Stage 1: FETCH - "Get the Instruction"

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:

1. Step 1: The address currently in the PC (Program Counter) is copied to the MAR (Memory Address Register)
2. Step 2: The address in MAR is sent along the Address Bus to main memory
3. Step 3: Memory locates the instruction at that address
4. Step 4: The instruction is sent back via the Data Bus to the MDR (Memory Data Register)
5. Step 5: The instruction in MDR is copied to the CIR (Current Instruction Register)
6. 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