[Beowulf] Question about amd64 architecture and floating point operations
hahn at physics.mcmaster.ca
Wed Nov 22 09:35:43 EST 2006
> and sun studio 11. The architecture has 64 bits integer registers and 128
> bits floating point registers,
no: it has SIMD registers which can be used as 2x64 or 4x32 FP
(or 16x8, 8x16, 4x32, 2x64 int).
> but my test programs in C just gave me the
> same precision that I got with an old athlon 2400 xp (32bits), that is, long
> double go only to 1x10^ 4961, even with the -m64 flag. I always imagined
> that I would get the double precision without the long double declaration
> (or, maybe, 40bits precision). What am I missing here?
eh? doubles always have 56b of precision (mantissa), but your 1x10^4961
refers only to the range of the exponent. for instance, you could dream up
an FP format that had 2 bits of mantissa and 126 bits of exponent, useless
though it would be...
if you are really after precision, you probably want to use x87-specific
80-bit FP, which has 64b mantissa iirc. again, that's orthogonal to the
range of exponent.
Beowulf mailing list, Beowulf at beowulf.org
To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
More information about the Beowulf