Int 15 Fn C4  - System - Programmable Option Select (ps50+)                [B]

   AH = C4h
   AL = subfunction
       00h return base POS register address
       01h enable selected slot for setup
       BL = slot number (1 to 8)
       02h disable setup for all slots

Return: CF set on error
   DX = base POS register address (if subfunction 00h)

See Also: AH=C6h