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        DOS 3.3+ Installable command - Install check

        Entry:
          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

        Note: This apparently 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.

        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