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From: daveo@Apple.COM (David M. O'Rourke)
Newsgroups: comp.sys.mac.hardware,comp.sys.mac.misc
Subject: Re: Addressable memory of 68000
Message-ID: <42648@apple.Apple.COM>
Date: 5 Jul 90 06:22:37 GMT
References: <C7SLP4D@xavier.swarthmore.edu> <90178.172523KPURCELL@LIVERPOOL.AC.UK> <268ACACA.44FD@intercon.com> <316@opusc.CS.SCAROLINA.EDU> <1990Jul4.003731.336@hellgate.utah.edu> <26708@netnews.upenn.edu>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 25

jeff@eniac.seas.upenn.edu.UUCP (Jeff White) writes:
>  On a related note, does anyone know if the 68000 can address more than 4 Megs
>of RAM (ie. is the present 4 Meg limit with Pluses and SE's a 68000 limit or
>just the way Apple designed those systems)?  With 7.0 requiring a minimum of
>2 Megs of RAM, I'm wondering whether Apple might design their newer machines to
>be able to handle more than 4 Megs, since 4 Meg under 7.0 may not leave a lot
>of extra memory for MultiFinder applications.

  The 68000 can address up to 16 megs of memory using it's 24-bit address bus.

  The 4 meg limitation is due to the way the Mac OS/Hard-ware addresses various
I/O devices and the ROM's.  I'm not positive, but I don't believe there is
a straight forward way to make a 68000 mac address more than 4 megs of memory
with out modification to the motherboard, and possibly changes through out the
ROMs.  I'm not a hardware person, and although I work at Apple I don't
go down deep into the OS {in fact I'm not ever sure I have access to do so, I'm
still a new person on the block.} so I'm not sure how deeply ingrained the
memory map is to the OS.

  Hope this helps.

-- 
daveo@apple.com                                               David M. O'Rourke
_______________________________________________________________________________
I do not speak for Apple in *ANY* official capacity.