🚀 go-pugleaf

cd2a+@andrew.cmu.edu (Court Demas) writes:


>I've heard many conflicting things about using the Vertical Retrace Interrupt -
>like that it can't be used with any IBM VGA's (some say just IBM's ISA VGA
>card - does such a thing exist?).

>It seems like this is available on EGA also.  I just don't understand how you
>can write an interrupt driver when all you know is the IRQ number.  Before
>I break out PC-Watch and try to locate this sucker, it would be cool if
>somebody else could explain.

Prentice Hall puts out a book called (I think) "Programming EGA/VGA displays"
or something similiar. Don't buy it; it is full of astounding typographical
errors, at least the copy I saw was. But it talks at length about snagging
the vertical and horisontal interrupts. There are apparently a *lot*
of pitfalls, not the least of which is that some (EGA?) cards were built
with the retrace-in-progress bit inverted, making it hard to code an interrupt
routine to do anything cleanly.

I forget which interrupt gets triggered when you enable vertical interrupts,
but I know that it is shared with other devices, so even when you do see
the interrupt go off you have to query the display board to see if it caused
the interrupt or not. Note that different boards trigger this interrupt
at different rates (anywhere between 50-70Hz?) 

In general, you have to have a pretty driving need to do things this way.
You can probably do just fine, generating your image as fast as you can
to one screen, flipping to it and generating the next image to the other
screen.
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"As sensitive and broad-minded humans, we must never allow ourselves to be
in any way judgmental of the religious practices of other people, even when
these people clearly are raving space loons."		Dave Barry
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