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  Description:  Programmable Option Select      [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 to return base POS adapter            and, if called with AL=00H
         register address                      DX=base POS adapter
     01H to enable slot                           register address
     02H to enable adapter                     If function unsuccessful
  BL=slot number (if AL=01H)                   Carry flag=set
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