🚀 go-pugleaf

then@lesti.hut.fi (Tomi  H Engdahl) writes:

>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.

This is implemented in REND386: you can get source code from 
sunee.uwaterloo.ca-- look for the devel3.zip file.  Look at the source
files sgsppt.c, sgint.c, and glovedel.asm.

The overheas is one interrupt per frame plus a 500 uS resync delay
every fifth frame.



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| My life is Hardware,                    |                              | 
| my destiny is Software,                 |         Dave Stampe          |
| my CPU is Wetware...                    |                              | 
| Anybody got a SDB I can borrow?         | dstamp@watserv1.uwaterloo.ca |
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