Registers and the fetch cycle
Distinguish an instruction's address, its contents and the CPU's working data.
What you will learn
- Explain the roles of PC, MAR, MDR and CIR.
- Outline a fetch sequence.
Registers hold different kinds of information
The program counter holds the address of the next instruction to fetch. The memory address register holds the address used for a memory access. The memory data register holds data being transferred to or from memory. The current instruction register holds the instruction being decoded or executed.
An accumulator is a working register used for intermediate arithmetic or logic results in some processor models. It does not replace the program counter.
Follow the information flow
In a simplified fetch cycle, the next instruction address is copied from the PC to the MAR. Memory returns the instruction through the MDR, and the instruction is placed in the CIR. The PC is updated so execution can continue; a branch may later change its value.
Exact micro-operation ordering depends on the processor model. The transferable skill is knowing which register stores an address and which stores the contents at that address.
Worked example
The next instruction is at address 240. Which register holds 240 before the fetch begins, and which register receives that address for the memory access?
Show the worked solution
- Before fetching, the PC identifies the next instruction address.
- That address is copied to the MAR for the memory operation.
- The instruction stored at address 240 is separate from the number 240.
Answer PC first, then MAR
Common mistakes
- An address is not the instruction stored there.
- The CIR stores the current instruction, not the next instruction's address.
What can you explain now?
Which register temporarily holds data arriving from memory?
Compare with the explanation
MDR
The memory data register carries the contents being transferred, whereas the MAR identifies a location.
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