Interval timers in both VIRTUAL and PROF modes take effect only when a process is executing. Information of such timers can be maintained in the PROC structure of each process. The (hardware) timer interrupt handler only needs to access the PROC structure of the current running process to decrement the timer count, reload the timer count when it expires and generate a signal to the process. The OS kernel does not have to use additional data structures to handle VIRTUAL and PROF timers of processes. However, REAL mode interval timers are different, because they must be updated by the timer interrupt handler whether the process is executing or not. Therefore, the OS kernel must use additional data structures for REAL mode timers of processes and take actions when their timers expire or are cancelled. In most OS kernels, the data structure used is a timer queue. We shall explain the timer queue in the programming project at the end of this chapter.
Source: Wang K.C. (2018), Systems Programming in Unix/Linux, Springer; 1st ed. 2018 edition.