Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site cvl.UUCP Path: utzoo!linus!decvax!decuac!cvl!tk 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