1.4.3 Timer

 We must ensuire that the operating system maintains control over the CPU. We cannot allow a user program to get stuck in an  infinite loop or to fail to call system services and never return control to the operating system. To accomplish the goal, we can use a timer. A timer can be set to interrupt the computert after a specifed period. The period may be fixed (for example, 1/60 second) or variable (for example, from 1 millisecond to 1 second). A variable timer is generally implemented by a fixed-rate clock and a counter. The operating system sets the counter. Every time the clock ticks, the counter is decremented. When the counter reaches 0, an interrupts at intervals from 1 millisecond to 1,024 millisecond clock allows interrupts at intervals from 1 millisecond to 1,024 milliseconds, in steps of 1 millisecond.

Before turning over control to the user, the operating system ensures that the timer is set to interrupt. If the timer interrupts, control transfers automatically to the operating system, which may treat the interrupt as a fatal error or may give the program more time. Clearly, instructions that modify the content of the timer are privileged.