|European Case Law Identifier:||ECLI:EP:BA:2006:T007404.20060518|
|Date of decision:||18 May 2006|
|Case number:||T 0074/04|
|IPC class:||G06F 7/48|
|Language of proceedings:||EN|
|Download and more information:||
|Title of application:||Floating-point processor with operand-format precision greater than execution precision|
|Applicant name:||Koninklijke Philips Electronics N.V.|
|Relevant legal provisions:||
|Keywords:||Inventive step (no)|
Summary of Facts and Submissions
I. This appeal is against the decision of the examining division to refuse European patent application No. 98117383.4.
II. The following documents will be referred to in the present decision:
D2: M. Darley et al., "The TMS390C602A Floating-Point Coprocessor for Sparc Systems", IEEE MICRO June 1990, p.36-47.
III. According to the decision appealed, taken on the file as it was standing and with reference to the previous official communications, the invention according to all claims was obvious in view of a combination of D1, regarded as the closest prior art document, with D2. The only difference between the invention and D1 was that software trap means were used for performing floating-point operations at the highest precision. Such traps were however conventional, as evidenced by D2, and thus a matter of normal design procedure.
IV. The decision was based on claims 1-8 as amended in accordance with the appellant's letter dated 24 February 2003. Independent claim 5 read:
" A floating-point method for performing a requested floating-point operation at a requested result precision on an operand set of operands to achieve a result, said requested result precision being selected from a set of plural precisions including a highest precision, said method comprising:
storing (S1) said operands in a highest precision format in respective registers of a floating-point processor;
determining (S2) an execution precision as a function of said requested precision and the maximum apparent precision of said operand set, said maximum apparent precision being a function of the number of trailing zeroes in the operand having said maximum apparent precision;
if said execution precision is less than said highest precision, performing (S3) said requested floating-point operation using a hardware execution unit of said floating-point processor; and
characterized by the steps of performing nominal execution at a precision lower than the format precision with which it stores operands;
if said execution precision is said highest precision, executing (S4) a trap and performing said requested floating-point operation in software."
Claim 1 was directed to a corresponding floating-point processor.
V. In the grounds of appeal the appellant argued essentially as follows:
Dl disclosed a floating-point processor that performed operations at three execution precisions, referred to as single, double and extended precision. An input format converter converted single or double precision source values to extended precision format but left extended precision format source values in extended format. The resulting operands stored in respective registers of the processor were in extended precision format. The processor furthermore had an execution unit that performed the operations at the precision determined by a mode selector. Independent of whether the mode selector selected single, double or extended precision, it was always the execution unit of the floating-point processor that executed the operation (p.8, l.15-22). Therefore Dl taught that the source values were stored in the registers of the execution unit at a precision equal to the highest execution precision at which the corresponding execution unit could perform an operation. In most applications the greater precisions were used more rarely than the lesser precisions. Extended precision, in particular, was encountered very infrequently in many computational environments, and it was therefore a disadvantage of the prior art floating-point processor according to Dl that it comprised a relatively complex execution unit, capable of executing extended precision operations.
D2 taught a person skilled in the art, faced with the problem of the prior art processor, that extended precision operations should be handled by a software trap instead of a dedicated processor. It also taught that the registers used by the dedicated processor should store source values in the double-precision format, ie the same precision as the highest execution precision at which the corresponding execution unit can perform an operation.
Hence, the person skilled in the art combining the teaching of Dl and D2 would arrive at a device comprising:
format conversion means for converting operands of an operand not in the double-precision format to the double-precision format;
register means for storing said operands of the operand set in the double-precision format;
an execution unit for performing an operation on the operands to yield an execution result, in case the required execution precision was not the extended precision;
trap means for executing a trap when the required execution precision was the extended precision.
The floating-point processor so obtained had the disadvantage that extended precision floating point operations were executed considerably slower, because it had no registers for storing extended precision values, and it had to load and store these values from and to the memory.
The floating-point processor according to claim 1 of the present application differed from what the person skilled in the art would obtain from documents Dl and D2 in that the format conversion means converted operands not in the extended-precision format to the extended-precision format. Furthermore, the register means stored the operands in the extended-precision format. On the one hand the invention used trap means for performing extended precision calculations which allowed a simplification of the execution unit and therefore reduced the cost of the implementation. On the other hand, as it could use extended precision registers there was no need to trap on high, eg extended, precision "load" and "store" instructions. This reduced latency and obviated the need for virtual registers in memory for storage of high precision operands and results. There was also a saving in complexity in that tag bits were not required to indicate the precision of stored operands (cf the present application, paragraph ). Consequently, the floating-point processor of claim 1 was faster than one that would result from a straightforward combination of Dl and D2.
Hence, the combination of measures as claimed in claim 1 entailed advantageous technical features while this combination could not be derived in an obvious way from the documents Dl and D2.
VI. By communication dated 18 January 2006, annexed to a summons to oral proceedings, the Board stated that the examining division's argumentation appeared convincing in spite of the appellant's counter-arguments and indicated the reasons for its view.
VII. By letter dated 10 April 2006, the appellant withdrew its request for oral proceedings and requested a decision in accordance with the state of the file.
VIII. Oral proceedings, at which the appellant was not represented, were held on 18 May 2006. It was verified that the appellant requested that the decision under appeal be set aside and a patent be granted on the application documents on file (cf the notice of appeal and the grounds, p.1). After deliberation the Board announced its decision.
Reasons for the Decision
1. The appellant argues that a combination of D1 and D2 would yield format conversion means converting operands to double-precision format, whereas according to the invention - and D1 - conversion is made to extended format. This argument is unusual in that a feature already contained in the closest prior art document, here D1, is said to involve a inventive step, but the point appears to be that the skilled person would have omitted this feature as a consequence of the combination with D2.
2. However, the appellant's reasoning is not convincing. The main advantage of the circuit in D1 is that latency can be reduced, for example when an operand is specified in extended precision (cf D1 p.6, l.9-15; p.9, l.28-30). To this aim, the operand's "apparent precision" is detected (cf p.5, l.9-11; paragraph bridging pages 7 and 8). This requires the operand to be stored at maximum precision, as explained in D1. It is not likely that the skilled person would have left out any of these crucial features when incorporating the teaching of D2 into D1, nor is it clear how the operands with extended apparent precision might otherwise be detected. D2 does not seem to contain any suggestion how to replace the detection of apparent precision. As to the appellant's argument that the prior art teaches that the format precision for storing operands should be equal to the nominal execution precision of the processor, ie double precision in the case of the invention, it is noted that D1 explicitly states that the mode selector determines the maximum precision between the requested result precision and the maximum apparent operand precision (p.5, l.13-15), which is not necessarily equal to the nominal execution precision of the processor. This quotation appears incompatible with the appellant's suggestion that the skilled person might opt for conversion to double-precision format.
3. For these reasons the subject-matter of claims 1 and 5 does not involve an inventive step (Article 56 EPC).
For these reasons it is decided that:
The appeal is dismissed.