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In article <4284@news.duke.edu> kds@phy.duke.edu (Stokes Kevin) writes:

>  I think the original posting was about the user of the VGA Card's hardware
>interrupt, not the timer chip.  The timer chip wouldn't stay in phase with the
>video retrace, and therefore wouldn't help.

There is a way to keep timer interrupt in phase with VGA hardware.
First you measure the time between VGA retraces. Then set the timer
to one-shot mode and set the pulse time a little more shorter than 
interrupt period. Then set the original IRQ0 interrupt to your own interrupt
routine. The you just send start signal to timer, and the system syncs
to VGA vertical retrace in few frames.

The interrupt routine does following:

1. Wait for vertical retrace to start
2. Send the start timer period signal to timer
3. (put here your own code)
4. Return from interrupt routine

I have heard that this system has been implemented with only few per cent
processing time overhed.

--

Tomi.Engdahl@hut.fi            "Don't force it; get a larger hammer"
then@vipunen.hut.fi