The total memory location required to feed the instruction in memory is called as instruction word size. The memory location of 8085 microprocessor can accommodate 8-bits of data. To store 16-bits data, they are stored in two consecutive memory locations (i.e. 2 Bytes).

According to the instruction word size in 8085 microprocessor, there are three types of instructions:

1. 1-Byte instruction
2. 2-Byte instruction
3. 3-Byte instruction

1 – Byte Instructions

• They include opcode and operands in the same byte.
• Operands are internal registers and coded into the instruction.
• Instructions require one memory location to store the single byte in the memory.

Note: Instructions having the only register or register pair as the operand is 1 – Byte Instructions. Instructions in the absence of operand are also 1 – Byte Instructions.

Examples:

MOV  B,C

LDAX  B

NOP

HLT

2 – Byte Instructions

• 1^st byte specifies opcode and 2^nd byte specifies operand.
• Instructions require two memory locations to store in the memory.

Note: Instructions having the 8-bit number either as an address or data as the operand is 2 – Byte Instructions.

Examples:

MVI  B, 26 H

IN  56 H

3 – Byte Instructions

• In a 3-byte instruction, the first byte specifies the opcode, and the following two bytes specify the 16-bit address.
• The 2^nd byte holds the low order address.
• The 3^rd-byte holds the high order address.
• Instructions require three memory locations to store the single byte in the memory.

Note: Instructions having the 16-bit number either as an address or data as the operand is 3 – Byte Instructions.

Examples:

LDA  2050 H

JMP  2085 H

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