samuels@halibut.nosc.mil (Lawrence J. Samuels) (06/14/91)
I'm sorry to repeat this 'frequently-asked question' (hey, maybe we need out _own_ FAQ list! -) ), but why, oh why, is it that 24-bit machines, such as my IIcx, can 'use' only 8MB of physical RAM? As we've all heard too many times, 2^24 = 16 MB, etc., but in 24-bit mode, RAM above 8 MB can't be used, as I understand. (Not fully accurate, but I want to keep the question brief.) I thank you in advance for an answer to this question. Perhaps I can offer you all an amusing, but somewhat sad story in return: Last week, my workplace had a Mac fair, vendors show up with products to sell us. Apple kindly sent a fellow to show all the good things about System 7. I hung around after the presentation to listen to the question asked. A friend of mine (possibly with ulterior motive) asked: "How much RAM can in use in my IIcx under System 7?" and I awaited the answers... "You can use up to 128 MB of RAM." said the Apple representative... ... ... (steam) ... I had to say something... finally, I sorted of sputtered, "But what about the old ROMs?" ( I didn't want to say 'dirty'.) I got a long, blank, and somewhat hostile look (my interpretation), then he said, "Well, 24-bit ROMs only let you use 8 MB of RAM." Disclaimer: maybe the guy didn't hear the original question, maybe he was trying to field a lot of stuff - but this is a touchy subject, you'd think he'd take greater care with a question oabout RAM use, just in case he'd say something wrong... or maybe I'm expecting too much. Anyhow, it _is_ sorta funny, isn't it? Thanks for any help with my original question! Larry Samuels samuels@nosc.mil
rmh@apple.com (Rick Holzgrafe) (06/18/91)
In article <4112@nosc.NOSC.MIL> samuels@halibut.nosc.mil (Lawrence J. Samuels) writes: > I'm sorry to repeat this 'frequently-asked question' (hey, maybe > we need out _own_ FAQ list! -) ), but why, oh why, is it > that 24-bit machines, such as my IIcx, can 'use' only 8MB of > physical RAM? As we've all heard too many times, 2^24 = 16 MB, etc., > but in 24-bit mode, RAM above 8 MB can't be used, as I understand. It's because addresses are used to access more than just RAM. Of the "missing" 8 Mb, 1 Mb is used to access ROM, 1 Mb is used to access hardware (sound chips, serial chips, etc.), and 1 Mb is assigned to each of the NuBus slots. That 6 Mb to the NuBus slots applies whether or not the slots are occupied, and whether or not your particular Mac actually has 6 slots. (Your IIcx has only three.) If you turn on VM, and cluster all your NuBus cards in the slots farthest from the power supply, the system will let you use those extra 1 Mb's for RAM, giving you more than 8 Mb of usable RAM. I'm not sure about IIcx's in particular, but a Mac II with a single video card in the far slot can access 13 Mb of RAM, *if* VM is turned on. Hope this helps. ========================================================================== Rick Holzgrafe | {sun,voder,nsc,mtxinu,dual}!apple!rmh Software Engineer | AppleLink HOLZGRAFE1 rmh@apple.com Apple Computer, Inc. | "All opinions expressed are mine, and do 20525 Mariani Ave. MS: 3-PK | not necessarily represent those of my Cupertino, CA 95014 | employer, Apple Computer Inc."