Path: utzoo!attcan!uunet!husc6!rutgers!cmcl2!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn )
Newsgroups: comp.sys.apple
Subject: Re: 65816 v. 680x0
Message-ID: <8878@smoke.BRL.MIL>
Date: 14 Nov 88 08:25:10 GMT
References: <8811132232.aa14565@SMOKE.BRL.MIL>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 39

In article <8811132232.aa14565@SMOKE.BRL.MIL> AWCTTYPA@UIAMVS.BITNET ("David A. Lyons") writes:
>>From:         Doug Gwyn <adm!smoke!gwyn@NYU.EDU>
>>[...] Also, even the 65816 still has too small a virtual
>>address space (64K 8-bit bytes).  Although one can cope with this by
>>using segmenting schemes, it does get in the way.
>Eh?  The 65816 has a 16 Meg ->physical<- address space and no
>support for virtual memory.  There are addressing modes that can
>access the whole thing, with no need for segmentation.  The only
>kind of segmentation you need is at a high level (the linker and
>loader level), since _executing code_ can't cross a 64K boundary.
>(This is a reasonable restriction, since code that _could_ cross a
>64K boundary would have to _always_ use JSL/RTL rather than JSR/RTS,
>pushing 3-byte rather than 2-byte return addresses on the stack.)

A virtual address space doesn't necessarily have anything to do
with virtual memory.  It's the addressable scope of program code
without having to alter any memory mapping tables or registers
(such as "base" or "bank" registers).

Although most accumulator-oriented 65816 instructions can be
used in a "long" (24-bit) mode, many useful instructions cannot.
In fact, the hodge-podge mixture of base instructions and
addressing modes has always been one of the annoying features
of the 65xx family architecture.

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
is a far cry from the 68000's simple addressing scheme.  And the
crucial role played by the 256-byte direct page in making many
addressing modes work leads to a turf fight that requires a lot
of unnecessary work to avoid.

The 68000 architecture has a few quirks of its own, but it is
MUCH cleaner in its overall design.

On the other hand, there's always the 808* family to poke fun at!
Almost anything looks good by comparison..