Path: utzoo!mnetor!uunet!iscuva!jimc
From: jimc@iscuva.ISCS.COM (Jim Cathey)
Newsgroups: comp.sys.mac
Subject: Re: Mac SE hardware question: 4.0 Meg max?
Message-ID: <1224@iscuva.ISCS.COM>
Date: 8 Mar 88 02:48:04 GMT
References: <799@bucket.UUCP> <7809@tut.cis.ohio-state.edu>
Organization: ISC Systems Corporation, Spokane, WA
Lines: 39

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.  

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