PEP 238 (revised)

[Bengt Richter]

On Mon, 06 Aug 2001 09:42:24 GMT, bokr at accessone.com (Bengt Richter) wrote:

>On Mon, 6 Aug 2001 01:41:15 -0400, "Tim Peters" <tim.one at home.com> wrote:
>
>>[Bengt Richter]
>>> UIAM, the way I read it, the only way you can get -0 from multiply
>>> is as a result of extreme underflow denormalizing to zero. Others
>>> give plain zero.  Of course, my intel pentium book is not the official
>>> ieee754 spec.
>>
>>I can't guess what you're reading.  Here from the FMUL docs, in the IA-32
>>Intel Architecture Software Developer's Manual, Volume 2: Instruction Set
>>Reference, available at, e.g.,
>>
>Sorry about the guesswork, specifics below ;-)
>
>>http://developer.intel.com/design/pentium4/manuals/245471.htm
>>
>>The last (byte-serving) link will get you to it quickly w/o enduring a 6.7MB
>>download of the whole thing:
>>
>>    The sign of the result is always the exclusive-OR of the source
>>    signs, even if one or more of the values being multiplied is 0 or
>>    [a graphic for infinity].
>>
>>Same as the 754 rules.
>>
>
>I was looking at (hardcopy)
>"Pentium Processor Family Developer's Manual
>Volume 3: Achitecture and Programming Manual
>1995"
>
>On page 7-11, "Table 7-8. Zero Operands and Results (Contd.)"
> +------------------+---------------------+-----------------------+
> | Operation        |       Operands      |     Result            |
> +------------------+---------------------+-----------------------+
  | Multiplication   | ±0 x ±0             | 0                     | Left out the op ;-/
> |                  | ±0 x ±0             | 0                     |
> |                  | ±0 x ±X, ±X x ±0    | 0                     |
> |                  | +X x +Y, -X x -Y    | +0[1]                 |
> |                  | +X x -Y, -X x +Y    | -0[1]                 |
> +------------------+---------------------+-----------------------+
>...
>[1] When extreme underflow denormalizes the result to zero
>
>I read the unadorned zeroes under results to mean the all-bits-zero
>representing zero (which is also a positive zero).
>
>(I hope '±' came through above, it's the symbol plus/minus, or
>'+' over '-' in the same character symbol).
>
>The instruction description for FMUL/FMULP/FIMUL on page 25-118
>does not help resolve what "0" above means. It does not mention
>the XOR sign rule.
>
>I did follow with a second post containing
>
>"My intel pentium book is plainly wrong, ignore previous doubt about *1.0 ;-)
> >>> math.atan2( 0.0,-0.0*1.0)
> 3.1415926535897931
> >>> math.atan2( 0.0, 0.0*1.0)
> 0.0"
>
>Since plainly there is a different effect for -0.0*1.0 and 0.0*1.0,
>both of which would seem to be covered by the second line in the table,
>unless Python is interfering, which I wouldn't think.
>
>It does that with python 1.5.2 on a '95-vintage P90 as well as on a more
>recent 300mhzPII with python 2.1, so I'd say the book is unclear at best ;-/
>
>Thanks for the intel URL. Hard copy can't compete up-to-datedness-wise.
>Although I my browser/acrobat combo just wants to download the whole thing.
>How do you get around that again?