What is the difference between a CPU Clock Rate and the Clock Cycle?

Question

I was told that the CPU Clock is like a doorbell that informs the CPU about an upcoming program instruction. Each time an instruction is finished by the CPU, the clock can be “rang” again.

I understand that the CPU needs to take some steps to process an instruction. When all of these steps are completed, is this known as one “CPU cycle” and the amount of cycles per second is the clock rate measured in hertz? So would a clock rate of 3 GHz actually mean that the CPU can perform 3 billion instructions per second?

I find the CPU cycles and the clock rate a little difficult to understand. Because I’ve assumed that one cycle equals one processed instruction so 1Hz = 1 instruction but I’ve also read online that one instruction doesn’t always take one cycle.

So what exactly is one CPU cycle then and how does this correspond to the clock rate?

In: Technology

Answer 1

> Because I’ve assumed that one cycle equals one processed instruction so 1Hz = 1 instruction but I’ve also read online that one instruction doesn’t always take one cycle.

It helps to know what a computer is doing. With a small number of the right electronic components (usually NAND gates in this case), you can add two binary digits simply by sending a pulse of electricity through the circuit. The answer just shows at the end of the circuit. If you want to add, say, a full 8-bit number (0-255), you can pack a bunch of these adders together in the right way. But again you send a pulse of electricity, you get the answer at the other end of the circuit.

This is one clock cycle, one instruction processed. You can do this with a breadboard and some transistors and such yourself, just solder everything together. A CPU is the same thing, it just has billions of those electronic components.

This one cycle equals one instruction thing is the goal of RISC chips. That means reduced instruction set. The processor is designed so it only has a small set of very simple instructions, and the goal is for each to take one clock cycle to complete, just like above.

Most processors are CISC, complex instruction set. They have a lot more instructions, some quite complex. A complex instruction will tell the processor to do something, and the processor may take many cycles to complete it.

So in general, consider clock cycle and rate to be the same thing.