Nature exponential function computing device and computing method thereof

A computing method adapted to a nature exponential function computing device, wherein the nature exponential function computing device performs natural exponential function computation on an input value conforming to a floating point number format, wherein the nature exponential function computing device comprises a first lookup table, a second lookup table, and an operation circuit, wherein the computing method comprises:

• storing a plurality of first natural exponent values by the first lookup table, and storing a plurality of second natural exponent values by the second lookup table;
• selecting a selected first natural exponent value from the plurality of first natural exponent values by the first lookup table according to an input exponent part of the input value;
• selecting at least one selected second natural exponent value from the plurality of second natural exponent values by the second lookup table according to the input exponent part and an input mantissa part of the input value, wherein the plurality of second natural exponent values are generated according to different powers of a natural constant, wherein the plurality of powers are respectively n powers of 2, wherein n is an integer;
• and
• operating on the selected first natural exponent value and the at least one selected second natural exponent value by the operation circuit to generate an output exponent part and an output mantissa part conforming to the floating point number format.