To: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Replacing arbitrary interrupt handlers
Reply-To: dmb AT ai DOT mit DOT edu
Date: Mon, 22 Feb 93 13:42:30 -0500
From: David Baggett <dmb AT case DOT ai DOT mit DOT edu>


A while back I asked how to intercept the timer interrupt from within a
DJGCC program, and several people pointed me to the source to gcrt0.o.
(Thanks!)

I've successfully inserted my own timer interrupt that chains properly
in my program, but I still have a few questions.  (BTW, I'm using v108
and not v109 since the timer code doesn't work with v109.)

1. In my timer interrupt I just update a global counter.  I run my
   interrupt at 5 times the normal rate and then chain to the old
   vector every 5th time to keep things kosher for DOS.  However, it
   seems that if I busy wait on the timer variable it doesn't get
   updated!  E.g., the code

	for (;;) {
		if (ticks > 100)
			break;
	}

   never ends.  I know the variable is getting updated properly because
   if I do

	for (;;) {
		vsync();	/* wait for vertical blank */

		if (ticks > 100)
			break;
	}

   it does terminate.  (vsync() just polls the VGA card to see if the
   vblank bit is set).

   Does anyone have any clue why this might be happening?  (It seems
   awfully bizarre to me.)

2. The code that steals the timer interrupt seems to be specific to
   go32.  Is there any way I can steal other vectors?  I need to grab
   the "terminate addresss" vector (called by DOS when the program
   terminates) and the mouse movement interrupt in particular.  I'd
   like to be able to do this without hacking go32.  Is it possible?
   (BTW, I don't care how disgusting the process is, as long as I can
   get something working. :)

Thanks!

Dave Baggett
dmb AT ai DOT mit DOT edu