martyl@bucket.UUCP (Marty Lee) (03/04/88)
With all this discussion about memory upgrades, I just read something from
APPLE that stated the SE cannot be upgraded beyond 4 Meg on the motherboard.
Even when 4 Meg and 16 Meg SIMMs come out they won't be usable on the SE.
Anybody out there know if this is true? Isn't the 68000 capable of addressing
16 Meg? With the screen, I/O and ROMs the SE has 8 Meg of addressable
memory space? Right? (Wrong?)
teksce (Fastest path)
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percivalsarrel@tut.cis.ohio-state.edu (Marc Sarrel) (03/07/88)
In article <799@bucket.UUCP> martyl@bucket.UUCP (Marty Lee) writes: > >Isn't the 68000 capable of addressing >16 Meg? With the screen, I/O and ROMs the SE has 8 Meg of addressable >memory space? Right? (Wrong?) Well, really the 68000 is cap _more_ than 16 Meg. Internally it has 32 address lines for a total of 2^32 = 4096 MegaBytes = 4 GigaBytes. However, in its 64-pin DIP package, only 23 of those address lines come out. (64 pins is about the largest DIP package you can have. Newer version in the 680xx family have non-DIP packages.) You may notice that 2^23 = 8 Meg. However, because the 68000 has a 16-bit wide data bus, the Mac can read in two bytes at a time, effectively doubling the address space. In essence, they use address lines 1 through 24 with line 0 always assumed to be zero. So, as you can see, the 4 Meg limit is not due to the 68000. It is due to the fact that the Apple ROMS start at 4Meg in the address If I remember correctly, 4Meg through 8Meg is reserved for ROM and every thing above that is used for memory mapped I/O. (with lots of duplication). Using 20/20 hindsight, you could say that this is very inefficient on Apple's part (you'd be right). But they obviously weren't thinking that anyone would need more than 4Meg of real RAM. Most Vaxen and the like don't have that much _real_ space. (of course they use virtual memory.) Does anyone in the know want to comment on Apple's design descisions? Hope this answered your question.-- Marc Sarrel The Ohio State University 611 Harely Dr #1 Department of Computer and Information Science Columbus, OH 43202-1835 sarrel@tut.cis.ohio-state.edu Disclaimer: Hey, what do I know? I'm only a grad student.
jimc@iscuva.ISCS.COM (Jim Cathey) (03/08/88)
In article <7809@tut.cis.ohio-state.edu> sarrel@tut.cis.ohio-state.edu (Marc Sarrel) writes: >>Isn't the 68000 capable of addressing >>16 Meg? With the screen, I/O and ROMs the SE has 8 Meg of addressable >>memory space? Right? (Wrong?) >...So, as you can see, the 4 Meg limit is not due to the 68000. It is due >to the fact that the Apple ROMS start at 4Meg in the address If I >remember correctly, 4Meg through 8Meg is reserved for ROM and every thing >above that is used for memory mapped I/O. (with lots of duplication). Using >20/20 hindsight, you could say that this is very inefficient on Apple's >part (you'd be right). The 4 Meg limit made perfect sense in the original Macs. It wasn't possible to place any more memory in them. Cheap decoding was entirely adequate, and the ROM was placed at the 4M point. Even in these machines nowhere in the Mac OS is this address 'fixed'. It should be possible (for Apple) to re-asssemble the ROM at the 12M point, hack a circuit board to change the decoder and have the thing come up. For the Mac SE, they probably should have done this, but perhaps by then there _were_ some things that made explicit assumptions about the ROM's location. (The thought police frown on this sort of thing.) Having Ex*el (or whatever) break because the ROM moved is powerful incentive to leave well enough alone. So this limits RAM on the SE. Still shouldn't be any inherent reason for an Apple machine to fully support some teensy bit less than 16M of RAM. Any votes for the Mac SE+? Another possiblilty...anyone out there ever heard of product strategies? Marketeers love this sort of stuff -- "I think people who want BIG machines should buy our Model II box" [you know, the one with the bigger profit margin!]. I personally have been forced to do, umm, _more interesting_ things than this by pressure from these quarters. +----------------+ ! II CCCCCC ! Jim Cathey ! II SSSSCC ! ISC Systems Corp. ! II CC ! TAF-C8; Spokane, WA 99220 ! IISSSS CC ! UUCP: uunet!iscuva!jimc ! II CCCCCC ! (509) 927-5757 +----------------+ "With excitement like this, who is needing enemas?"