Extended Memory with Teraspace Shared Memory Segments (iSeries)


Extended memory is the core of the SAP memory management system. Each SAP work process has a part reserved in its virtual address space for extended memory. You can set the size using the profile parameter em/initial_size_MB.


You can map the extended memory from the common resource (shared memory segment) to any work process (see graphic) at the same address in the virtual address space. This is important if you work with pointers in the ABAP program.


The following graphic shows two work processes that are accessing a common resource.

The common resource (teraspace shared memory segments) contains all the user contexts and is always mapped completely in the teraspace address space of each work process.

The graphic shows a context change or transfer:

·        The context for user x, mode y is made available in the extended memory of work process 1. The work process processes an inquiry of this context.

·        Analogous to this, the context for user n, mode m is made available in work process 2.

·        After processing, both work processes "put" the (changed) context back in the common resource by releasing the context again.

·        Then work process 1 can process the context for user n, mode m, and work process 2 can process the context for user x mode y.

The start address for the extended memory in teraspace is the same for all work processes.


The SAP system builds a layer on to the operating system functions for the page management of the extended memory. The extended memory is implemented as teraspace shared memory. This means the address space uses unprotected storage of the operating system as background memory. For more information, see the teraspace documentation for an iSeries operating system.