Compatibility: VGA
Expects: AH    1aH
AL    Subfn: 00H= Query current display combination
01H= Select DCC (activate a different display)
BX                BL=ID of active display;
BH=ID of inactive display
                                                                  
Returns: AL    1aH (if this fn is supported)
BL    ID of active display
BH    ID of inactive display
                                                                  
Info: This returns or updates the DCC byte at 0040:008a.

When AL=00H, the current DCC code is returned in BX.
When AL=01H, the BIOS examines the DCC table to see if the
specified combination is valid, and then deactivates the video
subsystem identified in BH and activates that in BL.

The following display ID codes are recognized

00H no display   (BH=00H if only one display is installed)
01H MDPA on monochrome screen
02H CGA color                  07H VGA analog monochrome
04H EGA color                  08H VGA analog color
05H EGA monochrome             0bH Model 30 analog monochrome
06H PGC color                  0cH Model 30  analog color

Notes: DCC handling is mainly a feature of PS/2 hardware; it was
designed as a means to disable the VGA controller on the PS/2's
motherboard in order to take advantage of future improvements in
video technology.

Not many VGA BIOSes provide full support of display combination
switching, though most support subfn AL=00H to indicate type 08H
in BL.

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INT 10H 1aH: Set or Query Display Combination Code