Virtual storage: The virtual storage address space is 2 gigabytes. The address space contains the commonly addressable system storage, the nucleus, and the private address space, which includes the user's region.
When a program is selected, the system brings it into virtual storage and divides it into pages of 4K bytes. The system transfers the pages of a program into central (real) storage for execution and out to auxiliary storage when not needed. Paging is done automatically; to the programmer, the entire program appears to occupy contiguous space in central storage at all times. Actually, not all pages of a program are necessarily in central storage at one time. Also, the pages that are in central storage do not necessarily occupy contiguous space.
Central (real) storage: Certain programs must have all their pages in contiguous central (real) storage while they are executing. They cannot be paged. These programs must be put into an area of virtual storage called the nonpageable dynamic area, whose virtual addresses are identical to real addresses.
Such programs are the only ones for which you should request central storage. To request central storage, code ADDRSPC=REAL on the JOB or EXEC statement and request the amount of central storage needed in a REGION parameter.