INT 21 U - OS/2 v1.x FAPI - OS/2 FILE SYSTEM JOIN/SUBST
        AH = 61h
        BP = 6467h ("dg")
        AL = function
            00h list
            01h add
            02h delete
        BX = drive number
        CX = size of buffer
        SI = type (0002h JOIN, 0003h SUBST)
        ES:DI -> buffer
Return: ???
Notes:  used by JOIN and SUBST to communicate with the OS/2 file system
        also supported by OS/2 v2.0+ Virtual DOS Machines

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