Understanding the use of central storage

MVS™ programs and data reside in virtual storage that, when necessary, is backed by central storage. Most programs and data do not depend on their real addresses. Some MVS programs, however, do depend on real addresses and some require these real addresses to be less than 16 megabytes. MVS reserves as much central storage below 16 megabytes as it can for such programs and, for the most part, handles their central storage dependencies without requiring them to make any changes.

The system uses the area of central storage above 16 megabytes to back virtual storage with real frames whenever it can. All virtual areas above 16 megabytes can be backed with real frames above 16 megabytes. In addition, the following virtual areas below 16 megabytes can also be backed with real frames above 16 megabytes:

SQA
LSQA
Nucleus
Pageable private areas
Pageable CSA
PLPA
MLPA
Resident BLDL

The following virtual areas are always backed with real frames below 16 megabytes:

V=R regions
FLPA
Subpool 226
Subpools 227 and 228 (unless specified otherwise by the GETMAIN macro with the LOC parameter)

When satisfying page-fix requests, MVS backs pageable virtual pages that reside below 16 megabytes with central storage below 16 megabytes, unless the GETMAIN macro specifies LOC=(24,31) or the PGSER macro specifies the ANYWHER parameter. If the GETMAIN or STORAGE macro specifies or implies a real location of 31, MVS backs pageable virtual pages with real frames above 16 megabytes even when the area is page fixed.