What makes a CPU fast

Two laptops on a shelf. Same price, same size, same screen, and one of them has a bigger number printed on the box. Everybody buys that one. Three numbers decide how quickly the thing in front of you gets its work done, and the one on the box is only ever one of them. We take them one at a time, and the first is the easiest of the three. It is the clock, the part that sends a steady beat out to everything else in the processor, simply beating faster.

Fifth of six in Inside the computer: the machine is built, and now we ask how fast it goes. If the loop itself is hazy, The fetch-execute cycle sets it up.

Two of the three are probably already yours. Clock speed, how fast the beat is. Number of cores, how many processors are on the chip. And the one that gets left out, cache size - the cache being the scrap of very fast memory sitting right beside the CPU. And the reports back that up. An examiner's report says candidates were often able to identify at least one characteristic of a CPU, most commonly the clock speed and number of cores. And one word in each of those is doing real work. The speed of the clock. The number of cores. The size of the cache. A marker reads the single word clock. Why can they not give it the mark? Because the word clock on its own does not say whether you mean the part or the measurement. Only the measurement answers a question about performance. An examiner's report is direct about it: some responses were not precise enough as to the characteristics, for example stating clock or core without reference to the speed of the clock, or the number of cores, which were too ambiguous. One note before we go on. Not every specification prints a statement about processor performance; the rows on screen show which ones do, and if yours does not, take this as useful background.

Clock speed first. The clock sends out its signal at regular intervals, and the speed is how many of those signals it sends each second. One beat is one cycle, and one cycle is roughly one step of the work. Hold that, because the rest follows from it. Look at the shape of that sentence. Something changes, and that means something else changes. A direction with no because in it is half an answer. A faster clock makes the computer faster is the version with no because. It hands the question back with different words. Double the clock speed and change nothing else. What happens, and why? More cycles every second, so more instructions carried out every second. The direction, with the reason attached to it.

Now the cores. A core is a processor in its own right, running its own fetch-execute loop, and one chip can hold several of them. Four cores means four instructions being worked on at the same time, as long as there are four separate jobs to hand out. So take a machine going from two cores to four. The program it is running was written to use one core, and one only. The count goes from two to four. What happens to performance, and why? Almost nothing. Three of the four sit idle, because an extra worker only helps when there is a separate job to hand them. So more cores is not automatically faster. A question that adds cores is usually asking whether the work can be split at all. It runs the other way too: raise the clock speed on a machine that is sitting waiting for data, and the extra beats are spent waiting faster.

Cache size next. The cache is a small amount of very fast memory close to the processor, holding data and instructions it is likely to want again. Every trip out to main memory is slow next to that. A bigger cache means more of what the processor wants is already close by. It does not mean the machine holds more of your files. Think of a shortcut rather than a cupboard. A bigger cache does not store more of your work. What does it buy? Fewer slow trips out to main memory, because more of what is wanted is already sitting close to the processor.

Now two of them at once, because a real machine never changes one thing on its own. Back to those two laptops on the shelf, same price. One has a fast clock and a very small cache. The other has a slower clock and a much larger cache. Which one would you pick, and does the clock or the cache decide it? It depends what it is being asked to do. A quick clock is wasted if the data is not there when the beat arrives, and a tiny cache is what leaves it missing. So the honest answer to most of these is it depends, followed by the reason. Name what the machine is for, then name the number that matters for it, because the number printed on the box is never the whole story. The answer has a shape. Name the measurement, then say and that means. Clock speed, and that means more instructions a second. Number of cores, and that means more jobs at once.

Three factors. I will ask, you answer, and then I will. What are the three characteristics, as phrases rather than single words? The three are clock speed, number of cores and cache size. The measuring word is part of every one of them. Now a reason. Why does a higher clock speed get more work done? More cycles in every second, so more instructions are carried out in every second. Now a new one. A game uses one core only. Does an eight-core chip help it? Barely. Seven of them have nothing to do, and the job still runs at the speed of the one doing it. Try the last one. A bigger cache saves the processor from doing what? The processor stops going out to main memory so often. The short trip gets taken more of the time.

Every one of these you are solid on is worth a thumbs up, and the ones without it are your own list of what to look at again. If that felt fast, that is normal, and a second pass fixes it.

Next in this chapter: Embedded systems, which is about the computers you own without ever calling them computers.

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