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From: roman@sigma.UUCP
Newsgroups: comp.sys.mac
Subject: Re: Mouse Anti-Freeze routine needed.
Message-ID: <1064@sigma.UUCP>
Date: Sat, 14-Feb-87 11:14:00 EST
Article-I.D.: sigma.1064
Posted: Sat Feb 14 11:14:00 1987
Date-Received: Sun, 15-Feb-87 08:12:22 EST
References: <678@bgsuvax.UUCP>
Reply-To: roman@sigma.UUCP (Bill Roman)
Organization: Summation, Inc., Kirkland, WA
Lines: 16
Summary: Try re-initializing the SCC

In article <678@bgsuvax.UUCP> denbeste@bgsuvax.UUCP (William C. DenBesten)
writes:
> [...] 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.

I did a little delving in volume III.  The mouse interrupt is (kludge!)
coming in on the SCC.  The "DCD" signals of channels A and B are used.
What is probably happening is that stray writes are wiping out the control
registers of the SCC.  Another possibility is that the processor priority
is being set to mask out this interrupt.  This seems less likely because
this would involve a stack frame mixup and would probably send execution
off to never-never land.

So what some wizard needs to do is find out how to get the SCC's state
restored to something which will re-enable this interrupt.