🚀 go-pugleaf

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.