Very simple: CPUs do not execute a constant number of instructions on each cycle. Some instructions are really slow, and their slowness is measured in cycles. Y

Very simple: CPUs do not execute a constant number of instructions on each cycle. Some instructions are really slow, and their slowness is measured in cycles. You can usually do one XOR in one cycle, because it's a really simple piece of circuitry. Some instructions have complicated circuitry, which means that it takes multiple pulses of electricity to move the input signals through all the logic gates inside. XOR can do it in one pulse, but a division might take 20 pulses. Cycles are just pulses. One important component of a program's performance, especially in numeric code, is how many instructions it manages to execute per cycle. If you have written your program's instructions in a 'good' order, then the CPU will e.g. not queue up many divisions in a row that depend on each other. It can do two different things at the same time. You can't do anything in less than one cycle, but you can do two of the same thing in the same cycle if you have two available execution units for that task. The most direct answer to your question is that different instruction sets have different ideas about how much work a single instruction should do. The basic divide is between CISC (complex instructions, x86 qualifies) and RISC (basic instructions, ARM). It is a dramatic oversimplification but one instruction in a CISC architecture might involve 10 instructions in a RISC. So you can't compare instruction counts between architectures. But you can compare cycles and still make a bit of sense. If this is interesting to you, you should try this game, where you can build a CPU purely out of the most basic logic gates: https://nandgame.com/ https://nandgame.com/