Abstract: A method for managing virtual address space in which programs designed for smaller virtual address spaces in the multiple virtual memory scheme can be collectively allocated to a single enlarged virtual address space in the single virtual memory scheme efficiently, without requiring any change in the programs themselves, such that the entire virtual address space becomes available in the compatible mode. In the method, the effective addresses to be used during an execution of a program designed for a smaller address space are calculated by combining an appropriate address base for this program specifying upper bits of the effective address and lower bits of the virtual addresses for a region of the enlarged address space to which this program is allocated. The address base may be replaced by the upper bits of the starting address of the program in the enlarged address space which are retained throughout the effective address calculation.