Opcode

An opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine language instruction that specifies the operation to be performed.

A machine instruction usually has:

• Opcode → the action (e.g., add, move, jump)
• Operands → the data or locations involved

Example

In a simplified assembly-like instruction:

ADD R1, R2

ADD → opcode (tells CPU to perform addition) R1, R2 → operands (the values/registers being used)

What opcodes do

Opcodes control all low-level operations, like:

• Arithmetic (ADD, SUB)
• Data movement (MOV, LOAD)
• Control flow (JMP, CALL)
• Logic (AND, OR, XOR)

Why they matter

• They’re the lowest-level instructions a CPU understands
• Compilers translate high-level code (like Python or C) into sequences of opcodes
• Different CPU architectures (x86, ARM, etc.) have different opcode sets

Example: x86 Machine Code

Here’s a simple assembly instruction:

MOV EAX, 1

Machine code (in hex):

B8 01 00 00 00

Breakdown:

B8 → opcode (means “move immediate value into EAX”) 01 00 00 00 → operand (the number 1, stored in little-endian format)

So the CPU reads:

“Take the value 1 and put it into register EAX”

Another Example

ADD EAX, EBX

Machine code:

01 D8

Breakdown:

01 → opcode for ADD D8 → specifies registers (EAX and EBX)

What It Looks Like in Memory

A program in memory is just a sequence of bytes:

B8 01 00 00 00 01 D8

The CPU reads byte by byte:

• Sees B8 → knows it’s a MOV instruction
• Reads next 4 bytes → operand
• Then sees 01 → ADD instruction
• Continues…

Key Insight

• Opcodes are just numbers (bytes)
• The CPU has a built-in “decoder” that interprets them
• Some opcodes are 1 byte, others are multiple bytes
• Extra bytes may define registers, memory addressing, etc.