7 Nov 1998 01:29:42 -0500

Related articles |
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[6 earlier articles] |

Re: floating point darcy@CS.Berkeley.EDU (Joseph D. Darcy) (1998-10-19) |

Re: floating point darcy@usul.CS.Berkeley.EDU (1998-10-24) |

Re: floating point comments@cygnus-software.com (Bruce Dawson) (1998-11-01) |

Re: floating point comments@cygnus-software.com (Bruce Dawson) (1998-11-01) |

Re: floating point darcy@usul.CS.Berkeley.EDU (1998-11-06) |

Re: floating point darcy@CS.Berkeley.EDU (Joseph D. Darcy) (1998-11-06) |

Re: floating point comments@cygnus-software.com (Bruce Dawson) (1998-11-07) |

Re: floating point eggert@twinsun.com (1998-11-19) |

Re: floating point dmcq@fano.demon.co.uk (David McQuillan) (1998-11-21) |

Re: floating point darcy@CS.Berkeley.EDU (Joseph D. Darcy) (1998-12-01) |

Floating Point jimp@powersite.net (Jim) (1999-11-02) |

From: | Bruce Dawson <comments@cygnus-software.com> |

Newsgroups: | comp.compilers |

Date: | 7 Nov 1998 01:29:42 -0500 |

Organization: | Cygnus Software |

References: | 98-09-164 98-10-018 98-10-040 98-10-120 98-11-015 98-11-031 |

Keywords: | arithmetic |

Joseph D. Darcy wrote:

*> When the x86's rounding precision is set to double, the answer is*

*> rounded to 53 significand bits (with extended exponent range).*

*> Likewise, when the rounding precision is set to float, only 24 bit*

*> significands are generated.*

Ah. My understanding was that when you set the rounding mode to double

the processor internally rounded the result to 64-bits and then to

53. This behaviour would mean that double rounding could occur even

when the precision was set to double and no values were written to

memory. However it looks like my understanding was wrong - I certainly

couldn't find any support for it in the Intel manuals. So I'll have to

partially retract my statement about nobody being happy with the x87 -

it doesn't implement double precision as badly as I had feared, since

the only unavoidable problem if you set the rounding to double is the

exponent range - which will rarely matter.

On a side note that I don't pretend to know the significance of, I

found this statement in the Pentium(r) manual:

Pentium(r) Processor Family Developer's Manual, Vol. 3, 6-11:

The precision-control bits affect the results of only the following

five arithmetic instructions: ADD, SUB(R), MUL, DIV(R), and

SQRT. No other operations are affected by PC.

Odd that the other arithmetic instructions are not affected.

--

Bruce Dawson, Cygnus Software Author of Fractal eXtreme for Win32

Get your 15 day trial copy of FX at: http://www.cygnus-software.com/

Write me at: comments@cygnus-software.com

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