Fetch-execute cycle registers

During the fetch-execute cycle, the processor uses the processor registers to hold data and addresses that it needs to access memory and execute instructions.

During the fetch stage:
- The program counter holds the address of the next instruction to be fetched from memory.
- We copy the program counter into the memory address register (MAR).
- The processor fetches the instruction from memory at the address in the MAR, and stores that instruction in the current instruction register (CIR).
- The program counter is incremented so it points to the next instruction for the next cycle.
During the execute stage:
- The processor might request to read some memory from the main memory, and it will use the MAR to hold the address of the memory it wants to read from, and the memory buffer register (MBR) to hold the data that it reads from memory.
- Or, it might want to write some data to memory, and it will use the MAR to hold the address of where it wants to write, and the MBR to hold the data that it wants to write to memory.

flashcards

Question	Answer
In the fetch-execute cycle, what do we do first with the program counter?	Copy its value into the memory address register.
What do we do to the program counter near the end of the fetch stage?	Increment its value
Why do we increment the value of the program counter after the fetch stage?	So that it points to the memory address of the next instruction to be executed.
What does the processor do after the value in the program counter is copied into the memory address register?	Fetches the data at that memory address, stores the instruction first in the memory buffer register, then copies that register to the current instruction register.
During the execute stage, what does the processor use the memory address register for?	To hold the address of memory it wants to read from or write to
During the execute stage, what does the processor use the memory buffer register for?	To hold the data that it reads from memory or wants to write to memory.