Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uwm.edu!uwvax!puff!rt3.cs.wisc.edu!blochowi
From: blochowi@rt3.cs.wisc.edu (Jason Blochowiak)
Newsgroups: comp.sys.apple
Subject: Re: Data bus size on the //gs (was Re: FPE and a transwarp gs)
Summary: There's only an 8 bit data path between the //gs and peripherals
Keywords: peripherals, coprocessor, //gs, Floating Point Engine, FPE
Message-ID: <3723@puff.cs.wisc.edu>
Date: 13 Nov 89 12:48:28 GMT
References: <2190.cortland.info-apple@pro-houston>
Sender: news@puff.cs.wisc.edu
Reply-To: blochowi@rt3.cs.wisc.edu (Jason Blochowiak)
Organization: U of Wisconsin CS Dept
Lines: 26

In article <2190.cortland.info-apple@pro-houston> jabernathy@pro-houston.cts.com (Joe Abernathy) writes:
>The FPE employs a 16-bit data bus.

	"Employs"? I haven't even seen the FPE, much less any spec sheets for
it, but... Unless I'm totally confused as to the //gs' design, it has an 8 bit
data bus, and a 16 bit address bus. The 24 bit addressing is generated by
sticking the bank number (the upper 8 bits of the 24 bit address) onto the data
bus at the right time. So, what this means (I'm assuming here that I'm not
totally lost, and I don't think I am) is that the FPE can talk to itself in
whatever fashion it likes (it could have a 128 bit data bus, for all the
external world cares), but it can only communicate with the //gs in discrete
8 bit chunks. Perhaps they've done something to make it look a little bit more
like 16 bits, possibly by grouping the I/O space addresses into words and
combining the the buffered values stored into the I/O space, but that still
doesn't get away from the fact that the //gs can only give it 1 byte at a time.

	If I'm wrong, I'd really like to hear about it, as it'd kind of change
things for this hardware project I'm working on - besides, Apple would
probably be interesting in knowing that their hardware manual for the //gs is,
at the very least, quite misleading.
 
>INET: jabernathy@pro-houston.cts.com

--
      Jason Blochowiak - blochowi@garfield.cs.wisc.edu or jason@madnix.uucp
       "Education, like neurosis, begins at home." - Milton R. Sapirstein