Selecting central storage backing
Virtual storage is first backed by central storage when it is:
- Referenced
- Obtained from a fixed storage subpool
- Page-fixed by a program using the PGSER macro
- Obtained from a pageable storage subpool (only in some cases).
When the storage is fixed, either implicitly through the subpool attribute, or explicitly through PGSER FIX, the LOC parameter, specified on the GETMAIN STORAGE, or CPOOL macro invocation used to obtain the storage, determines where the storage will be backed in central storage. Storage which is not fixed can be backed anywhere in central storage.
The STORAGE OBTAIN PAGEFRAMESIZE1MB option specifies to back storage by 1MB large page frames, if available.
The system will back the storage below 16 megabytes only if one
of the following is true:
- You specify LOC=24 when obtaining storage and the subpool is supported below 16 megabytes, and the subpool is not an SQA or LSQA subpool.
- You specify LOC=RES either explicitly or by default, your program resides below 16 megabytes, and the subpool is supported below 16 megabytes, and the subpool is not an SQA or LSQA subpool.
- You specify subpool 226, which is always backed below 16 megabytes.
You should not need central storage backing below 16 megabytes unless your program requires 24-bit real addresses.
The system may back fixed
virtual storage above 2 gigabytes if the subpool is not 226, and you
specify one of the following:
- LOC=(24,64)
- LOC=(RES,64)
- LOC=(31,64)
- LOC=(31,PAGEFRAMESIZE1MB)
- LOC=(EXPLICIT,64)
- LOC=(EXPLICIT,PAGEFRAMESIZE1MB)