-------------BEGINNING OF IR---------------- VHDL Issue Number: 2070 Language_Version VHDL-2002 Classification LRM Terminology, Grammar and Typographical Errors Summary Support for floating point denormal numbers Relevant_LRM_Sections Section 7.2.7 paragraph 3 and 3.1.4 paragraph 6 conflict with one another. Related_Issues Key_Words_and_Phrases IEEE 854, 754, floating point, denormal Authors_Name David Bishop Authors_Phone_Number 585-726-6788 Authors_Fax_Number Authors_Email_Address dbishop@vhdl.org Authors_Affiliation Eastman Kodak Authors_Address1 2400 Mt Read Blvd Authors_Address2 Rochester NY 14650-3006 Authors_Address3 Current Status: Analyzed Superseded By: ------------------------ Date Submitted: 28 July 2005 Date Analyzed: 26 October 2005 Author of Analysis: Chuck Swart Revision Number: 1 Date Last Revised: 26 October 2005 Description of Problem ---------------------- In some simulators 2.0**-1025 results in 2**-1025 (a denormal 64 bit IEEE 754 (854) floating point number. In others, it becomes zero. Note that this is very similar to the famous "Pentium bug", which is now built into the IEEE 1076-2002 LRM.... In a floating point number, if the exponent is zero, by IEEE 854 the number can become "denormal". This is a mechanism in floating point which allows you to express very small numbers (3.1.4 paragraph 6). However, in 7.2.7 paragraph 6 it says "if the exponent is negative then the result is the reciprocal of that obtained with the absolute value of the exponent." Proposed Resolution ------------------- Modify section 7.2.7 to refer to IEEE 854 on how to treat an exponent. VASG-ISAC Analysis & Rationale ------------------------------ First, Clause 3.1.4 states that VHDL uses "a representation for all floating_point types except for universal_real that conforms either to IEEE std 754 or to IEEE Std 854..." Note that only the representation is specified. An implementation is not required to use 754 arithmetic. This was intentional, since VHDL does not give access to INF, NAN, etc. Also, its not clear that every computer architecture/OS fully supports all arithmetic requirements imposed by 754/854. Second, the IEEE 754 (and 854) standards require support for +,-,*,/ and square root, but not exponentiation. This means that exponentiation may be implemented by a function, not by hardware. The accuracy of such a function is implementation dependent. Finally, the ISAC interprets the words in clause 7.2.7, including: "if the exponent is negative, then the result is the reciprocal of that obtained with the absolute value of the exponent." as the mathematical description of the operation, not as the specification of the algorithm for implementing exponentiation. In conclusion, the ISAC believes that the results of the exponentiation operator are implementation dependent. VASG-ISAC Recommendation for IEEE Std 1076-2002 ----------------------------------------------- No change to the language is needed. VASG-ISAC Recommendation for Future Revisions --------------------------------------------- No change to the language is needed. -------------END OF IR----------------Received on Wed Oct 26 14:56:45 2005
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