DATA PROCESSING APPARATUS

There is provided a data processing apparatus capable of increasing a number of general purpose registers while maintaining upper compatibility. Register designating information for designating a register is divided in two portions. The two portions are arranged in separate basic units on the basic units of an instruction code. When one instruction code is made ignorable and the ignorable instruction code is ignored, a control unit (CONT) executes register selecting operation by implicitly assuming predetermined register designating information. Thereby, when only a general purpose register (existing general purpose register) capable of being designated implicitly is used, the ignorable instruction code can be ignored and accordingly, the instruction codes are not increased. When an at least conventionally equivalent general purpose register is used, a conventionally equivalent instruction code may be used. By preventing the instruction codes from increasing, processing speed is not reduced.