Including Upper Memory Blocks

Many devices have a ROM that occupies a particular range in the device’s memory space. If a VDD knows it will not be accessing the ROM, it can call VDDIncludeMem to allow the VDM to use the address range of the ROM as an MS-DOS UMB. A VDD most likely calls VDDIncludeMem if it has chosen to implement application-based intercepts, as described in Application-Based Intercepts, where the VDD replaces most of the functionality originally in the device ROM. Since the ROM is no longer needed, its memory space can be reused by the VDM.