Putting application programs above the 16MB line

CICS® Transaction Server for z/OS®, Version 3 Release 1 keeps RMODE(ANY) application programs in the EDSA, which is in MVS™ extended virtual storage above the 16MB line. Work areas associated with the programs may also reside above the 16MB line.

Effects

It is possible to LINK or XCTL between 31-bit mode programs and 24-bit mode programs. You can convert programs to 31-bit mode programs and move them above the 16MB line to the extended private area. Moving programs above the 16MB line frees that amount of virtual storage below the 16MB line for other use.

See Using modules in the link pack area (LPA/ELPA) for information on using programs from the LPA or extended link pack area (ELPA).

Using the ELPA is usually better than using the extended private area when multiple address spaces are employed, because the program is already loaded when CICS needs it, and real-storage usage is minimized.

When running a CICS system with transaction isolation enabled, performance benefits can be gained by moving transactions and application programs above the line. Program work areas are then obtained from the EUDSA with a 1MB pagesize rather than the UDSA which has a 4KB pagesize.

Where useful

This facility is useful where there is demand for virtual storage up to the 16MB line and there is sufficient real storage.

Limitations

Because the purpose of using virtual storage above the 16MB line is to make the space below this available for other purposes, there is an overall increase in the demand for real storage when programs are moved above the 16MB line.

There is a restriction on the use of COMMAREAs being passed between programs running in 31-bit addressing mode and programs running in 24-bit addressing mode. COMMAREAs passed from a 31-bit program to a 24-bit program must be capable of being processed by the 24-bit program, therefore they must not contain 31-bit addresses: addresses of areas that are themselves above the 16MB line.

How implemented

Programs that are to reside above the 16MB line must be link-edited with the AMODE(31),RMODE(ANY) options on the MODE statement of the link-edit.

Related tasks
Virtual and real storage: performance considerations
Tuning CICS virtual storage
Splitting online systems: virtual storage
Setting the maximum task specification (MXT)
Using transaction classes (MAXACTIVE) to control transactions
Specifying a transaction class purge threshold (PURGETHRESH)
Prioritizing tasks
Adjusting the limits for dynamic storage areas
Using modules in the link pack area (LPA/ELPA)
Choosing aligned or unaligned maps
Defining programs as resident, nonresident, or transient
Allocating real storage when using transaction isolation
Limiting the expansion of subpool 229 using VTAM pacing
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