The fetch-execute cycle

The program you are running is sitting in main memory right now, and the processor is pulling it out one piece at a time. A few billion times a second it goes back for the next piece, and sometimes something travels the other way. What exactly is making those trips? It is only three moves, repeated for as long as the machine is on, and the first of them is simply going and getting something.

Halfway through Inside the computer: this is the fourth of the six. Anything unfamiliar here means The CPU and the von Neumann model is worth a look first.

Fetch, first. The next instruction is fetched to the processor from main memory. Decode. The instruction is decoded so the processor knows what it is being asked to do, and the control unit is the part that does the decoding. Third, execute: the instruction is carried out, and carrying it out may include reading from or writing to main memory. Fetch, decode or execute: at which stage does an addition actually happen? Execute. Fetch brought the instruction in, decode worked out what it was asking for, and the adding itself happens in execute. And then straight back to the top, for the next instruction. Say it in four parts, because the fourth is the one people drop: fetch, decode, execute, and straight back to the top. One thing about the name. Some specifications call this the fetch-execute cycle and one calls it fetch-decode-execute. They describe the same three stages and not one of them leaves decoding out. Use the name your own paper uses; the rows are on screen.

The loop itself is the easy half of this. What an exam question really tests is the exact words, and three sentences make that visible. Every one of them is a sentence a report quotes back. What separates a good one from a nil one is a word or two. Three sentences to correct. The memory is fetched into the processor. The information is decoded. The program is fetched from memory. All three are wrong. Which one needs more than a single word changed? The third one. The first two are single-word slips, and the last is wrong about how much arrives at once. So, take the first one. Memory is a place: things are fetched from it, and it is never the thing being fetched. And the machine moves data and instructions. Information is what data means to a person, and no mark scheme accepts it here. Both of those slips are on the record. An examiner's report says: two other common mistakes approaching this question were saying that the memory is fetched and using information in place of data. The program is fetched from memory: how would you rewrite it to say how much arrives? One instruction is fetched from memory, then the one after it. A program is read the way a book is read, a line at a time, and the loop is what gets through the rest of it. An examiner's report on another paper records the same thing: some candidates inaccurately identified that information was processed, or that programs were fetched from memory.

Now a question that is purely about main memory, the trips from the start of this video. Describe how main memory is used during the cycle. Write two sentences about main memory, and keep both of them on memory alone. Two things, and only two. Instructions are held in main memory and fetched from it, and executing an instruction may go back to read or write data. Anything else in your answer, about decoding or about the control unit, is a sentence about the cycle. Cross it out, because it was not what was asked. And here is an examiner's report on that same question: a lot of students described the FDE cycle rather than explaining how the main memory is used. And it says what that cost them: this prevented students from gaining the top marks on this question. The knowledge was there; the answer was about something else. So the fix is a read-back, and it is worth a minute in any exam. Write the answer, then read it against the question and delete every sentence that answers something else. One board also asks what a register - one of those tiny one-item stores inside the processor - is for at the point the cycle uses it. The memory address register holds the address being fetched. The program counter holds the address of the next instruction. A purpose each, and nothing moving anywhere. An examiner's report puts the gap plainly: candidates were often able to identify one or two registers that are used in the F-E cycle. Fewer candidates were able to give a purpose in the F-E cycle.

Three stages and a handful of words. See how many of them you can land first. What happens at fetch, in one sentence? The next instruction is fetched to the processor from main memory. Now execute. What happens there, and what might it go back to memory for? Carried out. That may also mean reading data from memory, or writing data back to it. Try an unseen one. An instruction says store this value in memory. Which stage writes it? At the execute stage. Storing a value is part of carrying the instruction out, so the write belongs there. One more. Which word never belongs in an answer about this cycle? Information. The machine fetches and processes data and instructions, and memory is fetched from, never fetched. And those trips from the start of this video are two sentences. Instructions are held in main memory and fetched from it, and carrying one out may go back to read or write data.

The thumbs up is your own tally: tap it on each one you have fully understood, and leave the rest for another day. It comes with practice, so a video you leave un-tapped today is just one to come back to.

Next in this chapter: What makes a CPU fast, which takes this same loop and asks how many of them fit into a second.

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