Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!mips!apple!dlyons
From: dlyons@Apple.COM (David A Lyons)
Newsgroups: comp.sys.apple2
Subject: Re: what to do with the Text Tools (multitasking tools)
Message-ID: <53393@apple.Apple.COM>
Date: 28 May 91 09:57:39 GMT
References: <767@generic.UUCP>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 37

In article <767@generic.UUCP> ericmcg@pnet91.cts.com (Eric Mcgillicuddy) writes:
>[toolbox] Calls do not use a processes zero page space for anything, they steal
>a bit of stack space, usually less than $20 bytes (Memory Manager is the only
>one I have seen in depth though).

Correct, tool calls frequently use space on the caller's stack (sometimes
much more than 32 bytes...more like half a K for Standard File, since it
calls the Dialog Manager, and the Dialog Manager uses a whole page).

But the caller's direct page is off-limits, and the D register could
actually point into ROM or softswitches, or overlap the stack in some
inconvenient way.  (That's why, in general, it is *not* okay to "borrow"
direct-page locations, preserving and restoring them, if you don't know
where D points!  Very unsafe!)

>[...]
>there is a flag that tells whether the tools are in use, something like the
>"prodos busy" flag I assume, if not then a small patch to the tool dispatcher
>and a global flag could easily be installed. If the flag is set, then the
>called process is immediately suspended

There is a system BUSY flag managed by calling INCBUSY and DECBUSY, and a
lot of tool sets use it.  Hopefully it is used everywhere it should be,
but I suspect there are a few non-reentrant calls that fail to use it, and
such problems probably won't show up until somebody stress-tests it by
writing something that depends on it.

Note that the BUSY flag is frequently nonzero for long periods of time.
For example, while a ChooseFont or Standard File dialog is up, or when
you're waiting for a key inside ReadChar.
-- 
David A. Lyons, Apple Computer, Inc.      |   DAL Systems
Apple II System Software Engineer         |   P.O. Box 875
America Online: Dave Lyons                |   Cupertino, CA 95015-0875
GEnie:DAVE.LYONS  CompuServe:72177,3233 Internet:dlyons@apple.com

My opinions are my own, not Apple's.