VXMS attempts to reserve all unreserved pages in the region of memory between 640KB and 1MB; this region is often termed the Upper Memory Area (UMA). The address ranges reserved in this manner will be used to simulate Upper Memory Blocks (UMBs). Note that this allocation scheme requires that VXMS be the last device driver loaded; any device drivers loaded after VXMS will not be able to reserve any addresses in the UMA.
When a UMB is not in use, its corresponding range of addresses is mapped to a black hole. When it is in use, the range of addresses corresponding to the UMB being allocated is mapped to a memory object outside the V86 mode address space which is allocated for this purpose. This is similar to the technique used to map objects in the HMA.
VXMS uses a delayed UMB allocation scheme. Unlike conventional XMS implementations, no UMBs are allocated until the first UMB request. Upon receiving the first UMB request, VXMS queries the UMB region to determine which address ranges are available, and reserves those ranges. This technique supports memory mapped devices which lie in the same region from which UMBs are taken. Advanced users can use Include/Exclude Regions in the DOS Settings feature to tell VXMS which ranges are not to be used.
Note that by default, all UMBs are owned by the DOS Emulation kernel and are not available for application use. If an application wishes to use UMBs, the DOS = NOUMB statement must be included in CONFIG.SYS or the application can't get any UMB because they are already used by DOS. Alternatively, the ownership of UMBs for a single VDM may be enabled or disabled using the DOS owns UMBs setting; see DOS Settings.
VXMS allows coexistence with EMS services, in that it queries VEMM before reserving address ranges, so that VEMM may reserve the space it requires for its frame. As such, it is possible that an application using both EMS and XMS services will execute and function correctly.
DOS Device Drivers
DOS device drivers may be loaded into UMBs, thereby conserving memory within the 640KB DOS application space; this support is functionally compatible with that provided by DOS 5.0. Loading a DOS device driver into a UMB requires a number of additional statements in CONFIG.SYS; an example is given in Figure "CONFIG.SYS - Loading Device Drivers into UMBs".
The first statement causes the VXMS virtual device driver VXMS.SYS to be loaded at initialization time. The second statement causes the ANSI.SYS device driver to be loaded into a UMB. The SIZE parameter ensures that the device driver is loaded into a UMB of the required size for its operation; if a UMB of this size cannot be allocated, the device driver is automatically loaded into low memory.
For DOS device drivers loaded into a specific VDM using DOS Device Drivers in the DOS Settings feature, the SIZE parameter is also supported. Specifying a SIZE parameter in DOS Settings will cause the device driver to be loaded into a UMB if possible; if UMBs are not present or if a sufficiently large UMB cannot be allocated, the device driver will be loaded into low memory.
Note that device drivers are always loaded into the largest available UMB; hence, in order to achieve efficiency in the utilization of UMBs, device drivers should be loaded in order of size, from largest to smallest. This is achieved by placing the DEVICE= statements in CONFIG.SYS, or names of the device drivers in the DOS Device Drivers setting, in that order.
The third statement allocates ownership of UMBs to the DOS kernel, and prevents applications from accessing UMBs. This statement sets the default for all VDMs; if an application running in a VDM requires UMBs, the default may be overridden for that VDM using the appropriate DOS Settings function.
Note that some device drivers may not function correctly in a UMB if they rely on having all memory above them available for their use. If incorrect operation of a device driver is experienced in a UMB, the value of the SIZE parameter should be increased by modifying the DEVICEHIGH statement in CONFIG.SYS or by altering the appropriate DOS settings for the VDM.
TSR Programs
TSR programs may also be loaded into UMBs in order to conserve DOS application space. TSR programs such as APPEND, which are loaded by default when a VDM is started, are loaded into a UMB where possible, thereby saving approximately 6KB of memory. Loading a TSR program into a UMB is performed from the DOS command line or from a batch file using the LOADHIGH command, as shown in Figure "LOADHIGH Command - Loading TSRs into UMBs".
Note that parameters for TSR programs are supported by the LOADHIGH command.
TSR programs which rely on having all memory above their location available for their use may not execute correctly when loaded in a UMB. In such cases, the TSR must be loaded into low memory.