Patent US 5,329,631

A microprocessor system is provided for extending an address space. To effectively use a program in the original address space and to reduce the processing time to return from the extended address space to the original address space, the microprocessor system includes a basic memory having a first address space for storing at least one program and a second address space for storing data. In addition, the microprocessor system provides an extended memory having an extended address space used in place of a part of the first address space, for storing at least one additional program and an address extending hard register for storing a flag and extending addresses of the extended memory. The flag indicates which part of the first address space and the extended address space is to be used, and the extending addresses being used to designate the extended memory when the flag indicates that the extended address space is to be used. Further, the microprocessor system includes a microprocessor, operatively connected to the basic memory, to the extended memory, and to the address extending hard register, for generating an address signal and for processing data. The address signal either the basic memory or the address extending hard register, and the program stored in the part of the first address space is transferred by the microprocessor to the second address space when the flag indicates that the part of the first address space is to be used.