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The Bookmark and the Whiteboard
The Bookmark and the Whiteboard
Imagine you're reading a recipe book while cooking. You use a bookmark to keep track of which step you're on (that's the Program Counter - always pointing to the next instruction). Meanwhile, you write calculation results on a whiteboard next to you (that's the Accumulator - storing intermediate results). Just as you move the bookmark down one step after reading each instruction, the PC increments after each fetch. And just as you erase and rewrite on the whiteboard with each new calculation, the ACC updates with each ALU operation.
What does PC stand for?: Program Counter
Computer Science glossary
- What does PC stand for?
- Program Counter
- What does ACC stand for?
- Accumulator
Deep Dive: Why These Registers Are Critical
Without the Program Counter, the CPU would have no idea what to do next. Every program is a sequence of instructions stored in memory. The PC ensures the CPU knows exactly which instruction to fetch next - it's the backbone of sequential execution. During normal operation, the PC increments by 1 each cycle, moving thro
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Exam Tips - PC and Accumulator
Common exam questions and how to answer them:
- "When does the PC change?" → During the FETCH stage, it increments by 1
- "When does the ACC change?" → During the EXECUTE stage, when ALU stores a result
- "What does the PC hold?" → The address (not the instruction itself!) of the NEXT instruction
- "What does the ACC hold?" → The result of the most recent ALU operation
Common mistakes to avoid:
- Saying PC holds "the current instruction" - it holds the ADDRESS of the NEXT instruction
- Saying PC increments during EXECUTE - it increments during FETCH
- Confusing ACC with other registers (MAR/MDR) - ACC is specifically for ALU results
- Forgetting that branch/jump instructions change the PC to non-sequential addresses
Now try it yourself
Quiz · Question 1 of 15
What does the Program Counter (PC) store?
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