Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
Int 15h Function C4h
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  Interrupt:   15h     Function:   C4h
 
  Title:       Programmable Option Select
 
  Description:                                                    [PS/2]
 
     Returns the base Programmable Option Select register address,
     enables a slot for setup, or enables an adapter.
 
     Input                                Output
 
     AH = C4h                             If function successful
     AL = Subfunction                       Carry flag: clear
        = 00h Return base POS adapter     And if called with AL = 00h
              register address              DX = Base POS adapter
        = 01h Enable slot                        register address
        = 02h Enable adapter
     BL = Slot number (if AL = 01h)       If function unsuccessful
                                            Carry flag: set
                                    -♦-