Instruction Formats

Instruction Formats

The 8085A instruction set consists of one, two and three byte instructions. The first byte is always the opcode; in two-byte instructions the second byte is usually data; in three byte instructions the last two bytes present address or 16-bit data.

1. One byte instruction :

For Example : MOV A, B whose opcode is 78H which is one byte. This instruction copies the contents of B register in A register.

2. Two byte instruction :

For Example : MVI B, 02H. The opcode for this instruction is 06H and is always followed by a byte data (02H in this case). This instruction is a two byte instruction which copies immediate data into B register.

3. Three byte instruction :

For Example : JMP 6200H. The opcode for this instruction is C3H and is always followed by 16-bit address (6200H in this case). This instruction is a three byte instruction which loads 16-bit address into program counter.

Review Questions

1. Describe the instruction format of 8085 microprocessor.

2. List different instruction formats.