Exam-style question
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Explain how the program counter, memory address register, memory buffer register and current instruction register can be used when an instruction is fetched.
Model answer
What a good answer should say
- The program counter contains the address of the next instruction.
- This address is copied to the memory address register, which identifies the memory location to be accessed.
- The instruction retrieved from memory is transferred to the memory buffer register and then copied to the current instruction register.
- The program counter is updated so that it contains the address of the next instruction.
Explanation
Why this works
A complete answer should identify the distinct roles of all four registers and describe their relationship during fetching. The program counter identifies the next instruction, the memory address register holds the address being accessed, the memory buffer register temporarily holds the instruction during transfer, and the current instruction register holds the instruction for decoding and execution.
Updating the program counter allows the processor to identify the following instruction.
Common mistake
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