MVDM provides support for applications which use the LIM EMS Version 4.0 and LIMA XMS Version 2.0 memory extenders to access more than 640KB of memory. The memory requested is allocated from OS/2 Version 2.0 system memory, and is managed by the operating system kernel; special hardware is not required. Each VDM has its own copy of EMS or XMS memory objects, and the objects of one VDM are protected from access by another VDM.
Support for these memory extenders is provided by two virtual device drivers, VEMM.SYS and VXMS.SYS. Unlike most virtual device drivers, these drivers do not have corresponding physical device drivers, but access the operating system's memory manager to handle memory allocation requests from applications.
MVDM supports the loading of DOS device drivers and TSR programs into XMS Upper Memory Blocks, in order to reduce memory consumption below the 640KB line, thereby leaving more base memory for applications. Loading of these programs into UMBs is supported by the DEVICEHIGH statement in CONFIG.SYS and the LOADHIGH command included in AUTOEXEC.BAT or executed from the command line.