Int 10 Fn 12  - Video - Alternate Function Select (ps,ega,vga,mcga) - Alt  [V]

   AH = 12h
   BL = 20h  select alternate print screen routine

Return: nothing
Desc:  installs a PrtSc routine from the video card's BIOS to replace the
     default PrtSc handler from the ROM BIOS, which usually does not
     understand screen heights other than 25 lines

Note:  some adapters disable print-screen instead of enhancing it

See Also: INT 05"PRINT SCREEN"