Stage 2: DECODE - "Understand the Instruction"

Stage 2: DECODE - "Understand the Instruction"

Purpose: Work out what the instruction means and what needs to be done

What Happens:

• The Control Unit examines the instruction in the CIR
• The instruction is split into two parts: opcode (what to do) and operand (what data to use)
• The CU works out what operation needs to be performed (e.g., ADD, LOAD, STORE, JUMP)
• The CU identifies what data or memory addresses are involved
• The CU prepares control signals to coordinate the necessary components

Example Instruction Breakdown:

• Instruction: "ADD 5" stored as binary in CIR
• Opcode: "ADD" - tells the CU this is an addition operation
• Operand: "5" - tells the CU to add the number 5
• CU Decision: "Prepare ALU for addition, get current ACC value, add 5 to it"

No data changes during DECODE: This stage is purely about understanding and preparing - no actual calculations happen yet, and registers like PC and ACC don't change.