Re: precise floats

[Gene Ouye <geneo@rational.please_no_unsolicited_mail.com>]

Robert Dewar wrote:
 [...]
> However, the above definitions are most certainly inadequate on the x86.
> This machine fully supports the 80-bit floating-point format corresponding
> the optional IEEE extended type. Indeed all arithmetic in the fpt chip is
> in fact performed in this mode.

I've gone through the 486, Pentium, and Pentium II manuals looking for
something that shows the 80-bit floating-point format is "fully
supported" and I don't see it.  True, all FP operations are performed on
numbers internally converted to the 80-bit format, and it is possible to
load and store 80-bit FP numbers, but all the other FP operations FP
numbers can only take operands from memory that are 64 bits or smaller
(eg, FADD, FCOM, FCOMP, FDIV, FDIVR, FMUL, FSUB, FSUBR).

 [...]
> Note: some people sometimes get confused into thinking that somehow the
> 80-bit format is "reserved" and should not be used. This derives from the
> intention in the IEEE-754 standard that the extended format be used for
> the purposes of getting accurate 64-bit results (e.g. in the log/exp
> case above).

I am not confused between the IEEE-754 standard and the Intel
Architecture Software Developer's Manual Volume 2: Instruction Set
Reference or the Pentium Processor Family Developer's Manual Volume 3:
Architecture and Programming Manual.  Those Intel manuals and others I
have examined do not make any claims that the 80-bit FP type is "fully
supported", and an examination of all the operations available to the
Pentium or 486 programmer does not lead me to the conclusion that the
80-bit FP type is "fully supported".

What am I missing?

Gene Ouye