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From: brianp@ernie.Berkeley.EDU (Brian Peterson)
Newsgroups: comp.sys.atari.st
Subject: strange bug?
Message-ID: <21951@ucbvax.BERKELEY.EDU>
Date: Wed, 25-Nov-87 08:24:10 EST
Article-I.D.: ucbvax.21951
Posted: Wed Nov 25 08:24:10 1987
Date-Received: Sat, 28-Nov-87 18:32:33 EST
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: brianp@ernie.Berkeley.EDU.UUCP (Brian Peterson)
Organization: University of California, Berkeley
Lines: 31

I wrote a little routine which just jumped to an exception handling routine,
and put the address of my routine in the exception vector in place of the
original.  If the exception was the divide by zero, it worked as expected.
When it was the midi/kboard exception, the results were strange.
My routine was like this:
	move.l #fc281c a0
	jmp (a0)

with the routine's address in $118.  (actually, I wrote it in Forth,
so it was all backwards.)  The result varied depending on whether
I used a0, a1, or a2.

    a0 caused the computer to just die,
    a1 caused one of [illegal instruction, address error, bus error]
	at random to happen, 
    a2 caused beeps. 

The a1 and a2 ones did their thing some but not all times I hit a
computer key, and also did their things sometimes when a midi byte
went in.  The a2 one also made blocks of randomly colored pixels
occaisionally, which were the width of the screen, and went from some point
on the screen down to just above the next prompt-n-cursor on the screen.
I could fix the a2 version by replacing fc281c in 118.  The a1
one wouldn't let me type that many consecutive characters per prompt.

Does anyone know what would cause that behaviour?  Might it be
fried hardware, buggy roms, bugs in my software (Forthmacs by Mitch Bradley -
quite a useful package), or something I don't know about how exceptions
happen and interact and whatever ?????   Thanx for any info!

	Brian Peterson, ...!ucbvax!ucbernie!brianp (brianp@ernie.berkeley.edu)