Xref: utzoo comp.sys.mac.hardware:4346 comp.sys.mac.misc:1337
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From: ccw@nvuxr.UUCP (christopher wood)
Newsgroups: comp.sys.mac.hardware,comp.sys.mac.misc
Subject: Re: Addressable memory of 68000 (was Re: New Macs)
Message-ID: <1782@nvuxr.UUCP>
Date: 13 Jul 90 12:49:34 GMT
References: <316@opusc.CS.SCAROLINA.EDU> <1990Jul4.003731.336@hellgate.utah.edu> <26708@netnews.upenn.edu> <1990Jul5.063108.14564@chinet.chi.il.us>
Reply-To: ccw@nvuxr.UUCP (22456-christopher wood)
Followup-To: comp.sys.mac.hardware
Organization: Bell Communications Research, NJ
Lines: 33

In article <1990Jul5.063108.14564@chinet.chi.il.us> magik@chinet.chi.il.us (Ben Liberman) writes:


>If my memory (no pun intended ;-) serves me, the 68000 will directly address
>16,777,216 bytes of ram, rom...whatever (24 bit address).  The newer Motorola
>chips will address 4,294,967,296 (that's 4 Gigabytes - a 32 bit address)

>The 4 meg limit on SE's is just the way the board is designed (there are prob.
>memory upgrade mods. out there to get you more).

Same with a plus, if anyone cares.  The motherboard was laid out for 4
SIMMs, either 256K or 1 M SIMMs.  Give them credit for thinking ONE
generation ahead...

>The current Mac OS memory limitation of 8 meg. is a matter of Apple design.
>OS space (rom and hardware address?) were mapped to start at 8 meg. because,
>at the time, it was inconceivable that anyone could want more memory than that!

This was a design decision made on the origional 128K Macintosh.  The
68000 had 24 bit address (16 M addresses), (Hmm, doesn't the processor
also have 16 bit data paths, so that it could address 32 M 8-bit bytes?)
so they divided the address map in half, with the lower 8 M for RAM, and
the upper 8M for ROM, IO, etc.  

>Computers - Life in the fast lane
>	------------    ------------   ----------------------
>	Ben Liberman    USENET         magik@chinet.chi.il.us
>	                GEnie,Delphi   MAGIK


-- 
Chris Wood     Bellcore     ...!bellcore!nvuxr!ccw
                         or nvuxr!ccw@bellcore.bellcore.com