Re: Higher resolution timers

["Vesa Jääskeläinen" <chaac@nic.fi>]

Marco Gerards wrote:
> Hi Colin,
> 
> Colin D Bennett <colin@gibibit.com> writes:
>> What would be the best way to do this? Possibilities that I am aware of
>> are: 
>>  * RDTSC instruction (must calibrate with RTC at startup?)
>>  * HPET (complex to use?)
>>  * RTC (can we set the timer interrupt rate to >18 Hz?)
>>
>> Does anyone have any thoughts on these options?
>>
>> I think that using the TSC (w/ RDTSC instruction) and calibrating it
>> with a quick 2-3 RTC tick loop at startup might be the easiest option.
> 
> Only something that really returns a time seems like a good choise to
> me.  AFAIK RDTSC returns clock cycles that passed since the system was
> switched on.  This means that code will run faster on a processor with
> a higher clock speed.  Am I mistaken?

That's way you calibrate it :) Then you can get pretty good accuracy :)

> So I prefer something that can be used to wait a microsecond and not a
> certain amount of clock cycles.  Using the RTC seems a good choice,
> but we have to keep in mind the BIOS uses it as well.

Default rate for RTC is bad for any accuracy. At least in this case.

> 
> Isn't HPET only available on newer processors?

Yes.

But the idea is to use best possible resolution available. So if choice 
A is not there then fallback for choice B. Of course in some cases 
animation will not be so smooth. But at least it will work.