INT 2F U - DOS 3.3+ internal - INSTALLABLE COMMAND - INSTALLATION CHECK
        AX = AE00h
        DX = FFFFh
        CH = FFh
        CL = length of command line tail (4DOS v4.0)
        DS:BX -> command line buffer (see below)
        DS:SI -> command name buffer (see below)
        DI = 0000h (4DOS v4.0)
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 buffer:
Offset  Size    Description
 00h    BYTE    max length of command line, as in INT 21/AH=0Ah
 01h    BYTE    count of bytes to follow, excluding terminating 0Dh
      N BYTEs   command line text, terminated by 0Dh

Format of command name buffer:
Offset  Size    Description
 00h    BYTE    length of command name
 01h  N BYTEs   uppercased command name (blank-padded to 11 chars by 4DOS v4)