Stage 2: DECODE - "Understand the Instruction"
Part of Fetch-Decode-Execute Cycle — GCSE Computer Science
This key facts covers Stage 2: DECODE - "Understand the Instruction" within Fetch-Decode-Execute Cycle for GCSE Computer Science. Revise Fetch-Decode-Execute Cycle in Systems Architecture for GCSE Computer Science with 15 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 5 of 9 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 5 of 9
Practice
15 questions
Recall
12 flashcards
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.