Compatibility: EGA VGA
See: BL=10H EGA/VGA (get EGA info)
BL=20H EGA/VGA (use alternate print screen)
BL=30H     VGA (set text-mode scan-lines)
BL=31H     VGA (enable default palette loading)
BL=32H     VGA (enable access to video)
BL=33H     VGA (enable gray-scale summing)
BL=34H     VGA (enable cursor emulation)
BL=35H     VGA (PS/2 display switching)
BL=36H     VGA (screen refresh on/off)

These functions are put in place by ROM-Scan when an EGA  or VGA  card is
installed and are built into the standard ROMS for PS/2 systems.  They are
not available on CGA  or MDA  systems.

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INT 10H 12H: EGA/VGA Special Functions/Alternate Select