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INT 2F - Multiplex - DOS 3.3+ internal - SET FASTOPEN ENTRY POINT
        AX = 122Ah
        BX = entry point to set (0001h or 0002h)
        DS:SI -> FASTOPEN entry point
                (entry point not set if SI = FFFFh for DOS 4+)
Return: CF set if specified entry point already set
Notes:  entry point in BX is ignored under DOS 3.30
        both entry points set to same handler by DOS 4.01

DOS 3.30 FASTOPEN is called with:
        AL = 01h  ???
            CX = ??? seems to be offset
            DI = ??? seems to be offset
            SI = offset in DOS CS of filename
        AL = 02h  ???
        AL = 03h  open file???
            SI = offset in DOS CS of filename
        AL = 04h  ???
            AH = subfunction (00h,01h,02h)
            ES:DI -> ???
            CX = ??? (subfunctions 01h and 02h only)
Returns: CF set on error or not installed
Note: function 03h calls function 01h first

PCDOS 4.01 FASTOPEN is additionally called with:
        AL = 04h ???
            AH = 03h
            ???
        AL = 05h ???
        AL = 0Bh ???
        AL = 0Ch ???
        AL = 0Dh ???
        AL = 0Eh ???
        AL = 0Fh ???
        AL = 10h ???