If a setup a timer to tick 1000 times a second, does it actually tick at this rate? Is it guaranteed to give 1000 ticks per second? Will the timer update more than one time per tick? Like in the code below, will tick_counter++ be 1000 before an update and 1020 afterward?

void updateTickCounter()  {
    tick_counter++;
}

Under Windows, on both really old and new systems, I've had Allegro timers going at rates of 10,000 BPS with no problems. Allegro uses the Windows high-performance timers when possible, (which is practically true for all systems), however, Allegro's timers update at their own rate, so if they fall behind they have to catch up, which means the accuracy of the timer will be at the mercy of how often Allegro updates.

IE: If you have a 10,000 BPS timer, but Allegro is only updating its timers once every hundredth second, your timer will update 100 times every hundredth second. Like this:

// TimerTick runs at 10,000 BPS
void TimerTick (void)
{
    timer_count++;
}

// Time 0.000, Timer Count = 0  , Should Be = 0
// Time 0.005, Timer Count = 0  , Should Be = 50
// Time 0.010, Timer Count = 100, Should Be = 100
// Time 0.015, Timer Count = 100, Should Be = 150
// Time 0.020, Timer Count = 200, Should Be = 200

If you want extreme precision, you'll need to access the Windows high-performance timers yourself. In the interim though, using an Allegro timer set to something high works well enough though and I've never encountered problems as such. You'll never be able to perceive this accuracy error unless you're trying to vsync to a 120 Hz CRT monitor signal or something.

--- Kris Asick (Gemini)
--- http://www.pixelships.com

I'm consistantly getting resolution of about 4 milliseconds ie a timer set to 1000 BPS ticks a minmum of 4 times so I guess that means it updates 250 beats a second.

Last time I checked, the allegro timer maxed at about 100 bps. I made a wrapper back then which uses QPC if available, and allegro timers otherwise. It's not hard.