Path: utzoo!mnetor!tmsoft!torsqnt!jarvis.csri.toronto.edu!rutgers!mit-eddie!genrad!decvax!ima!esegue!johnl
From: johnl@esegue.segue.boston.ma.us (John R. Levine)
Newsgroups: comp.arch
Subject: Re: Instruction (dis)continuation (
Message-ID: <1989Sep20.150557.5158@esegue.segue.boston.ma.us>
Date: 20 Sep 89 15:05:57 GMT
References: <2353@oakhill.UUCP> <261500010@S34.Prime.COM> <34701@apple.Apple.COM> <31361@ucbvax.BERKELEY.EDU> <2128@munnari.oz.au>
Reply-To: johnl@esegue.segue.boston.ma.us (John R. Levine)
Organization: Segue Software, Cambridge MA
Lines: 27

In article <2128@munnari.oz.au> ok@cs.mu.oz.au (Richard O'Keefe) writes:
>...  Note that "memory mapped I/O" has two faces:
>    -- device registers appear as memory locations TO THE CPU
>    -- device registers appear as memory locations TO THE BUS
>It would be possible to have a machine with special
>	input	DeviceAddress, Register
>	output	DeviceAddress, Register
>instructions which the CPU, cache, and so on "knew" about, but which
>appeared ON THE BUS just like memory references that miss the cache.
>Whether that would be a good thing is another question again.

It's a fine idea.  On the 8086 and its descendants, hence on the IBM PC,
excuse me, Industry Standard Architecture, bus, I/O cycles and memory cycles
are the same except for a line that says whether it's an I/O or a memory
address.  I/O addresses are never cached, of course.

The PDP-11 was the first machine to use memory-mapped I/O (the first one I
know about, anyway.)  By convention, all device registers were mapped in the
highest 8K bytes of the address space, and caches knew not to cache
addresses in that range.  On the Q-Bus, the second version of the PDP-11
bus, there is even a line that says that the current address is in the top
8K.  They intended it to make it easier to decode device addresses, but it
is equally useful to distinguish between I/O and memory.
-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 492 3869
johnl@esegue.segue.boston.ma.us, {ima|lotus}!esegue!johnl, Levine@YALE.edu
Massachusetts has 64 licensed drivers who are over 100 years old.  -The Globe