Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!amdcad!sun!pitstop!sundc!seismo!uunet!munnari!otc!metro!ditsyda!megadata!eve!andrew
From: andrew@eve.oz (Andrew McRae)
Newsgroups: comp.arch
Subject: Re: RISC & context switches
Message-ID: <103@eve.oz>
Date: 15 Feb 89 00:59:07 GMT
References: <89Feb12.125852est.10867@ephemeral.ai.toronto.edu>
Organization: Megadata P/L, North Ryde, Sydney, Aust.
Lines: 33

From article <89Feb12.125852est.10867@ephemeral.ai.toronto.edu>, by bradb@ai.toronto.edu (Brad Brown):
>> [ Discussion about multiple register sets ]
> 
>This is not a bad idea if you have the silicon to do it (as other posters
>have pointed out.)  Actually it's been used in some designs.  The most
>interesting is actually the IBM 8100, a fast transaction processing machine
>which is kind of old and has now been discontinued.

The Concurrent Computer Corp. 3200 series has multiple register
sets (up to 16), the operative set selected by a 4 bit field in the
processor status register. Generally these are not used to speed
context switching between processes, but to allocate one set to the
user level processes, and the other sets to the OS. Each interrupt
level had a different register set, so that no register saving had
to occur at interrupt service time, and there was no need to save
user registers during kernel operations.

This register swapping was
tied up with the architecture (e.g. at interrupt time some of
the registers had useful values stored in them by the microcode
such as vector number, device address, previous status/program
counter etc). Not being a stack based machine, the register
swapping tended to be fundamental to the way the OS did things (I'm
speaking of course for the native OS/32, not Unix).

I'm suprised that this idea (different register sets for different
interrupt levels) has not been taken up in some of the more modern
architectures, but there are problems (try doing a splX, and see
if your registers hold the same values...). 

Andrew McRae			inet:	andrew@megadata.oz{.au}
Megadata Pty Ltd,		uucp:	..!uunet!munnari!megadata.oz!andrew
NSW    AUSTRALIA

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