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Classification of Instruction Set of 8085 Microprocessor

  • Instruction format

    The format of 8085 microprocessor instructions is shown in table below - Instruction format of 8085 microprocessor.

    Opcode
    Operand
    Operand - 1
    Operand -2
    8 – bits                                                  8 – bits                                                   8 - bits

    The instruction has two parts, Opcode and Operand.

    Opcode : Represents the operation to be performed on the operand. It is also called mnemonic. Operand : Data or address is given in this part. If the operand is an 8-bit data, only Operand-1 is present in the instruction. If the operand is a 16-bit data or address, Operand-1 and Operand-2 are specified in the instruction. Both Operand-1 and Operand-2 are optional.



    Classification of instructions based on size

    There are three groups of instructions in 8085 microprocessor based on the length or size of the instruction. They are,

    1. Single byte (or 1 byte) instructions
    2. Two byte instructions
    3. Three byte instructions

    Single byte instructions

    This type of instruction has only Opcode and the operand is specified within the Opcode itself.

    Example :

    i) MOV B, C
    ii) ADD B

    Two byte instructions

    This type of instruction has Opcode and one operand. The first byte represents the Opcode and the second byte represents the 8-bit operand data or 8-bit port address.

    Example :

    i) MVI A, 50H
    ii) OUT 50H

    Three byte instructions


    This type of instruction has Opcode and two operands. The first byte represents the Opcode, the second byte presents the lower order 8-bits of data or address and the third byte represents the higher order 8-bits of data or address.

    Example :

    i) STA 5000H
    ii) LXI B, 5000H

    Classification of instructions based on function:

    There are 246 instructions (74 types) in the 8085 microprocessor. Based on the function of the instruction, the instructions are classified into the following five types.

    1. Data transfer instructions
    2. Arithmetic instructions
    3. Logic and bit manipulation instructions
    4. Branch instructions
    5. Machine control instructions

    1. Data transfer instructions

    These instructions move (or copy) data from source to destination. The source and destination are registers and memory. Memory to memory transfer is not possible. After the data transfer, the content of the source is not modified and the earlier content of the destination is altered. No flags are affected. 

    Examples : 1) MOV A, B 2) MOV A, M

    2. Arithmetic instructions

    Arithmetic operations like addition, subtraction, increment and decrement are performed by this category of instructions. One of the operand is taken from the Accumulator and the other operand may be from registers or memory. The result of the arithmetic operations is stored in the Accumulator. All the flags are affected.

    Examples : i) ADD B ii) INR A

    3. Logic and bit manipulation instructions

    Logical functions like AND, OR and EX-OR are performed by this instructions. All logic functions are performed in relation with the contents of the Accumulator. All the flags are affected.

    Examples : 1) ANA B ii) CMA

    4. Branch instructions

    Branch instructions change the sequence of the program execution unconditionally or conditionally. The condition of flags is used to take the decision for conditional branches. No flags are affected.

    Examples : i) JMP 5000H ii) JNZ 5000H

    5. Machine control instructions

    The instructions dealing with interrupt handling and system operations are classified into this category. No flags are affected.

    Examples : i) HLT ii) EI

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