Stage 1: FETCH - "Get the Instruction"
Part of Fetch-Decode-Execute Cycle — GCSE Computer Science
This key facts covers Stage 1: FETCH - "Get 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 4 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 4 of 9
Practice
15 questions
Recall
12 flashcards
Stage 1: FETCH - "Get the Instruction"
The Fetch-Decode-Execute (FDE) cycle is the three-stage sequence every CPU uses to process instructions. Fetch: the PC holds the address of the next instruction → copied to MAR → instruction retrieved from RAM into MDR → copied to CIR → PC increments. Decode: the Control Unit interprets the instruction in the CIR. Execute: the ALU carries out the operation and stores the result in the Accumulator. This cycle repeats continuously — every program instruction goes through exactly these three stages, billions of times per second.
Purpose: Retrieve the next instruction from main memory (RAM)
Detailed Step-by-Step Process:
- Step 1: The address currently in the PC (Program Counter) is copied to the MAR (Memory Address Register)
- Step 2: The address in MAR is sent along the Address Bus to main memory
- Step 3: Memory locates the instruction at that address
- Step 4: The instruction is sent back via the Data Bus to the MDR (Memory Data Register)
- Step 5: The instruction in MDR is copied to the CIR (Current Instruction Register)
- Step 6: The PC is incremented (+1) to point to the address of the next instruction
Key Registers Involved:
- PC: Holds address of next instruction, then increments
- MAR: Receives the address from PC, sends it to memory
- MDR: Receives the instruction from memory
- CIR: Holds the current instruction being processed
Memory trick: "Please Make Me Coffee In Pots" = PC → MAR → Memory → (back to) MDR → CIR → PC+1