Patent ID: 8694567

Claim:
An arithmetic unit comprising: an input unit which receives an argument of a function represented in a floating-point format that includes an exponent field and a mantissa field; a converter which converts the exponent field included in the input argument according to an exponent conversion rule defined in accordance with the function; a storage which stores a table that includes a plurality of indexed values obtained by approximating the mantissa field included in the argument according to a mantissa conversion rule defined in accordance with the function; a retrieving unit which derives two indexes of the table, namely, a value approximating at least the mantissa field of the input argument by high-order bits of the mantissa field and a value obtained by adding 1 to the approximate value, and which retrieves two values from the table stored in the storage, by using the two indexes thus derived; and a deriving unit which derives a return value of the function represented in the same floating-point format as that of the argument, in accordance with the converted exponent field and the two values retrieved, and which outputs the return value, wherein the number of indexes in the table stored in the storage is defined to be a sum of 1 and the maximum number taken by the approximate value of at least the mantissa field of the argument, by adding an index obtained by adding 1 to the maximum approximate value of at least the mantissa field of the argument, a curve that connects true solutions between the respective two indexes acquired by the retrieving unit is a concave downward curve, and a straight line that connects the respective values for the two indexes stored in the storage is located below the curve, and the curve and the straight line are closest to each other at a location other than the two indexes.