What is the difference between clock cycle and clock period?

Question

I'm reading Computer Organization and Design Sixth edition by Patterson and they define clock speed as this:

Almost all computers are constructed using a clock that determines when events take place in the hardware. These discrete time intervals are called clock cycles (or ticks, clock ticks, clock periods, clocks, cycles). Designers refer to the length of a clock period both as the time for a complete clock cycle (e.g., 250 picoseconds, or 250 ps) and as the clock rate (e.g., 4 gigahertz, or 4 GHz), which is the inverse of the clock period.

What is the difference between clock cycle and clock period then?

My book says the clock period is the length of the clock cycle... but it also says the clock cycle to be the time for one clock period. Isn't this circular?

How I understood it was that a clock cycle was a pulse of the CPU, and the clock period was the length of time of that pulse. But this is a well known text so I think I'm the one who doesn't understand.

Answer 1

Both the terms can generally be used for the same meaning however there might be a slight difference in the context of where it is used.

Consider the clock of the cpu which provides timing signals to coordinate all hardware, then while referring to the time interval of the clock after which it takes a transition is called its clock period. This is in general for any clock which provides a timing signal and is not restricted to only clocks providing timing signals to a cpu.(Think of the wall clock, the period of its second hand is 60 seconds)

And when we talk in terms of cpu architecture (like pipelining concepts), we use the term clock cycles to denote the time taken to complete 1 instruction(or 1 microinstruction). It kind of provides a layer of abstraction from the working of the clock(If lets say you change the clock, its clock period might change, but still in cpu terms we would still refer to it as 1 clock cycle).

Answer 2

First of all there is a distinction between clock cycle of a computer and central processing unit frequency. In general frequency and Time (i.e period) is directly related i.e 2 seconds is equivalent to 1/2 Hz. This means it took 2 seconds between two clock ticks or signals and frequency is handy to describe what happen in the context of in one second. In this case, only half of that signal are transmitted. clock speed usually are much more slower than the CPU speed because they are used to coordinate the timing between microchips let say between CPU and RAM. This is because they have different frequency (i.e speed). For example if your CPU want to read data in the memory (for example RAM) it needs to have ideal timing to read in that RAM's address pin otherwise it may read a false data. computer clocks are a different type of entity than the processor itself. It is just like a normal clock to regulate things. If you make a deeper reading, there are many events that happens within one clock cycle, so this gives you as a benchmark tool like when to read data and write data to RAM for example from CPU.