Subject: Re: 240/260 CPU hacking....
To: Toru Nishimura <nisimura@itc.aist-nara.ac.jp>
From: John Maier <jmaier@midamerica.net>
List: port-pmax
Date: 03/27/2001 20:29:22
> People who are familiar with the evolution of Personal Iris, Indigo
> and Indy product family would have some clue about that.  Sillicon
> Graphics dumped all of R3000 products and switched whole to R4x00.
> R4000 upgrade was done by entire PCB replacement.  However, the
> hardware subcomponents of Indigo/Indy followed continuous design
> modification path in priciple, probably, to maintain SGI's awesome
> software product assets.

I'm sure that when SGI/MIPS designed the R4x00, they had the R3000
replacement in mind, thus the 32bit mode.  Also considering the
future of a true 64 bit SGI hardware/IRIX.

I need to check but I think I read, in 64bit hardware mode, the IRIX loads
the HI and LO words of a 64bit data/instruction with the 32bit
code/data of a 32bit app, thus being able to run 32bit apps even faster
than a 32bit 2000/3000 or 4000(32bit hardware mode) x2 time faster.

(don't hold me to that, I may be thinking of the Z800 which uses a similar
idea :-)

>
> IRIX eventually evolved into a stable 64bit OS which runs 32bit
> applications, then NetBSD could do.

Sure, the R4x00, even in 64bit mode can run 32bit MIPS II code.  The
problem is, if the R4x00 is initalized in 32bit mode (via a small serial
PROM) it can't use it's 64bit MIPS III instuctions efectively.  i.e. running
64bit instructions and data on a 32bit bus requires two words per
instruction.
This would be the biggest stumbling block in getting a NetBSD 64bit running
on a /260.
Well not stubling block but it would run slow (half as fast as 32bit
instuctions).

However, I agree that a true 64bit architecture MIPS based computer would be
able to
run 32bit MIPS apps with out problem, if not faster that a 32bit MIPS
counter part.

It's really all quite ingenious how they designed the R4x00 to be a
chameleon, it's
a 32bit and 64bit machine all at once.  The more I read, the more respect I
have for it.

John Maier
Administrator
Midamerica Internet Services
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