Message-Id:	<1.5.4.32.19970425080251.002ea448@ubeclu.unibe.ch>
Mime-Version:	1.0
Date:	Fri, 25 Apr 1997 10:02:51 +0200
To:	tgrand AT canvaslink DOT com
From:	Roger Noss <noss AT pupk DOT unibe DOT ch>
Subject:	Re: timer interrupt - sample code anyone?
Cc:	djgpp AT delorie DOT com

At 04:28 PM 4/24/97 -0400, tgrand AT canvaslink DOT com wrote:
>
>Then there is a function called _vga_vsync(), which simply calls
>_vsync_out() and then _vsync_in().  So I think this is how Allegro is
>waiting until the monitor has finished drawing a frame... (at least in
>VGA mode anyway.)

The Allegro web pages also discuss some "vertical retrace emulator" which
may be built on these functions.  I'm not sure what exactly the emulator
does, and I want the actual retrace, not just an emulated one.  Anyway, the
web page for it says that it won't work in Win95.  My rule of thumb is, if
not Win95, then probably not NT.

>And as for your readings in NT, are you sure you're really being able
>to access the -real- 0x3CA, or could NT be secretly doing some emulation
>or something?  I say this only because I remember that OS/2 used to have
>a DOS session setting that was something like
>VERTICAL_RETRACE_EMULATION.

I have no way of knowing what NT is doing.  I also posted a puzzling test of
uclock() which suggests that NT is stealing resources at about 10 ms per
call, but there has been no response on the list.  My intuition is that NT
must let port reads go through or it would be very difficult to write device
drivers.

Roger Noss

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