Path: utzoo!attcan!uunet!peregrine!elroy!ames!lll-tis!oodis01!uplherc!wicat!meph!gsarff
From: gsarff@meph.UUCP (Gary Sarff)
Newsgroups: comp.sys.apple
Subject: Re: 65816 v. 680x0
Message-ID: <00331@meph.UUCP>
Date: 15 Nov 88 21:16:36 GMT
References: <871@dogie.edu> <8878@smoke.BRL.MIL>
Organization: WICAT Systems Inc. Orem, Utah (Engineering-R & D)
Lines: 37

In article <871@dogie.edu>, terranova@vms.macc.wisc.edu (John Terranova) writes...
>In article <8878@smoke.BRL.MIL>, gwyn@smoke.BRL.MIL (Doug Gwyn ) writes...
>>The various strange addressing quirks once, for example, limited
>>APW C arrays to 64Kb in early releases (now arrays are permitted
>>to exceed 64Kb, at the expense of slower code generated to access
>>them).  The typical continual shuffling of the data bank register
> 
>You are complaining about a 64K limit on arrays and, for that matter,
>single code segments?!?  You should consider yourself fortunate.  On a
>Macintosh (some claim a superior machine to the GS) you are restricted
>to 32K arrays, 32K code segments and 32K of global data.  Sounds to me
>like the 65816 compares fairly favorably (in this respect, atleast) to
>the 68000.

This is a problem with the Macintosh memory management scheme that apple chose
and the C compiler, not the 68000 as such.  They chose to do without an MMU
on the Macintosh (yes 680x0 systems can have mmu's, I have some on my
systems) and thus forcing the code to be position independent, as you no
doubt know.  Therefore the compiler must generate all code and data ref's
as offsets from the pc or someother register, and the 68000 only allows
8 or 16 bit signed offsets, thus the 32K limit.  This is a limitation of
the 68000 IN THIS MODE, but the 68000 has other addressing modes, and
code and data can be larger and do not have to worry about crossing some
64K boundary as the 65816 does with its 16 bit pc register that doesn't
increment across bank boundaries.


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