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Path: utzoo!watmath!clyde!burl!ulysses!ucbvax!ucdavis!lll-crg!well!espen
From: espen@well.UUCP (Peter Espen)
Newsgroups: net.micro.mac
Subject: Re: Re: 1.5 Meg Upgrade from MacMemory
Message-ID: <240@well.UUCP>
Date: Sun, 27-Oct-85 18:56:26 EST
Article-I.D.: well.240
Posted: Sun Oct 27 18:56:26 1985
Date-Received: Tue, 29-Oct-85 01:16:49 EST
References: <501@ihwpt.UUCP> <2092@amdahl.UUCP> <509@ihwpt.UUCP> <523@ihwpt.UUCP>
Distribution: net
Organization: Whole Earth Lectronic Link, Sausalito, CA
Lines: 34
Summary: A Question

In article <523@ihwpt.UUCP>, dafa@ihwpt.UUCP (David Fay) writes:
> After talking further with MacMemory and with my hardware friends,
> I would like to clarify several issues discussed in my
> previous submissions on the MacMemory 1.5 Megabyte upgrade.
> 
> As several people pointed out, I was confused about how screen memory
> location is determined. In fact, screen memory location is determined
> entirely by hardware address decoding. However, in order that programs
> can know about the location of screen memory, the ROM puts a pointer to
> it in the low memory location ScreenBase. This is done by figuring out how
> much memory the Mac has, and then assuming screen memory starts a
> certain distance below the top of memory. Currently, the ROM assumes
> that a Mac can have only 128K or 512K. If you increase memory beyond
> 512K, the current ROM will set ScreenBase incorrectly, so you must
> patch it to point to the actual location of the screen memory. This is
> done by the ROMs that Levco and others provide with their upgrades.
>
 
	If the screen memory location is determined entirely by hardware
address decoding, then how can the Apple ROM alter the screen memory
location to point to the top of RAM for a 512K Mac? I did the Dr. Dobbs
128 to 512K upgrade on my Mac and I didn't alter any of the hardware
address decoding for screen memory at all. The only thing that the
upgrade did was add the 256K memory chips and add the address decoding
for address lines A17 and A18 to select Memory address line MA8 when
 needed. My screen memory never-the-less is right up there at 7a700
as it should be. I always assumed that was because the ROM was detecting
that I had more than 128K of memory and so was writing the ScreenBase
pointer as 7a700. I still don't understand why that ScreenBase pointer
can't be made to point to whatever the top of memory is. Perhaps 
someone out there can clear this up for me. Thanks!
		Peter Espen