The four registers behind every instruction: PC, MAR, MDR, CIR

By the end of this lesson

By the end you can state what the Program Counter (PC), Memory Address Register (MAR), Memory Data Register (MDR) and Current Instruction Register (CIR) hold, and explain why the CPU needs all four.

The problem these four solve

To run one instruction the CPU must answer four questions:

Where is the next instruction? → the PC knows.
Which memory cell do I talk to? → the MAR holds that address.
What came back from memory? → the MDR holds the value.
What am I actually executing? → the CIR holds the instruction.

Each register answers exactly one question, so the CPU never has to pause and work out what is what.

One question per register — address, address, data, instruction.

The four, one at a time

Program Counter (PC). Holds the address of the next instruction to be fetched. After each fetch it is incremented (add 1) so it points at the following instruction. A branch or jump changes the PC instead.
Memory Address Register (MAR). Holds the address of the memory location about to be accessed — the cell being read from, or written to.
Memory Data Register (MDR). Holds the data moving between the CPU and memory: the instruction (or value) just read in, or the value about to be written out. It is also called the memory buffer register.
Current Instruction Register (CIR). Holds the instruction currently being decoded and executed, taken from the MDR.

Definition

Word

The natural chunk of data a processor handles at once — the width of its registers and data bus, typically 32 or 64 bits. Addresses and instructions fit in a word too.

Worked example: Where is 101 in all of this?

Using the program from the Von Neumann lesson (instructions at addresses 100, 101, 102), say what each of the four registers holds at the moment just before the CPU fetches the instruction at address 101.

Have a go first?

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Apply

One-line answerQuestion 3 · Recall

Answer the 3 questions above to continue.