Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ukma!rutgers!cmcl2!yale!mfci!colwell
From: colwell@mfci.UUCP (Robert Colwell)
Newsgroups: comp.arch
Subject: Re: RISC & context switches
Message-ID: <640@m3.mfci.UUCP>
Date: 11 Feb 89 17:23:39 GMT
References: <784@atanasoff.cs.iastate.edu>
Sender: colwell@mfci.UUCP
Reply-To: colwell@mfci.UUCP (Robert Colwell)
Organization: Multiflow Computer Inc., Branford Ct. 06405
Lines: 37

In article <784@atanasoff.cs.iastate.edu> hascall@atanasoff.cs.iastate.edu (John Hascall) writes:
>
>  One of the benifits of a simple instruction set (RISC) is that it
>  frees up chip area for more registers.  I think some papers have
>  proposed register counts > 100, what is the largest number of
>  general purpose registers in an existing chip?

You're apparently talking about single-chip micros.  That's not the
only domain in which RISC/CISC concepts are interesting, and I think
once you leave the single-chip domain, your premise isn't obviously
correct.

>  I seem to recall there was (is?) a TI processor which had all of
>  its registers in memory except 1 register which pointed to
>  the other registers, so a context switch was just save/restore
>  that one register.  Could a similar concept be implemented
>  with all the registers in the chip?

I think this was the TI 9900, the first 16-bit micro, which for some reason
didn't seem to catch on very well.  It did indeed have all its registers in
main memory.  And this isn't as dumb an idea as it first appears -- you need
far fewer address bits to refer to a register than to memory addresses, so
having "registers" that reside in memory is still better than no "registers"
at all.  The BellMac-8 microprocessor from Bell Labs, ca 1977-1979, borrowed
this idea.  I'm not sure of the TI chip, but Bell's also had the overlapped
sliding register window for parameter-passing that later showed up again in
the RISC-I from Berkeley.

The Bellmac-8 also had one of the nicest assemblers I've seen -- had lots of
high level constructs like if-then-else, while, do-until, switch, etc.  If
you really wanted one-for-one mapping of code to machine you didn't have to
use those features, but it was often very nice to have them.

Bob Colwell               ..!uunet!mfci!colwell
Multiflow Computer     or colwell@multiflow.com
175 N. Main St.
Branford, CT 06405     203-488-6090