A computer system, having a non-volatile storage unit (152), a main storage unit (151), and a data processor (102) including a memory management unit (102A) for managing a program stored in the non-volatile storage unit and the main storage unit to transfer a program stored in the non-volatile storage unit to the main storage unit, wherein the memory management unit (102A) includes a program storage control function of storing a program subjected to predetermined data conversion and a program yet to be subjected to predetermined data conversion in the non-volatile storage unit, and a function of combining programs subjected to predetermined data conversion so as not to bridge over a boundary between blocks at the execution of the program storage control function, as well as, at a first access to a certain block, expanding all the data included in the block to a corresponding block of the main storage unit.