Francisco Javier Mesa-Martinez
lefa at ucsc.edu
Tue May 6 15:56:18 CDT 2003
On Tue, 6 May 2003, Jochen Kunz wrote:
> My thought was that they have a (slightly) other architecture but
> carried on the FP format. Like Alpha inhirited some VAX float formats.
The Cray FP format is designed for speed over precision. IEEE is designed
for precision with reduced range. CRAY FP can be fun sometimes, i.e. it is
like a box of chocolates you never know what you are gonna get.
The IEEE rules are pretty strict, rounding, normalization and underflow
can be pretty slow due to HW requirements. Very few micros do actually
implement full IEEE 754 support. BTW I believe IEEE FP is based on Intel's
work. Cray's approach was to make the format map into a design that would
have the shortest critical paths possible, so you get speed but you sacrifice
things like predictable rounding. So you have to take Cray with a grain of
salt, i.e. lots of pre and post processing required.
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