The CPU and the von Neumann model

Picture the inside of a processor as four empty boxes, with one wire still loose: the one coming out of the clock. Where does that wire go, and what one verb belongs in each of the four boxes? We get there one box at a time. The processor is the chip that carries out the instructions, and the first step is not a part at all - it is the idea behind the whole machine.

This is the second of six videos in Inside the computer, and it is the one the other five lean on. If any of this feels unfamiliar, start with Hardware and software.

Main memory holds what the processor is working on right now. Instructions in some of its cells, and the data those instructions work on in others. Nothing on the outside of a cell says which is which. Both kinds sit in the same memory, and the processor fetches both out of it. That is the stored program idea. Stored program means the program is held in memory rather than wired into the machine, and the design built around it carries the name of the man who wrote it down: the von Neumann model. Why does holding the program in memory let one machine do many different jobs? Because changing the job is only loading different numbers. No wires move, nothing is rebuilt, and the same parts do something else entirely. One word in the name misleads. Stored does not mean saved on a disk for later; it means the program is sitting in the memory the processor reads from.

The parts inside only make sense once you know the job they share. The processor, the central processing unit, the CPU on any diagram, fetches an instruction from memory, works out what it says, and carries it out. That loop is called the fetch-execute cycle, and it has a video to itself next. Today it is only a name, because every part below has a place in it. The control unit decodes each instruction and sends out the signals that make the other parts act. It directs. The arithmetic logic unit is where the arithmetic and the logical comparisons are carried out. It calculates. Two sentences come back instead of those two, and neither earns anything. The control unit controls the CPU, and the control unit executes the instructions. Take the sentence the control unit executes the instructions. Which part is it really describing? The arithmetic logic unit does it. Executing the working is its job, and the control unit only decided that working was needed. An examiner's report names that exact swap: a common error was stating that the Control Unit actually performs the FDE cycle, or that it executes the instructions. One verb each, never swapped: the control unit directs, the arithmetic logic unit calculates. And controls the CPU simply puts the word from the name back into the answer.

Two more parts to place, and each one arrives the same way: a box, and the verb that goes on it. A register is a very small, very fast store inside the processor. That is half an answer, and the missing half is what it is for. An examiner's report names the half that goes missing: commonly the register was just described as fast memory without saying what it is used for. A value has been fetched and the instruction is only half done. Which part is holding it, and for how long? A register, and that is its entire job. One item, held for exactly as long as the instruction takes. Some boards name four particular registers and ask what each one holds. One states in its own specification that knowledge of specific registers is not required, and your board's row is on screen now. If those four are yours, the video after this one is called The registers: MAR, MDR, PC and Accumulator. The clock now, and it has a job rather than a display. Whatever it puts out, every other part in there moves in step with it. Two answers come back that earn nothing. The clock measures how long each instruction takes, or the clock shows you the time. Both of those are wrong. What is the clock sending out, and how often? An electronic signal, at regular intervals. Picture a metronome rather than a wristwatch, with every part moving on the beat. An examiner's report records the clause students drop: some students, while correctly stating that the clock sent out an electronic signal, omitted to say that the signal was sent at regular intervals. So three words do the work there. At regular intervals, said every single time the clock comes up.

Next, the wires between all of this. A bus is a collection of wires through which data or signals are transmitted from one component to another. Some boards name three of them, and each name says what it carries. The address bus carries where, the data bus carries what, and the control bus carries when and how. A bare bus is never the whole answer. Write which one it is every time, because those are three different answers. Same trap with the control unit: it is CU on a diagram, and writing CPU there names the whole box instead of the part inside it. One line is left to draw, from the clock. Does it go to a bus, or to the control unit? To the control unit. A bus carries things between parts, and the clock is not a thing being carried; it is the beat the control unit works to. An examiner's report on a diagram question records it plainly: many candidates erroneously connected the clock to the buses. One board also counts the cache as part of the processor. It is a small amount of very fast memory, holding data and instructions the processor is likely to want again. An examiner's report is strict about the wording: the cache was often wrongly stated as holding information rather than correctly stating that it held data or instructions. Data or instructions, then, and never information. That word turns up again in the video on the cycle, and it is wrong there too.

That is every part named. One question before the picture goes up. A program checks whether one number is bigger than another. Which part does the comparing? The arithmetic logic unit. Logic is in its name, and a comparison is a logical operation rather than a sum. Now the picture from the start, finished. Main memory sits outside the processor, with the buses running between the two. Inside sit the control unit, the arithmetic logic unit, the registers and the clock. And the cache joins them for the one board that counts it, with the clock's wire running to the control unit. One box, one verb. The control unit directs, the arithmetic logic unit calculates, the register holds, the clock beats. A box with no verb beside it is half an answer.

Five parts of the processor, five jobs. See if you can beat me to each one. The control unit: what does it do, without using the word control? It decodes each instruction and sends out the signals that make the other parts act. Now the clock. What does it send out, and how often? It sends out an electronic signal at regular intervals, and that steady beat keeps everything in step. Next one, and it is unseen. A phone updates and can do something new: which idea explains that? The stored program idea. The instructions are numbers in memory, so new numbers means new behaviour, with nothing rewired. Now the last one. Half an answer says fast memory. What is the half that is missing? What it is for. Holding one single item while an instruction is carried out.

Thumbs up on this one if you have got it, and the ones above it too, so you can see at a glance what is left. Anything that has not settled is worth a second pass, and a saved video is easy to find again.

Next in this chapter: The registers: MAR, MDR, PC and Accumulator, which opens four of the boxes you have just met.

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