Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!rutgers!phri!roy
From: roy@phri.UUCP (Roy Smith)
Newsgroups: comp.arch
Subject: Re: more Observability
Message-ID: <3954@phri.UUCP>
Date: 25 Aug 89 01:33:13 GMT
References: <5945@pt.cs.cmu.edu> <21547@cup.portal.com> <1063@cernvax.UUCP>
Reply-To: roy@phri.UUCP (Roy Smith)
Organization: Public Health Research Inst. (NY, NY)
Lines: 18

In article <1063@cernvax.UUCP> hjm@cernvax.UUCP (Hubert Matthews) writes:
> So, please, no more write-only registers or similar gate saving tricks.

	My impression of Unibus read-only and write-only registers sharing
the same address was that it wans't meant to save gates, but address space.
On a Unibus, you only have (if I remember correctly) 4K of I/O space and
that space was sparsely used, in an effort to make auto-configure possible
(i.e. if there is something at address 77XXXX, it must be a YY-11).  With
32-bit address spaces common today, this is probably not necessary.

	In a sense, DEC did the same with their pdp-11 opcodes; "add byte"
was really "subtract word".  Given the confines of a 16 bit word, I suspect
they did the right thing, but again, it's probably not necessary today.
-- 
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
{att,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu
"The connector is the network"