Path: utzoo!utgpu!watserv1!ria!uwovax.uwo.ca!7103_2622
From: 7103_2622@uwovax.uwo.ca (Eric Smith)
Newsgroups: comp.sys.atari.st
Subject: Re: Why does <Esc> work? :-)
Message-ID: <1991Mar2.183903.8724@uwovax.uwo.ca>
Date: 2 Mar 91 23:39:03 GMT
References: <706256@fiction>
Lines: 23

In article <706256@fiction>, Daniel_Roedding@fiction.ms.sub.org writes:
> csbrod@immd4.informatik.uni-erlangen.de (Claus Brod) writes:
>> Daniel_Roedding@fiction.ms.sub.org writes:
>> >I re-disassblemled (ouch!) the DeskTop's media change force routine. The
>> >DeskTop expects A0 to be zero after a GEMDOS call. Is this always true?
>> To my knowledge, certainly not.
> 
> So then I'd say that it is really random whether the <Esc> function works
> or not. Or is it defined that Fopen() clears A0? If not, the desktop cer-
> tainly would crash!! (3 Bombs, I tried it by changing A0)
> 

In practice, all the trap #1 calls seem to leave A0 alone (i.e. its value
on exit is the same as the value on entrance). However, since they are
documented as potentially destroying A0-A2 and D1-D2, relying on this
behaviour is *very* *very* bad and will break someday (as I've been told
when I've done this :-). Since new versions of TOS will presumably include
new versions of the desktop, I guess that is one application that can afford
to ignore this caveat; it is rather strange that it does, though.
-- 
Eric R. Smith                     email:
Dept. of Mathematics            eric.smith@uwo.ca
University of Western Ontario   7103_2622@uwovax.bitnet