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From: tk@cvl.UUCP (Tharakesh Siddalingaiah)
Newsgroups: net.micro.mac
Subject: Re: Re: 1.5 Meg Upgrade from MacMemory
Message-ID: <909@cvl.UUCP>
Date: Tue, 29-Oct-85 08:38:14 EST
Article-I.D.: cvl.909
Posted: Tue Oct 29 08:38:14 1985
Date-Received: Fri, 1-Nov-85 00:17:48 EST
References: <501@ihwpt.UUCP> <2092@amdahl.UUCP> <509@ihwpt.UUCP> <523@ihwpt.UUCP> <240@well.UUCP>
Reply-To: tk@cvl.UUCP (Tharakesh Siddalingaiah)
Distribution: net
Organization: Computer Vision Lab, U. of Maryland, College Park
Lines: 48

In article <240@well.UUCP> espen@well.UUCP (Peter Espen) writes:
>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 
>
> 


The hardware was always look for screen memory starting at location 7A700
but since you orignaly had 128k of memory the 2 MSB wern't being decoded
so it seemed that the screen was at 1A700. I belive the ROM did a similar
sort of thing i.e. put the address 7A700 in the ScreenBase pointer and the
decodeing logic took care of the rest.

-tk
-- 
 Tharakesh Siddalingaiah ---
      University of Maryland Computer Vision Lab,     (301) 454-4526
      ARPA:tk@cvl   CSNET:tk@cvl   UUCP:{seismo,allegra,brl-bmd}!umcp-cs!cvl!tk