PURPOSE: To reduce a program area by providing in a data processor a means for storing coded data a means for arithmetic processing which corresponds to a data form according to its storage contents.

CONSTITUTION: When a data form setting instruction is executed, a data form specified in the instruction is coded and outputted to an internal data bus 1 and at the same time, a write control signal 3 to a code storage circuit 2 is generated to fetch coded data form information on the bus into the code storage circuit 2. The information written in the circuit 2 is held until the data form setting instruction is executed, and then supplied, as it is, to an instruction execution control circuit 4. According to the data form information, this circuit 4 performs arithmetic processing which corresponds to a data form specified by an arithmetic processing circuit 5. Consequently, the instruction system of the data processor is made simple and regular, and simular processing parts in a program are made common to reduce the capacity of a program memory.