PURPOSE: To form a serial interface with small hardware without deteriorating the executing efficiency of an executing part, by applying the macroservice processing to perform transmission/reception of the serial data.

CONSTITUTION: This serial data processor contains the memory parts 102 and 103 which store programs and various data, an executing part 101 which interprets the contents of programs and executes them, a status register 200 which stores the status information showing the executing state, and a serial interface part 400 which transmits and receives the serial data. The part 101 interrupts execution of a program while holding the status information showing the program executing state into the register 200 and transmits and receives the serial data by a serial data processing request signal received from the part 400. Then the part 101 restarts the program execution. The part 400 produces repetitively the processing request signals by the prescribed timing until the transmission and reception of data are through. Thus the serial data are intermittently processed.