PURPOSE: To extend a user data part and a user stack part by using the same control program by equalizing the extending directions of an allocate data part and the user stack part.

CONSTITUTION: The user space extending system is constituted of a whole user data part 1, a load data part 2 within it, the extending direction 3 of the allocate data part, the user stack part 4, and the extending direction 5 of the user stack part 4. Then, when a program is loaded on a main storage, its initial data is loaded to the load data part 2 in the user data part 1, and besides, an initial user stack area is taken for the user stack part 4, and afterwards, when the request of the extension of the data area is issued during the execution of the program, the allocate data part is extended toward the extending direction 3. On the other hand, the user stack part 4 is extended to the same direction 5 as the direction 3 as occasion demands with the execution of the program, and since the extending direction is equal, the extension can be controlled by the same program.