INT 14h,  7Eh    Install an "external application" function

      Entry:  AL         Code assigned to external application

              DX         Offset of application entry point

              ES         Segment of application entry point

      Exit:   AX         1954h

              BL         Code assigned to application (same as input AL)

              BH         01h - Installation was successful
                         00h - Installation failed

    This call is used by external application code  (special screen drivers,
    modem code, database code, etc) to link into the INT 14h service for use
    by multiple applications. The "error return" (BH=0 with AX=1954h) should
    mean that  another application layer has  already been installed at that
    particular code. Codes 80h through BFh should be supported.

    External application codes 80h-83h are  reserved by FOSSIL developers for
    re-organizing FOSSIL services by type (comm, screen, keyboard, system).

    Installed application code will be entered, via a FAR call, from the INT
    14H dispatcher whenever it is entered with AH=(application code).

    If the value returned in AX from this function is not 1954h, the service
    code that is trying to be installed should bring up its own INT 14h code
    that can service INT 14h functions 7h-BFh (80h-BFh are "applications").