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  Description:  Extended Communications Port Control      [PS/2]
 
  Reads or sets the modem control register (MCR) for the specified serial
  communications port.
 
  Input                                        Output
  ═════════════════════════════════            ══════════════════════════════
  AH=05H                                       If called with AL=00H
  AL=subfunction                               BL=modem control register
     00H to read modem                            contents (see above)
         control register                      If called with AL=01H
     01H to write modem                        AH=port status
         control register                         (see Int 14H Function 00H)
  BL=modem control register                    AL=modem status
     contents (if AL=01H)                         (see Int 14H Function 00H)
     Bit(s)     Significance
     ══════     ═══════════════════
     0          data-terminal ready
     1          request-to-send
     2          Out1
     3          Out2
     4          loop (for testing)
     5-7        reserved
  DX=communications port number
     (0=COM1, 1=COM2, etc.)
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