Instruction Set of 8085

 Instruction Set of 8085

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 memory location.	2. Load register B with the data byte 32H.
3. Between a memory location and a register.	3. From a memory location 2000H to register B.
4. Between an I/O device and the accumulator.	4. From an input keyboard to the accumulator.

Table: - 8085 Data transfer instruction set summary

Opcode	Operand	Functions	Clock cycle	Number of bytes
MOV	Rd, Rs	Move register to register	4	1
MOV	M, Rs	Move register to memory	7	1
MOV	Rd, M	Move memory to register	7	1
MVI	Rd, Data	Move immediate register	7	2
MVI	M, Data	Move immediate memory	10	2
LDA	16 bit address	Load A direct	13	3
LDAX	B	Load A indirect	7	1
LDAX	D	Load A direct	7	1
LXI	B, 16 bit Data	Load immediate register Pair B & C	10	3
LXI	D, 16 bit Data	Load immediate register Pair D & E	10	3
LXI	H, 16 bit Data	Load immediate register Pair H & L	10	3
LXI	SP, 16 bit Data	Load immediate stack pointer	10	3
LHLD	16 bit address	Load H & L direct	16	3
STA	16 bit address	Load A direct	13	3
STAX	B	Store A indirect	7	1
STAX	D	Store A indirect	7	1
SHLD		Store H & L direct	16	1
XCHG		Exchange D & E H & L registers	4	1

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.

Table: - 8085 Arithmetic instruction set summary

Opcode	Operand	Functions	Clock cycle	Number of bytes
ADD	R	Add register to A	4	1
ADD	M	Add memory to A	7	1
ADC	R	Add register to A with carry	4	1
ADC	M	Add memory to A with carry	7	1
ADI	8-bit data	Add immediate to A	7	2
ACI	8-bit data	Add immediate to A with carry	7	2
DAD	B	Add B & C to H & L	10	1
DAD	D	Add D & E to H & L	10	1
DAD	H	Add H & L to H & L	10	1
DAD	SP	Add stack pointer to H & L	10	1
SUB	R	Subtract register from A	4	1
SUB	M	Subtract memory from A	7	1
SBB	R	Subtract register from A with borrow	4	1
SBB	M	Subtract memory from A with borrow	7	1
SUI	8-bit data	Subtract immediate from A	7	2
SBI	8-bit data	Subtract immediate from A with borrow	7	2
INR	R	Increment register	4	1
INR	M	Increment memory	10	1
INX	B	Increment B & C registers	6	1
INX	D	Increment D & E registers	6	1
INX	H	Increment H & L registers	6	1
INX	SP	Increment stack pointer	6	1
DCR	R	Decrement register	4	1
DCR	M	Decrement memory	10	1
DCX	B	Decrement B & C registers	6	1
DCX	D	Decrement D & E registers	6	1
DCX	H	Decrement H & L registers	6	1
DCX	SP	Decrement stack pointer	6	1
DAA		Decimal adjustment	4	1

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.

Table: - 8085 Logical instruction set summary

Opcode	Operand	Functions	Clock cycle	Number of bytes
CMP	R	Compare register with A	4	1
CMP	M	Compare memory with A	7	1
CPI	8 bit data	Compare immediate with A	7	2
ANA	R	AND register with A	4	1
ANA	M	AND memory with A	7	1
ANI	8 bit data	AND immediate with A	7	2
ORA	R	OR register with A	4	1
ORA	M	OR memory with A	7	1
ORI	8 bit data	OR immediate with A	7	2
XRA	R	Exclusive OR register with A	4	1
XRA	M	Exclusive OR memory with A	7	1
XRI	8 bit data	Exclusive OR immediate with A	7	2
RLC		Rotate A left	4	1
RRC		Rotate A right	4	1
RAL		Rotate A left with carry	4	1
RAR		Rotate A right with carry	4	1
CMA		Complement A	4	1
CMC		Complement carry	4	1
STC		Set carry	4	1

Table: - 8085 Stack/PUSH and POP instructions set summary

Opcode	Operand	Functions	Clock cycle	Number of bytes
PUSH	B	Push register pair B and C on stack	12	1
PUSH	D	Push register pair D and E on stack	12	1
PUSH	H	Push register pair H and L on stack	12	1
PUSH	PSW	Push accumulator A and Flags on stack	12	1
POP	B	Pop register pair B and C off stack	10	1
POP	D	Pop register pair D and E off stack	10	1
POP	H	Pop register pair H and L off stack	10	1
POP	PSW	Pop accumulator A and Flags off stack	10	1
XTHL		Exchange top of stack H and L	16	1
SPHL		H and L to stack pointer	6	1

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.

Table: - 8085 JUMP instruction set summary

Opcode	Operand	Functions	Clock cycle	Number of bytes
JMP	16-bit address	Jump unconditional	10	3
JC	16-bit address	Jump on Carry	7/10	3
JNC	16-bit address	Jump on no Carry	7/10	3
JP	16-bit address	Jump on positive	7/10	3
JM	16-bit address	Jump on minus	7/10	3
JZ	16-bit address	Jump on zero	7/10	3
JNZ	16-bit address	Jump on no zero	7/10	3
JPE	16-bit address	Jump on parity even	7/10	3
JPO	16-bit address	Jump on parity odd	7/10	3

Table: - 8085 CALL and Return instruction set summary

Opcode	Operand	Functions	Clock cycle	Number of bytes
CALL	16-bit address	Call unconditional	18	3
CC	16-bit address	Call on Carry	9/18	3
CNC	16-bit address	Call on no Carry	9/18	3
CP	16-bit address	Call on positive	9/18	3
CM	16-bit address	Call on minus	9/18	3
CZ	16-bit address	Call on zero	9/18	3
CNZ	16-bit address	Call on no zero	9/18	3
CPE	16-bit address	Call on parity even	9/18	3
CPO	16-bit address	Call on parity odd	9/18	3
RET		Return unconditional	10	1
RC		Return on Carry	6/12	1
RNC		Return on no Carry	6/12	1
RP		Return on positive	6/12	1
RM		Return on minus	6/12	1
RZ		Return on zero	6/12	2
RNZ		Return on no zero	6/12	2
RPE		Return on parity even	6/12	1
RPO		Return on parity odd	6/12	1

Machine Control Operations

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

Table: - 8085 I/O and machine control instructions set summary

Opcode	Operand	Functions	Clock cycle	Number of bytes
EI		Enable interrupts	4	1
DI		Disable interrupts	4	1
NOP		No-operation	4	1
HLT		Halt(Power Down)	5	1
RIM		Read interrupt mask	4	1
SIM		Set interrupt musk	4	1
IN		Input	10	1
OUT		output	10	1

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.

REFERENCES

1. R. S. Gaonkar, Microprocessor Architecture, Programming, and Applications with the 8085, Fifth Edition, Penram International Publishing (India) Private Limited.
2. S Ghoshal, Microprocessor Based System Design, Macmillan India Limited, 1996
3. M. Mano, Digital Logic and Computer Design, Prentice – Hall India
4. B. Ram - Fundamentals of Microprocessor and Microcontrollers
5. “Microprocessors: Principles and Applications” by A Pal
6. “Microprocessors and Microcontrollers : Architecture, Programming and Interfacing Using 8085, 8086 and 8051” by Soumitra Kumar Mandal
7. “Introduction to Microprocessors and Microcontrollers” by Crisp John Crisp
8. “Microprocessors And Microcontrollers” by A Nagoor Kani
9. “Microprocessors And Microcontrollers : Architecture, Programming and System Design 8085, 8086, 8051, 8096” by KRISHNA KANT
10. “8 - Bit Microprocessor” by Vibhute