Instruction Format

 

What are Instruction Formats

Instruction includes a set of operation codes and operands that manage with the operation codes. Instruction format supports the design of bits in an instruction. It contains fields including opcode, operands, and addressing mode.

The instruction length is generally preserved in multiples of the character length, which is 8 bits. When the instruction length is permanent, several bits are assigned to opcode, operands, and addressing modes.

The function of allocating bits in the instruction can be interpreted by considering the following elements −

• Number of addressing modes
• Number of operands
• Number of CPU registers
• Number of register sets
• Number of address lines

The figure displayed the general IA-32 (Intel Architecture- 32 bits) instruction format. IA-32 is the instruction format that can Intel’s most outstanding microprocessors. This instruction format includes four fields, such as opcode field, addressing mode field, displacement field, and immediate field.

The opcode field has 1 or 2 bytes. The addressing mode field also includes 1 or 2 bytes. In the addressing mode field, an instruction needs only one byte if it uses only one register to generate the effective address of an operand.

The field that directly follows the addressing mode field is the displacement field. If an effective address for a memory operand is computed using the displacement value, then it uses either one or four bytes to encode. If an operand is an immediate value, then it is located in the immediate field and it appears either one or four bytes.