Knowledge Organiser: Program Counter and Accumulator

Program Counter (PC): Register that holds the address of the NEXT instruction to be fetched
Accumulator (ACC): Register that stores the result of the most recent ALU operation
Increment: Increase by 1 — the PC increments after each fetch to point to the next instruction
Branch instruction: Changes the PC to a non-sequential address, creating a jump in program flow

The PC holds the ADDRESS of the next instruction — NOT the instruction itself
The PC increments during the FETCH stage (not execute)
The ACC is updated during the EXECUTE stage when the ALU completes a calculation
Branch/jump instructions overwrite the PC with a new address (not sequential +1)
The ACC is controlled by the ALU; the PC is controlled by the Control Unit
Each new ALU result overwrites the previous ACC value

PC = bookmark (tracks your place in the program); ACC = whiteboard (stores current working result)
PC changes during FETCH; ACC changes during EXECUTE — never mix these up
Without the PC, the CPU would not know which instruction to fetch next
The PC enables sequential execution — programs run instruction by instruction in order

Saying the PC holds the current instruction: The PC holds the address of the NEXT instruction — the instruction itself is fetched from memory into the CPU
Confusing when PC increments: The PC increments during the FETCH stage, not after execution — it points ahead immediately
Thinking the ACC permanently stores results: The ACC is overwritten by every new ALU operation — it only holds the most recent result
Saying jumps "break" the PC: Branch/jump instructions simply load a new address into the PC — it still works normally, just from a different address

Program Counter & Accumulator, Knowledge Organiser