SAP 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 (see Virtual Address Space in a Work Process). You can set the size of extended memory using the profile parameter em/initial_size_MB: Extended Memory Pool Size. Under Windows, further memory is assigned dynamically as needed, and you can also set this amount.
You can map the extended memory from the common resource onto any work process, and after onto another process (see graphic) on the same address in the virtual address space. This is important if you work with pointers in the ABAP program.
Depending on the operating system, how you implement the extended memory is different. Platform-Specific Description of Memory Management
The section Implementation explains the implementation for the different platforms in more detail.
The SAP system builds a layer on to the operating system functions for the page management of this memory. This extended memory is implemented as an unnamed mapped file. This means the address space uses the paging file or uses the swap space of the operating system as background memory.