Extended Memory Support

The OS/2 Version 2.0 Multiple Virtual DOS Machines architecture provides support for the LIMA Extended Memory Specification Version 2.0 specification, in a similar manner to that provided for LIM EMS Version 4.0, using normal system memory and emulating XMS functions. The following discusses how MVDM support for the extended memory specification has been implemented.

The extended memory specification manages three different kinds of memory:

  • High Memory Area (HMA)

  • Upper Memory Blocks (UMBs) in the Upper Memory Area (UMA)

  • Extended Memory Blocks (EMBs).

    Each of these areas is discussed as they relate to the implementation of expanded memory support for VDMs in OS/2 Version 2.0. Figure "Memory Map of Areas Supported by Extended Memory"below shows where these memory areas or blocks reside in memory.

    For more information regarding the Expanded Memory Specification, refer to DOS Protected Mode Interface.

    The OS/2 Version 2.0 LIMA XMS emulation provides the following functions:

  • Implements all LIMA XMS Version 2.0 functions.

  • Provides each VDM with a separate XMS emulation. Each VDM has its own High Memory Area, Upper Memory Blocks and Extended Memory Blocks; hence features such as interprocess communication work as if each VDM was running on a different real 80386. A VDM therefore cannot affect the availability of extended memory objects in other VDMs or access memory owned by other VDMs.

  • Provides configurable limits for how much XMS memory is available across all VDMs as well as a limit per-VDM. The DOS Settings feature can override the per-VDM limit, subject to the constraint given by the overall limit, and can disable XMS altogether for a particular VDM if its installation conflicts with the program being run in the VDM.

  • XMS can be removed and the operating system can run without loading XMS in any VDM session.

    Applications which use extended memory may use the XMS support in the same manner as in a native DOS environment. In addition, portions of the DOS operating system, device drivers and TSR programs may be loaded into extended memory, thereby conserving memory within the DOS application address space for application use.

    Note that older applications which access extended memory directly, rather than through an extended memory manager, may not be compatible with the XMS support under MVDM. For example, Microsoft Windows Version 2.x cannot make use of extended memory in a VDM.

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