Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site wanginst.UUCP Path: utzoo!linus!wanginst!vishniac 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