How is CPU usage calculated?

Question

On my desktop, I have a little widget that tells me my current CPU usage. It also shows the usage for each of my two cores.

I always wondered, how does the CPU calc

Answer 1

This is my basic understanding from having a little exposure to similar code. Programs like Task Manager or your widget access system calls like NtQuerySystemInformation() and use the information gathered from the OS to make the simple calculation of the percent of time a CPU is idle or being used (in a standard amount of time). A CPU knows when it is idle so it can therefore determine when it's not idle. These programs can indeed get clogged up...my crummy laptop's Task Manager freezes all the time when calculating CPU usage when it's topping out at 100%.

A cool bit of example code can be found on MSDNs website that shows function calls for calculating CPU usage for a set of instructions: http://msdn.microsoft.com/en-us/library/aa364157(VS.85).aspx

What these system calls do is access kernel code I believe... which is beyond the scope of my understanding.