PURPOSE: To decrease the number of processing steps by using a single bit of an idle field of a microcode to add +1 and -1 to the 15th bit of a computing element.

CONSTITUTION: A digital signal processor consists of a program ROM 1, a data ROM 2, a RAM 3, an address counter 4, saving registers 5 and 6 for counter 4, the storing (output) registers 12 and 13 for arithmetic result of a computer element 11. A microcode for this arithmetic includes a field to prescribe the contents of the arithmetic to be processed and a field to prescribed the contents to be used for operations. Then a single bit of an idle field of the microcode is used to control +1 and -1 to be added to the 0-th bit as well as +1 and -1 to be added to the 15th bit respectively.