Instruction and Instruction Format in 8085

Definition of Instruction in 8085 Microprocessor

Instruction is a command or a binary pattern given to a computer to perform specific operation on given data/Operand. In 8085 Microprocessor, Instructions are of 8 bit. There are 74 types of 246 instructions.

The group of instructions including it's Size, Machine cycles, and T-States is called Instruction set. The entire group of instructions called the instruction set determines what functions the micro processor can perform. It has two parts Opcode and Operand.

• Opcode specified how to manipulate data/operand.
• Operand may be data or address.

Opcode	Operand

Learn Microprocessor[Beginner to Advanced]

1. Instruction Cycle: The processing time required to complete the execution of an instruction is known as Instruction Cycle. It consists of:
• Fetch Cycle
• Decode Cycle
• Execute Cycle
• Instruction Cycle are Basically the group of Machine cycles like Instruction Fetch cycle, Memory Read cycle, Memory write Cycle, I/O write etc.
2. Machine Cycle: The time taken by processor to access memory location, IO ports or to acknowledge an interrupt once. 
• It is basically the combination of T-States.
3. T-States: The sub division of Operation per one Clock Cycle of processor's clock. 
• These sub division are internal states synchronized with system clock and each T states precisely equal to one clock period. 

Instruction Format(Types of Instruction on the basis of Size):

Instruction Format is defined as the different types of Instruction on the basis of size. There are three types of instruction format in 8085 microprocessor.

1. One Byte Instruction:
• These Instruction use a total memory of one-byte(8 bit).
• These Instructions consists of the opcode only in the same byte.
• For example:
• MOV A,B
• ADD B
• RLC

• Opcode

2. Two Byte Instruction:
• These instructions use a total memory of two bytes. 
• The first byte specifies the opcode and second byte specifies the 1 byte operand(data).

• Opcode	Operand(data)

• For Example:
• ADI 40H
• IN 01H
• MVI A,32H
3. Three Byte Instruction:
• These instructions use a total memory of three bytes.
• The first byte specifies the opcode and the remaining two bytes specifies the 16-bit address i.e. second byte specifies the lower order address and third byte specifies the higher order address.

• Opcode	Operand(address)

• For Example:
• LDA 2050H
• LXI H, 2040H
• STA 2070H 

Related Posts: 

• 8085 Microprocessor Features, Pin Diagram, Architecture
• 8086 Microprocessor Features, Architecture
• All 8085 Instructions with Operand, Size, Addressing Mode, Machine Cycle, T-States
• Timing Diagram [8085 Microprocessor]