DosAsyncTimer starts a single-interval timer. The timer runs asynchronously to the calling thread, and posts an event semaphore when the specified time interval expires.

Time intervals for DosAsyncTimer, DosStartTimer, 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, 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.