From:	Shawn Hargreaves <Shawn AT talula DOT demon DOT co DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Newbie needs Help! Allegro + W95 :-(
Date:	Fri, 10 Jan 1997 19:30:07 +0000
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	<q01J6aAwwq0yEw3v AT talula DOT demon DOT co DOT uk> <5b2fb7$cps$1 AT troll DOT powertech DOT no>
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Ove Kaaven writes:
>>doesn't, which is very irritating (if only IBM had seen fit to include a
>>retrace interrupt in the VGA design!).
>
>This may not have been in the *original* IBM VGA card, but according
>to my old book in programming for PC video subsystems, the EGA, the
>VGA, and the MCGA supports generating a Vertical Interrupt on IRQ2
>whenever vertical retrace starts, and explains how this is done. On
>the EGA/VGA, these interrupts appears to be controlled by the CRTC
>Vertical Retrace End register (CRTC register 11h) bits 4 and 5, though
>I haven't really studied this in detail.

Some VGA clones do have this ability, but the majority don't. And most
SVGA chipsets don't support it (again there are exceptions, eg. the ATI
Mach 64, but every card does it differently and there is no standard way
to access it). It doesn't strike me as a very good idea to use a feature
that is only available on maybe 10% of the hardware around...

/*
 *  Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
 *  Ghoti: 'gh' as in 'enough', 'o' as in 'women', and 'ti' as in 'nation'.
 */

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