A Level Computer Science OCR Practice Exam

What happens to the Program Counter after an instruction is executed?

1. It resets to zero

2. It increases by 1

3. It decreases by 1

4. It holds the last executed instruction address

The correct answer is: It increases by 1

After an instruction is executed, the Program Counter (PC) typically increases by 1 to point to the next instruction in the sequence. This incrementing mechanism allows the CPU to follow the flow of program execution efficiently. The Program Counter is crucial for ensuring that the processor executes instructions in the correct order as specified by the program. When an instruction is completed, the increment ensures that the next instruction in memory is fetched for execution. This behavior is fundamental to how most instruction sets are designed—sequential instruction execution is the norm unless explicitly modified by control flow instructions (such as jumps or branches). In contrast, a reset to zero or a decrease would disrupt the flow of execution, potentially causing the CPU to re-execute previous instructions or to point to invalid instructions, leading to errors or unpredictable behavior. Holding the last executed instruction address does not reflect the typical behavior of the Program Counter, which is designed to always point to the next instruction to be executed rather than the one just completed.