Processor storage overview

Processor storage only consists of real storage (formerly called central storage) in the z/Architecture® mode. This section provides an overview of real storage. Note that unlike the combination of central and expanded storage in the ESA/390 environment, expanded storage is not supported in the z/Architecture mode.

The system uses a portion of both central storage and virtual storage. To determine how much central storage is available to the installation, the system's fixed storage requirements must be subtracted from the total central storage. The central storage available to an installation can be used for the concurrent execution of the paged-in portions of any installation programs.

The real storage manager (RSM) controls the allocation of central storage during initialization and pages in user or system functions for execution. Some RSM functions:

Allocate central storage to satisfy GETMAIN requests for SQA and LSQA.
Allocate central storage for page fixing.
Allocate central storage for an address space that is to be swapped in.

If there is storage above 16 megabytes, RSM allocates central storage locations above 16 megabytes for SQA, LSQA, and the pageable requirements of the system. When non-fixed pages are fixed for the first time, RSM:

Ensures that the pages occupy the appropriate type of frame
Fixes the pages and records the type of frame used

Pages that must reside in central storage below 16 megabytes include:

SQA subpool 226 pages.
Fixed pages obtained using the RC, RU, VRC, or VRU form of GETMAIN if one of the following is true:
- LOC=24 is specified.
- LOC=RES, the default, is either specified or taken, and the program issuing the GETMAIN resides below 16 megabytes, runs in 24-bit mode, and has not requested storage from a subpool supported only above 16 megabytes.
Fixed pages obtained using the LU, LC, EU, EC, VU, VC, or R form of GETMAIN.
Storage whose virtual address and real address are the same (V=R pages).

Pages that can reside in central storage above 16 megabytes include:

Nucleus pages.
SQA subpools 239 and 245 pages.
LSQA pages.
All pages with virtual addresses greater than 16 megabytes.
Fixed pages obtained using the RC, RU, VRC, or VRU form of GETMAIN if one of the following is true:
- LOC=(24,31) is specified.
- LOC=(RES,31) is specified.
- LOC=31 is specified.
- LOC=(31,31) is specified.
- LOC=RES, the default, is either specified or taken, and the program issuing the GETMAIN resides above 16 megabytes virtual.
- LOC=RES, the default, is either specified or taken, and the program issuing the GETMAIN resides below 16 megabytes virtual, but runs in 31-bit mode and has requested storage from a subpool supported only above 16 megabytes.
Any non-fixed page.

Note: The system backs nucleus pages in real storage below 2 gigabytes. You can however, back SQA and LSQA pages above 2 gigabytes when you specify LOC=(24,64) or LOC=(31,64).

Each installation is responsible for establishing many of the central storage parameters that govern RSM's processing. The following overview describes the function of each area composing central storage.

The primary requirements/areas composing central storage are:

The basic system fixed storage requirements — the nucleus, the allocated portion of SQA, and the fixed portion of CSA.
The private area fixed requirements of each swapped-in address space — the LSQA for each address space and the page-fixed portion of each virtual address space.

Once initialized, the basic system fixed requirements (sometimes called global system requirements) remain the same until system parameters are changed. Fixed storage requirements (or usage) will, however, increase as various batch or time sharing users are swapped-in. Thus, to calculate the approximate fixed storage requirements for an installation, the fixed requirements for each swapped-in address space must be added to the basic fixed system requirements. Fixed requirements for each virtual address space include system storage requirements for the LSQA (which is fixed when users are swapped in) and the central storage estimates for the page-fixed portions of the installation's programs.

The central storage for the processor, reduced by the global fixed and paged-in virtual storage required to support installation options, identifies the central storage remaining to support swapped-in address spaces. The total number of jobs that can be swapped in concurrently can be determined by estimating the working set (the amount of virtual storage that must be paged in for the program to run effectively) for each installation program. The working set requirements will vary from program to program and will also change dynamically during execution of the program. Allowances should be made for maximum requirements when making the estimates.