Re: DECstation 2100 floating point

[George Harvey <fr30%dial.pipex.com@localhost>]

On Mon, 21 Mar 2011 20:40:14 +0000
George Harvey <fr30%dial.pipex.com@localhost> wrote:

> I've been giving 5.1 a try on my DECstation 2100 (PMIN, R2000 CPU). I
> had a bit of trouble finding a disk that the ROM would boot from but
> apart from that, the network install ran smoothly. However, running a
> few test programs, I'm seeing a lot of floating point errors. For
> example, the 'flops' benchmark from pkgsrc returns mostly 'nan' and
> 'inf' values (see sample output below). If I run the same tests on my
> 5000/133 (3MIN, R3000 CPU), then they work normally so I'm wondering
> what the difference is between R2000 and R3000 floating point behaviour?

A few more details...

If I compile 'flops' with no optimisation then it works on my 2100, if
I use -O or -O2 then it produces 'nan' and 'inf' results. However, if I
copy the binaries over to my 5000/133 then both optimised versions work
normally. So it looks as though GCC is generating code that works on a
R3000/R3010 combo but doesn't work on a R2000/R2010. I've had a hunt
round the 'net and the R2000 and R3000 are supposed to be
instruction-set compatible so I don't understand why I'm seeing
different results.

George