Path: utzoo!mnetor!uunet!husc6!bbn!rochester!stuart
From: stuart@cs.rochester.edu (Stuart Friedberg)
Newsgroups: comp.arch
Subject: Re: negative addresses
Message-ID: <9581@sol.ARPA>
Date: 11 May 88 01:53:50 GMT
References: <2393@uvacs.CS.VIRGINIA.EDU> <389@attila.weitek.UUCP>
Organization: U of Rochester, CS Dept., Rochester, NY
Lines: 19
Summary: BBN Butterfly uses all of the 68K 16-bit signed address

In article <389@attila.weitek.UUCP>, mahar@weitek.UUCP (Mike Mahar) writes:
> A pretty good arguement can be made that the 68000 is a signed address
> machine.  And the address displacements are signed.  There is even a short
> absolute addressing mode. It uses an absolute 16-bit signed address.

Right.  And a machine that makes use of that is the BBN Butterfly
multiprocessor.  Both the "positive" and "negative" portions of that
signed address space are used to efficiently access "Subspace Zero", where
magic memory mapped functions, implemented by a bit-slice coprocessor,
hang out.  However, the machine presents a conventional memory map (all
positive) to programmers, so Subspace Zero addresses have to be mapped
in at the very bottom and very top of the address space.

For this particular purpose, it would have been more symmetric, but far
less conventional, to regard memory space as signed.  It would have
been far more conventional, and convenient, FOR THIS PARTICULAR PURPOSE
if the 68000 provided a 16-bit absolute unsigned address, instead.

Stu Friedberg  {ames,cmcl2,rutgers}!rochester!stuart  stuart@cs.rochester.edu