“The computer includes a timer which functions like a stopwatch on a smartphone. A timer is a variable that is set to a period of time, for example, 0.5 s, which is represented as an integer number milliseconds or microseconds (0.5 s is 500 ms). The hardware clock of the computer causes an interrupt at fixed intervals and the operating system decrements the value of the timer. When its value goes to zero, we say that the timer has expired; an interrupt occurs.”