The existing virtual video device driver, which forms the base for the new support, is aware of all VGA registers. Therefore, the first consideration is to identify the ranges of port addresses that are not part of the register set. All extended registers must be shadowed when a DOS session is in the background and saved/restored, including extensions to indexed registers already in the VGA range and non-indexed registers.

Currently, most chip sets have some means of mapping the video buffer into the A000-BFFF address range, or alternatively, access to video memory through port I/O. If the buffer is memory-mapped into some other region below 1MB, that part of the DOS session address space must be reserved by the virtual video device driver to handle the page faults generated as a result of access to those addresses by the DOS session.

Support for coprocessed chip sets requires even further consideration. A DOS session can switch from foreground to background at any time. If a DOS session application has initiated a coprocessor operation that has not completed when the virtual video device driver receives control to perform a switch to the background, some strategy must be adopted to ensure that, for instance, no data is lost by the DOS session. Obviously, this is relevant only if data is required from the application in a system memory-to-video memory transfer, or the reverse. Foreground and background switches are further described later in this chapter.