Every computer motherboard has a clock pulse generator. This is simply some circuitry that generates clock pulses. Most fundamentally these pulses are used to drive the CPU - this is why we care about the clock speed of a CPU. Given two otherwise equal processors, the one that is capable of being driven at a faster clock speed will be the more powerful one.
In order to implement the functions listed in the introduction, we need to know about the passage of time. To accomplish this, the hardware is set up so that the clock generates an interrupt some number of times per second. The operating system must expect and handle these periodic interrupts. By counting the interrupts, the O/S detects the passage of time.
The rate of clock interrupts is determined by hardware. On the PDP/11, a clock interrupt is posted 60 times per second. Some hardware setups post interrupts at a much faster rate. Because of the frequency of clock interrupts, clock interrupts are most certain to be missed if we turn off interrupts for too long.
On the PC, the processor clock operated at 4.77 MHz. Current PC-type machines operate much, much faster, anywhere from 33 MHz on a slow 486, to over 500 MHz on a Pentium III. The clock interrupt occurs 100 times per second in PC-XINU, as shown in the clock interrupt handler.
