Systems ArchitectureTopic Summary
Knowledge Organiser: Program Counter and Accumulator
Part of Program Counter & Accumulator · GCSE GCSE Computer Science revision
This topic summary covers Knowledge Organiser: Program Counter and Accumulator within Program Counter & Accumulator for GCSE Computer Science. Revise Program Counter & Accumulator in Systems Architecture for GCSE Computer Science with 15 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 8 of 8 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 8 of 8
Practice
15 questions
Recall
12 flashcards
Knowledge Organiser: Program Counter and Accumulator
Key Terms
- 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
- CIR: Current Instruction Register — holds the instruction currently being decoded/executed
- 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
Must-Know Facts
- 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
Key Concepts
- 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
Common Mistakes
- Saying the PC holds the current instruction: The PC holds the address of the NEXT instruction — the current instruction is in the CIR (Current Instruction Register)
- 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