This key facts covers Stage 3: EXECUTE - "Do the Instruction" within Fetch-Decode-Execute Cycle for GCSE Computer Science. Revise Fetch-Decode-Execute Cycle in 3.4 Computer Systems for GCSE Computer Science with 16 exam-style questions and 12 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 6 of 10 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Stage 3: EXECUTE - "Do the Instruction"
Purpose: Carry out the actual operation specified by the instruction
What Happens Depends on Instruction Type:
Arithmetic/Logic Operation (e.g., ADD, SUBTRACT, AND):
- The ALU performs the calculation or logic operation
- The result is stored in the Accumulator (ACC)
- Example: ADD 5 → ALU adds 5 to current ACC value → new total goes to ACC
Data Transfer (e.g., LOAD, STORE):
- LOAD: Data is copied from memory to a register (usually ACC)
- STORE: Data is copied from a register to memory
- Example: LOAD 200 → fetch data from memory address 200 → copy to ACC
Control Flow (e.g., JUMP, BRANCH, CALL):
- The PC is changed to a different address (not just +1)
- This allows loops, IF statements, and function calls
- Example: JUMP 100 → PC is set to 100 → next FETCH will get instruction from address 100
Different instructions take different amounts of time: A simple ADD might take 1 clock cycle, but a MULTIPLY might take several cycles.
Practice questions for Fetch-Decode-Execute Cycle
What does FDE stand for in computer science?
Describe the three stages of the FDE cycle.