DosStartTimer starts an asynchronous, repeated-interval timer, and posts an event semaphore each time the specified time interval expires.

Time intervals for DosStartTimer, DosAsyncTimer, and DosSleep are specified in milliseconds; however, it is important to recognize that the actual duration of the specified time interval will be affected by two factors:

• First, the system clock keeps track of time in less precise units known as clock ticks. The duration of a clock tick depends on the frequency of the system-clock interrupt that is used by your computer. (To determine the duration of the clock tick on your computer, issue DosQuerySysInfo and examine the timer-interval field.)

  Because clock ticks are less precise than millisecond values, any time interval that is specified in milliseconds will be rounded up to the next clock tick.

• Second, because the system is a priority-based, multitasking operating system, there is no guarantee that a thread will resume immediately after the timer interval expires. If a higher-priority process or thread is running, or if a hardware interrupt occurs, the timed thread blocks. (To minimize the inaccuracy caused by preemptive scheduling, an application can dedicate a thread to managing time-critical tasks and then raise that thread to a higher priority.)

These factors usually cause the timer interval to be longer than requested; however, it will generally be within a few clock ticks.