PURPOSE: To express a floating decimal point having the sufficient number of valid digits by expressing a numerical value in a list type data expressing form by the address value of an unused storage space in the storage space of a computer.

CONSTITUTION: When expressing the floating decimal point as a list element, it is necessary for the machine word expression of the floating decimal point in the computer to satisfy a certain condition. This is the condition of setting the position of a mantissa part at the right edge of a machine word and storing valid figures from the left end bit and under such an assumption, the floating decimal point is expressed by utilizing the address value of the unused storage space in the logical storage space. In this case, the unused storage space is a storage space excluding a physical storage space from the logical storage space, and the expression range of the floating decimal point is from a MAX LOGICAL ADDRESS to (MAX LOGICAL ADDRESS)/2+MAX PHYSICAL ADDRESS, namely, it has the half size of the logical storage space.