Interrupt 2Fh Function 4000h

mov ax, 4000h ; Enable VM-Assisted Save/Restore

int 2fh

mov [ModesSupported], al ; video modes supported while in background

Enable VM-Assisted Save/Restore directs the virtual-display device (VDD) to notify the virtual machine (VM) application whenever the VDD needs to access the video hardware registers. The VDD returns a value specifying the number and type of video modes the VDD supports when the VM application is in the background.

A VM application calls this function during its initialization.

Parameters

This function has no parameters.

Return Value

The return value is one of the following values, if successful.

Value

Meaning

01h	No modes virtualized in background.
02h	Only text modes virtualized in background.
03h	Only text and single-plane graphics modes virtualized.
04h	Only text, single-plane, and VGA multiplane graphics modes virtualized.
0FFh	All supported video modes virtualized.

Otherwise, the function returns zero in the AL register if virtualization is not supported.

Comments

When a VM application calls this function, the VDD disables I/O trapping of
unreadable registers. Thereafter, the VDD calls Save Video Register State and
Restore Video Register State (Interrupt 2Fh Functions 4005h and 4006h) if it needs to access the video registers. The VM application must provide an appropriate interrupt handler to process these functions.

When an VM application calls this function, the VDD saves the current state of the video registers. The VDD uses this saved state later to restore video registers before it calls Notify Foreground Switch and Restore Video Register State (Interrupt 2Fh Functions 4002h and 4006h).

After a VM application calls Enable VM-Assisted Save/Restore, the VDD no longer saves video memory across screen switches; it becomes the application's responsibility to completely reinitialize video memory after a Notify Foreground Switch request.