Re: CRAY

"glen herrmannsfeldt" <gah@[EMAIL PROTECTED]
> wrote in message
news:P7ednZuBjMKrsRzVnZ2dnUVZ_gWdnZ2d@[EMAIL PROTECTED]
> robin wrote:
>
> (snip, someone wrote)
>
> >>When you multiply two 48 bit values (the Cray
> >>mantissa) you potentially get a 96 bit product.  The
> >>Cray hardware only computed about 60 bits in the product
> >>and added in a couple of "statistical" round to make up
> >>for the missing 30 or so bits.  This was then trimmed down
> >>to 48 bits.  It was accurate to 48 bits in the majority
> >>of cases.  I think it was always accurate to 46 or so
> >>bits.  Whether that is foolish or broken is a judgment
> >>call.
>
> > I'd call it broken.
> > Makes it difficult to get integer results.
> > Makes it difficult to get a double precision product.
>
> There are many algorithms where the result depends on the
> average precision of a large number of steps, and many of
> those were popular on Cray machines.

Multiplication produces an exact result.  Always.
That exact result can be truncated or rounded when
the number of product bits exceed requirements.
It is unlike division, which may not produce an exact result.

>  When the choice was
> to run the problem, or nothing at all, one took what was
> available.   The Cray-1 single precision was 64 bits with,
> I believe, 15 for exponent.  I believe double precision
> in software, probably to satisfy the Fortran standard.

As I said, having a wrong result for single precision
makes it difficult to obtain double precision result.