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From: dafa@ihwpt.UUCP (David Fay)
Newsgroups: net.micro.mac
Subject: Re: Re: 1.5 Meg Upgrade from MacMemory
Message-ID: <522@ihwpt.UUCP>
Date: Fri, 18-Oct-85 00:11:14 EDT
Article-I.D.: ihwpt.522
Posted: Fri Oct 18 00:11:14 1985
Date-Received: Mon, 21-Oct-85 03:38:24 EDT
References: <501@ihwpt.UUCP> <2092@amdahl.UUCP> <509@ihwpt.UUCP> <1201@wanginst.UUCP>
Distribution: net
Organization: AT&T Bell Laboratories
Lines: 20

Having spent some time talking to MacMemory and to my hardware friends,
I'd like to clarify several issues that were confused in my previous
submissions on the MacMemory 1.5 Meg upgrade.

First, as several people pointed out, I was most definitely confused
about how screen memory location is determined. In fact, screen memory
location is determined entirely by hardware address decoding. But in
order for programs to know where screen memory is located, a ROM
routine figures out where screen memory is and sets a pointer to it
in the low memory location ScreenBase. It does this by figuring out
how much memory is in the Mac and then assuming that screen memory must be a certain distance below the top
of memory
either 128K or 512K. Therefore, >512K upgrades must patch the
ScreenBase pointer with the correct location if they set 
-- 
--------------------
David Fay
AT&T Bell Laboratories
Naperville, IL 60566
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