Path: utzoo!utgpu!watmath!clyde!att!rutgers!mailrus!iuvax!pur-ee!hankd
From: hankd@pur-ee.UUCP (Hank Dietz)
Newsgroups: comp.arch
Subject: Memory-Mapped vs. Memory-Controlled (was Re: ZISC computers)
Summary: Dumb trivia and the NorthStar FPB
Keywords: ZISC
Message-ID: <9924@pur-ee.UUCP>
Date: 29 Nov 88 21:32:02 GMT
References: <22115@sgi.SGI.COM> <278@antares.UUCP> <2958@ima.ima.isc.com>
Organization: Purdue University Engineering Computer Network
Lines: 17

You don't even have to be memory-mapped to get operands to a functional unit
which is out there somewhere.  The old NorthStar Horizon FPB -- a BCD
floating-point S100 board made of TTL (circa 1974) -- wasn't memory-mapped,
but was controlled by a memory reference.  First, you'd touch a location,
then the FPB would actually watch bus references and would load things into
the FPB as it saw them being loaded into the CPU...  yes, I know that sounds
like the bus loading is wrong, but it did work....  Supposedly, the idea was
to save a few T-states over the usual memory-mapped technique:  8080-family
CPUs were not good at multi-byte memory-to-memory moves, but could do a
sequence of memory-to-register loads fairly fast.  Results were read from
the FPB in a similar way.

Actually, I recall quite a few things being memory-controlled in the above
sense, but not memory-mapped.  I just thought it might be worth mentioning
this alternative, however "kludgey" it might seem....

						-hankd@ee.ecn.purdue.edu