Hello Mr. Bertho,
Thanks for the tips. However, I suspect that knowing when exactly
vertical retraces occur is still relevant to modern monitors (and OSes),
where one needs to prevent "screen tearing" and other nasty artefacts.
So I would think it is still useful to know how to do it right.
Thank you!
--
https://github.com/tkchia
On 07/01/2018 09:45 PM, Bertho Grandpied via Freedos-devel wrote:
On Sat, 30 Jun 2018 ,David McMackins wrote :
> Anyway, I'm figuring out VGA and VESA graphics programming, but one
> thing I can't find is how to wait for an interrupt to tell me when the
> vblank period starts so that I can vsync.
The vertical interrupt was indeed part of IBM's
original video systems for the PC (MDA,CGA,
EGA), up to and including the original VGA for
the PS/2, but don't count on it on any modern
VGA,SVGA, etc (in-)compatibles ! The main use
for monitoring that interrupt (where available) was
in order to avoid so-called "snow" effects, but that
would be before fast, dual-ported "VRAM" were
introduced. There is no need to coordinate
video memory accesses to/from CPU and the
video adapter any more; the only remaining use
for hooking vertical retrace I can think of is
in order to measure vertical retrace frequencies,
and that doesn't require the interrupt - simply poll
the V-retrace bit = bit 3 at input port 3DAh
(or 3BA in B/W mono).
Anyway... if your, presumably very old, VGA card
did support the Vertical interrupt, it should be routed
as IRQ 9, interrupt #71h, on an AT-compatible (IRQ 2
on an original PC or PC/XT). On the EGA/VGA, you'd
have to ENABLE that V-retrace interrupt explicitly,
by REsetting bit 5 of the CRTC register #11h, and
then hook the HW interrupt in your programme, of
course.
In any case you are welcome to try your HW for
generating that interrupt, using the preceding indications,
there are no risks in testing IMHO : but there is
also very little chance of success on any PC video
system less than 25 yrs old :=)
HTH anyway, and happy hackings
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