Instruction Format

When the assembler processes an Instruction it converts the instruction from its mnemonics form to standard machine language format called the "Instruction format". In the process of conversion the assembler must determine the type of instruction, convert symbolic labels and explicit notation to a base/displacement format, determine the lengths of certain operands and parse any literal and constants.

An instruction format defines layout of bits of an instruction, in terms of its constituent parts.

An instruction format must include an opcode and implicitly or explicitly, zero or more operands.

Each explicit operand is referenced using one of addressing modes.

Format must, implicitly or explicitly, indicate addressing mode for each operand.


For most instruction sets, more than on instruction format is used.

Concept of operation code and operand code:
The CPU understands only the binary language. The machine code is the binary format of an instruction to be executed by the CPU. It has two parts: Operand and Opcode.

Operand Code
Operand Code is the part of instruction that specifies the data or the memory location of the data on which the operation will be performed.
Types:

• Immediate
• Register
• Memory address
• Implied

Opcode
Also known as the operation code, Opcode is part of instruction that specifies the operation to be performed by the instruction. n bit opcode is used to represent 2n instructions.