AH = C2h
        AL = subfunction
            00h ???
               BH = ???
            01h set something to default values
            02h ???
               BH = ???
            03h ???
               BH = ???
            04h points to RET instruction
            05h same as 01h, but also takes input value in BH
               BH = ???
            06h ???
               BH = 01h or 02h stored, otherwise ignored
            07h ???
               ES:BX = pointer to ???
               Return: AL = 00h
Note: the values in BH for those functions that take it as input are stored
      in different locations for each subfunction
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