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From: vishniac@wanginst.UUCP (Ephraim Vishniac)
Newsgroups: net.micro.mac
Subject: Re: Re: Re: 1.5 Meg Upgrade from MacMemory
Message-ID: <1254@wanginst.UUCP>
Date: Tue, 29-Oct-85 13:27:34 EST
Article-I.D.: wanginst.1254
Posted: Tue Oct 29 13:27:34 1985
Date-Received: Thu, 31-Oct-85 23:34:12 EST
References: <501@ihwpt.UUCP> <2092@amdahl.UUCP> <509@ihwpt.UUCP> <523@ihwpt.UUCP> <240@well.UUCP>
Distribution: net
Organization: Wang Institute, Tyngsboro, Ma.  01879
Lines: 23

> 	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 

I believe the answer is that the video hardware, even on 128K Macs, 
addresses the screen at the "Fat Mac" location (7A700).  Wraparound
does the rest.

-- 
Ephraim Vishniac
  [apollo, bbncca, cadmus, decvax, harvard, linus, masscomp]!wanginst!vishniac
  vishniac%Wang-Inst@Csnet-Relay