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From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Question - Use VBL or Def. Task?
Message-ID: <33883@cup.portal.com>
Date: 14 Sep 90 04:53:55 GMT
References: <90256.130529TAB113@psuvm.psu.edu>
Organization: The Portal System (TM)
Lines: 22

SysBeep, according to IM-III, is a trap that can move or purge
memory.  Thus, it can't be called from a completion routine or
a VBL task.

When you are playing around with something and want to know if
it gets exectuted (e.g., a completion routine, a VBL task, a
slot interrupt, or whatever), do something like this:

	move.l	ScrnBase, a0
	add.l	#1, (a0)

or, from Think C

	++*(long *)ScrnBase;

(don't trust me on the syntax for this...and be aware that MPW
C handles low memory globals differently).

Then, just watch the upper left corner of the screen to see if your
code is getting called.

						Tim Smith