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From: eacj@batcomputer.tn.cornell.edu (Julian Vrieslander)
Newsgroups: comp.sys.mac
Subject: Re: Mac SE - compatibility (where's the horizontal retrace bit?)
Message-ID: <458@batcomputer.tn.cornell.edu>
Date: Thu, 19-Mar-87 20:07:56 EST
Article-I.D.: batcompu.458
Posted: Thu Mar 19 20:07:56 1987
Date-Received: Sat, 21-Mar-87 09:16:28 EST
Reply-To: eacj@batcomputer.UUCP (Julian Vrieslander)
Organization: Theory Center, Cornell University, Ithaca NY
Lines: 20
Keywords: Mac SE, compatibility, horizontal retrace

I have encountered a pretty arcane incompatibility between the Plus and the SE.
On the Plus, you can read bit 6 in VIA register B to get at the horizontal synch
signal, which is also the clock for the sound generator DAC.  I have been using
this bit as a time base for a very tricky custom sound driver.

In the SE, that bit no longer follows the horizontal retrace (I think it is used
for reporting disk drive interrupts now).  In fact, I remember seeing somewhere
that the VIA chip used in the SE is a special version of the SY6522, modified
for Apple.

I was expecting to write a new driver for the Mac II, but I was surprised to
find that my program died on an SE.  Is there any other place in the SE that
software can read the status of the horizontal retrace?  Or should I warm up my
soldering iron?  

-- 
Julian Vrieslander, Neurobiology & Behavior, Cornell Univ., Ithaca, NY 14853    
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