/Add Buff

The 8085 Bus Structure
A15

Address Bus (16bit)

A0

Memory

I/P

8085
MPU

D7
D0

O/P

Data Bus (8bit)
Control Bus (8bit)

8085 Bus Structure
• Address Bus : Consists of 16 address lines: A0 – A15
Address Bus : Consists of 16 address lines: A
– Address locations: 0000 (hex) – FFFF (hex)
– Can access 64K ( = 216 ) bytes of memory, each byte has 8 bits
(
) y y, y

– Can access 64K × 8 bits of memory
– Use memory to map I/O, Same instructions to use for accessing I/O devices and memory
/

• Data Bus : Consists of 8 data lines: D0 – D7
–O
Operates in bidirectional mode t i bidi ti l d
– The data bits are sent from the MPU to I/O & vice versa
– Data range: 00 (hex) – FF (hex)
Data range: 00 (hex) – FF (hex)

• Control Bus:
– Consists of various lines carrying the control
Consists of various lines carrying the control signals such as read / write enable, flag bits

8085 Registers
• Registers:
– Six general purpose 8‐bit registers: B, C, D, E, H,L
– Combined as register pairs to perform 16‐bit operations: BC, DE, HL
– Registers are programmable (load, move, etc.)

• Stack Pointer (SP)
( )
• Accumulator & Flag Register
– (Zero Sign Carry Parity AuxCarry)
(Zero, Sign, Carry, Parity, AuxCarry)

B

C

D

E

H

L
SP
PC

• Program Counter (PC)
–C
Contains the memory address (16 bits) of the i h dd (16 bi ) f h instruction that will be executed in the next step.

How instruction executed
How instruction executed
• All instructions (of a program) are stored in memory. memory
• To run a program, the individual instructions must be read from the memory in sequence, must be read from the memory in sequence and executed.
– Program counter puts the 16‐bit memory address of the
Program counter puts the 16 bit memory address of the instruction on the address bus
– Control unit sends the Memory Read Enable signal to access the memory the memory
– The 8‐bit instruction stored in memory is placed on the data bus and transferred to the instruction decoder
– Instruction is decoded and executed

Instruction Set of 8085
Instruction Set of 8085
• Arithmetic Operations
– add, sub, inr/dcr

• Logical operation
– and, or, xor, rotate, compare, complement

• Branch operation ranch operation
– Jump, call, return

• Data transfer/Copy/Memory operation/IO
Data transfer/Copy/Memory operation/IO
– MOV, MVI, LD, ST, OUT

Copy/Mem/IO operation
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MVI R, 8 bit // load immediate data
MOV R1, R2 // Example MOV B, A
,
// p ,
MOV R M // Copy to R from 0(HL Reg) Mem
MOV M R // Copy from R to 0(HL Reg) Mem

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LDA 16 bit // load A from 0(16bit)
STA 16 bit // Store A to 0(16bit)
STA 16 bit // Store A to 0(16bit)
LDAX Rp
// load A from 0(Rp), Rp=RegPair
STAX Rp
// Store A to 0(Rp)
LXI Rp 16bit // load immediate to Rp

• IN 8bit
IN 8bit // A
// Accept data to A from port 0(8bit) td t t Af t 0(8bit)
• OUT 8 bit // Send data of A to port 0(8bit)

Arithmetic Operation
• ADD R
// Add A = A + B.reg
//
• ADI 8bit // Add A= A + 8bit
• ADD M
ADD M // Add A=A + 0(HL)
// Add A=A + 0(HL)
• SUB R
// Sub A = A ‐B.reg
• SUI 8bit // Sub A= A ‐ 8bit
• SUB M // Sub A=A ‐ 0(HL)
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INR R // R = R+1
INR M
INR M // 0(HL)=0(HL)+1
// 0(H ) 0(H )
DCR R // R = R‐1
DCR M // 0(HL)=0(HL)‐1
INX Rp
// Rp=Rp+1
DCX Rp
// Rp=Rp‐1

Other Operations
Other Operations
• Logic operations
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–
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–

ANA R ANI 8bit ANA M
ANA R
ANI 8bit ANA M
ORA, ORI, XRA, XRI
CMP R // compare with R with ACC
//
p
CPI 8bit // compare 8 bit with ACC

• Branch operations
– JMP 16bit, CALL 16 bit
– JZ 16bit, JNZ 16bit, JC 16bit, JNC 16 bit
– RET

• Machine Control operations
– HLT NOP POP PUSH
HLT, NOP, POP, PUSH

Assumption
• RAM Memory is interfaced
RAM Memory is interfaced
• Instructions are stored in memory
• O
One I/O display port is interfaced to display
/O di l i i f d di l data of ACC

Simple Assembly Program
Simple Assembly Program
MVI A, 24H // load Reg ACC with 24H
MVI B , 56H // load Reg B with 56H
ADD B // ACC= ACC+B
OUT 01H // Display ACC contents on port 01H
HALT // End the program
//
p g

Result: 7A (All are in Hex)
DAA operation for Decimal Adjust A+6=10H
DAA operation for Decimal Adjust A+6=10H

Flowchart to multiply two number
Flowchart to multiply two number
Start
LDA 2000 // Load multiplicant to accumulator
LDA 2000 // Load multiplicant to accumulator
MOV B,A // Move multiplicant from A(acc) to B register
LDA 2001 // Load multiplier to accumulator
MOV C,A // Move multiplier from A to C
MOV C,A // Move multiplier from A to C
MVI A,00 // Load immediate value 00 to ACC
MVI A 00 // Load immediate value 00 to ACC
ADD B // Add B(multiplier) with A
DCR C // Decrement C, it act as a counter

JNZ L // Jump to L if C!=0
STA 2010 // Store result in to memory
HLT // End

Code to multiply two number
Code to multiply two number
LDA 2000 // Load multiplicant to accumulator
MOV B,A // Move multiplicant f
MOV B A
// M l i li from A(acc) to B register
A( ) B i LDA 2001 // Load multiplier to accumulator
MOV C,A // Move multiplier from A to C
MOV C A // Move multiplier from A to C
MVI A,00 // Load immediate value 00 to a
L: ADD B // Add B(multiplier) with A
//
( p )
DCR C // Decrement C, it act as a counter
JNZ L // Jump to L if C reaches 0
STA 2010 // Store result in to memory
HLT // End

Factorial of a Program
Factorial of a Program

LAST:
END:

LXI SP, 27FFH ; Initialize stack pointer
LDA 2200H ; Get the number
CPI 02H ; Check if number is greater than 1
CPI 02H ; Check if number is greater than 1
JC LAST
MVI D, 00H ; Load number as a result
MOV E, A
MOV E A
DCR A
MOV C,A ; Load counter one less than number
CALL FACTO ; Call subroutine FACTO
;
XCHG ; Get the result in HL // HL with DE
SHLD 2201H ; Store result in the memory // store HL at 0(16bit)
JMP END
LXI H, 000lH ; Store result = 01
SHLD 2201H
HLT

Sub Routine for FACTORIAL
Sub Routine for FACTORIAL
FACTO:
BACK:

LXI H, 0000H
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MOV B, C ; Load counter
DAD D // double add ; HL=HL+DE
DCR B
JNZ BACK ; Multiply by successive addition
XCHG ; Store result in DE // HL with DE
DCR C ; Decrement counter
CNZ FACTO ; Call subroutine FACTO
CNZ FACTO C ll b i FACTO
RET ; Return to main program

8085 Simulator & Kit
8085 Simulator & Kit
• 8085 Simulator is available
– Course website

• 8085 Kit is available in HW Lab (CS422)
– First test the program on Simulator and then go for the HW
– Sometime Kit have Driver, IDE and Assembler