INT FD - TFPCX - INSTALLATION CHECK
        AH = function (see separate entries below)
Program: TFPCX is an interface between modem and terminal program for packet-
          radio communications
Notes:  the installation check consists of testing for the string "N5NX" three
          bytes beyond the interrupt handler; INT FD is the default, but may
          be changed, so the full installation check consists of scanning
          for the signature
        TFPCX returns AX=FFFFh on any unsupported function call
SeeAlso: AH=01h,AH=03h,AH=FEh

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