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INT 2F - Multiplex - DOS 3.3+ internal - INSTALLABLE COMMAND - INSTALL CHECK
        AX = AE00h
        DX = FFFFh
        DS:BX -> command line
Return: AL = FFh if this command is a TSR extension to COMMAND.COM
        AL = 00h if the command should be executed as usual
Notes:  This call provides a mechanism for TSRs to install permanent extensions
          to the command repertoire of COMMAND.COM.  It appears that
          COMMAND.COM makes this call before executing the current command
          line, and does not execute it itself if the return is FFh.
        APPEND hooks this call, to allow subsequent APPEND commands to execute
          without re-running APPEND

Format of command line:
Offset  Size    Description
 00h    BYTE    max length of command line, as in INT 21/AH=0Ah
 01h    BYTE    count of bytes to follow
      N BYTEs   command line text, terminated by 0Dh