The fetch–execute cycle in register transfer notation

By the end of this lesson

By the end you can describe the stages of the fetch–execute cycle and write them in register transfer notation — the notation the syllabus requires you to use.

How to read register transfer notation

Register transfer notation writes an arrow between two locations. It reads left to right: "the contents of the left-hand one are copied into the right-hand one."

Notation	Meaning
`PC → MAR`	Copy what is in the PC into the MAR
`MEM[MAR] → MDR`	Copy the contents of the memory cell whose address is in the MAR into the MDR
`PC + 1 → PC`	Add 1 to the PC and store the answer back in the PC
`MDR → CIR`	Copy what is in the MDR into the CIR

Two things to notice: nothing is moved — a copy is made, so the source keeps its value — and MEM[...] means "fetch from memory", not "a register called MEM".

The two stages

The CPU repeats the same loop forever (until you switch it off):

Fetch — get the next instruction out of memory.
Execute — carry it out, then go back to fetch again.

That is the whole machine. Everything else — branching, interrupts, waiting — is a detail inside these two stages.

Fetch, execute, repeat — four transfers to fetch, then decode and execute.

Worked example: Running `ADD 7` from address 101

The PC holds 101, and address 101 holds ADD 7. Write the full cycle for this one instruction.

Have a go first?

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Apply

Spot the errorQuestion 3 · Stretch

Answer the 3 questions above to continue.