What's the algorithm behind sleep()?

Question

Now there\'s something I always wondered: how is sleep() implemented ?

If it is all about using an API from the OS, then how is the API made ?

Does it all bo

Answer 1

Essentially, yes, there is a "special gizmo" - and it's important for a lot more than just sleep().

Classically, on x86 this was an Intel 8253 or 8254 "Programmable Interval Timer". In the early PCs, this was a seperate chip on the motherboard that could be programmed by the CPU to assert an interrupt (via the "Programmable Interrupt Controller", another discrete chip) after a preset time interval. The functionality still exists, although it is now a tiny part of a much larger chunk of motherboard circuitry.

The OS today still programs the PIT to wake it up regularly (in recent versions of Linux, once every millisecond by default), and this is how the Kernel is able to implement pre-emptive multitasking.