INT 21 - TopWare Network OS v5.10+ - RECEIVE OF USER-DEFINED PACKETS
        AX = FF9Ah
        ES:BX -> buffer for user-defined packet (see below)
Return: nothing

Format of user-defined packet:
Offset  Size    Description
 00h    BYTE    FFh
 01h    WORD    (call) length of data field plus 3
                (return) length of received message (0000h if none received)
 03h    BYTE    destination ID (FFh for broadcast message)
 04h    BYTE    sending station ID
 05h    BYTE    type code (11h; all other codes reserved for TopWare)
 06h  N BYTEs   received message

.