INT 2F - 4DOS.COM v2.1+ - API
        AX = D44Dh
        BH = function
            00h installation check
                Return: AX = 44DDh
                        BL = major version number
                        BH = minor version number
                        CX = PSP segment address for current invocation
                        DL = 4DOS shell number (0 for the first (root) shell,
                             updated each time a new copy is loaded)
            01h (internal, v2.1-3.03) terminate current copy of 4DOS
                Return: nothing
                (internal, v4.0) ???
                Return: AX = 44DDh
                        ES:BX -> data area (see below)
            02h ???
                DX = ???
        ---v2.1-3.03 only---
            03h EXEC program
                CX:DX -> EXEC record
            FEh deallocate shell number (passed through to root shell)
                ???
            FFh allocate shell number (passed through to root shell)
Note:   bug in v3.00 will crash system if unrecognized value in BH
SeeAlso: AX=D44Eh,AX=E44Dh,INT 21/AX=4403h
Index:  installation check;4DOS|installation check;NDOS

Format of EXEC record:
Offset  Size    Description
 00h    WORD    offset of ASCIZ program name in same segment as EXEC record
 02h    WORD    offset of DOS commandline in same segment as EXEC record
 04h    WORD    segment of environment for child process (see INT 21/AH=26h)

Format of 4DOS v4.0 data area:
Offset  Size    Description
 00h  2 BYTEs   ???
 06h    WORD    XMS handle for swapping
        ???

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