The first goal of virtual video device driver support is to maintain the integrity of the operating system, and then to maintain the integrity of the DOS session. A DOS session has almost total access to the video hardware when running full-screen (foreground) and can put it into a state that prevents successful return to the desktop. An application can program registers into any state and use on-chip hardware-assist or coprocessed capabilities. A DOS session with full access to the video hardware can make use of all the capabilities provided.

While the DOS session remains full-screen, none of this is a problem. However, when a user wants to switch to another session, the adapter state must be saved and then restored accurately when it is brought into the foreground again. (Requirements for the application to continue to execute in the background will be addressed later in this chapter.) Assume that if the DOS session application is operating in an enhanced mode (above standard VGA), it is put into a suspended state, that is, it is not permitted to execute while in the background.

A virtual video device driver works hardest when switching from background to foreground, or the reverse. At these times, the entire state of the adapter, register, and video memory contents must be saved and restored.

In addition, if the base video subsystem is not adapter- or chip set-aware, the adapter must be placed (by default) into a clean state. A problem could occur if the new foreground session, after leaving an enhanced-mode DOS session, is a protect-mode session and no base video support exists for protect mode. Notice that the desktop is just another protect mode session that also might be running in an enhanced mode. This enhanced-mode support might be localized to the desktop display driver. Do not confuse this with protect-mode support.

CAUTION:
Any time a virtual video device driver requires register access to determine chip revisions, or for whatever reason, that process can occur in the background. All registers read from or written to should be restored, including their indexes, if appropriate.