ROM BIOS provides many function calls which are typically used by a DOS application program. To maintain the virtualization concept and ensure compatibility with applications which access BIOS or its functions, these functions are emulated by a virtual device driver.
The VBIOS virtual device driver contains a mechanism to map and initialize the system ROMs (including the ROM and EBIOS data areas) and the interrupt vector table into memory within each virtual DOS machine. This is done at VDM creation time, before any other virtual device drivers are loaded. This allows other virtual device drivers to hook interrupt vectors and use VBIOS services. Certain BIOS interfaces are not emulated directly, but are passed to other routines which provide improved performance or functionality. For example, the video interface routines provided by ROM BIOS are powerful but extremely slow. In order to increase the performance of the video output, the video virtual device driver intercepts the ROM BIOS video interrupt (INT 10h) and performs the requested operations directly, providing improved performance.