Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!usenet.ins.cwru.edu!cwjcc!ncoast!allbery
From: allbery@NCoast.ORG (Brandon S. Allbery)
Newsgroups: comp.sys.mac
Subject: Re: ShortCut vs. Boomerang: Opinions?
Message-ID: <1990Jan24.001455.19300@NCoast.ORG>
Date: 24 Jan 90 00:14:55 GMT
References: <44959@improper.coherent.com> <13977@fluke.COM> <2940@murtoa.cs.mu.oz.au> <4520@sbcs.sunysb.edu>
Reply-To: allbery@ncoast.ORG (Brandon S. Allbery)
Followup-To: comp.sys.mac
Organization: North Coast Public Access UN*X, Cleveland, OH
Lines: 38

As quoted from <4520@sbcs.sunysb.edu> by vallon@sbmiclr.cs.sunysb.edu (Justin Vallon):
+---------------
| In article <44959@improper.coherent.com>, dplatt@coherent.com (Dave
| Platt) writes:
| > [Comments about Boomerang]
| >
| > Back around 2.0B5 or so, I installed the Mr. Bus Error INIT, which
| > repeatedly stomps location 0 with an odd doubleword... this rapidly
| > detects code which is dereferencing an empty-but-not-null handle (very
| > much a no-no).  Boomerang immediately began suffering from bus errors,
| > aborting the current application.
| 
| (I assume that Mr. Bus Error is something like a VBL task that writes
| to location $0 every interrupt).  I have two questions.
| 
| BTW, what's the point of being odd?  Isn't a bus error enough?  Anyway,
| 68020's and beyond don't generate address errors, right?
+---------------

Most 680x0 instructions are word or longword-oriented; an odd address passed
to those instructions will cause a bus error.

Quite a few applications, INITs, etc. have a tendency toward "unsafe" memory
practices; in particular, referencing a nil handle.  Mr. Bus Error and similar
INITs are designed to catch these by causing address 0 to contain an invalid
address (see above), so operation on the value will cause a system bomb
and/or your favorite debugger to come up.  (I strongly suggest you use MacsBug
or TMON, etc. with such an INIT....)

This helps detect problems which could, undetected, corrupt random memory and
thereby cause even worse problems.

++Brandon
-- 
Brandon S. Allbery    allbery@NCoast.ORG, BALLBERY (MCI Mail), ALLBERY (Delphi)
      uunet!cwjcc.cwru.edu!ncoast!allbery ncoast!allbery@cwjcc.cwru.edu
*(comp.sources.misc mail to comp-sources-misc[-request]@backbone.site, please)*
*Third party vote-collection service: send mail to allbery@uunet.uu.net (ONLY)*