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From: dgold@apple.UUCP
Newsgroups: comp.sys.mac
Subject: Re: Mouse Anti-Freeze routine needed.
Message-ID: <459@apple.UUCP>
Date: Tue, 10-Feb-87 13:21:28 EST
Article-I.D.: apple.459
Posted: Tue Feb 10 13:21:28 1987
Date-Received: Wed, 11-Feb-87 20:23:57 EST
References: <678@bgsuvax.UUCP>
Reply-To: dgold@apple.UUCP (David Goldsmith)
Organization: Apple Computer Inc., Cupertino, USA
Lines: 29

In article <678@bgsuvax.UUCP> denbeste@bgsuvax.UUCP (William C. DenBesten) writes:
>I have run across an interesting, but apparently not uncommon, problem.  In
>a program that I am writing, I have found a way to freeze my mouse pointer.
>...
>I suspect that the problem involves the vertical retrace interupt becoming
>disabled, but I haven't yet delved into that chapter of Inside Macintosh.
>...
Since the mouse is connected through the SCC, mouse freezing is almost always
a result of a wild memory reference.  The Zilog SCC is very sensitive, and
almost any read or write to one of its registers will disable it.  Since
the Macintosh only looks at a few of the address lines for I/O references,
it's quite easy for a wild reference to wind up hitting the SCC.

Check your program for access to NIL handles or pointers, or uninitialized
pointer variables.  If you have access to a Macintosh XL (Lisa), running
your program on it is a good way to find such errors, as wild memory
references tend to cause bus errors or other problems because of the XL's
different hardware (the mouse is handled in a totally different fashion,
so mouse freezing shouldn't be a danger).


-- 
David Goldsmith
Apple Computer, Inc.
MacApp Group

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