Path: utzoo!mnetor!uunet!husc6!tut.cis.ohio-state.edu!sarrel From: sarrel@tut.cis.ohio-state.edu (Marc Sarrel) Newsgroups: comp.sys.mac Subject: Re: Mac SE hardware question: 4.0 Meg max? Message-ID: <7809@tut.cis.ohio-state.edu> Date: 7 Mar 88 02:02:34 GMT References: <799@bucket.UUCP> Organization: The Ohio State University Dept of Computer and Information Science Lines: 31 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.