INT 21 - TopWare Network OS v5.10+ - SEND MESSAGE
        AX = FF80h
        DH = FFh
        DL = destination address (FFh for broadcast)
        CX = message length (max 2000)
        DS:SI -> message to be sent (see below)
Return: nothing
Program: TopWare Network Operating System is manufactured by Grand Computer
          Company
Notes:  this function is supported on both Workstations and the server
        there is no guarantee that the message will be received correctly, or
          at all, by the destination

Format of message:
Offset  Size    Description
 00h    BYTE    type code
                07h TopSend
                11h user application
                other reserved for TopWare
 01h    var     data
Note:   sending messages with a type code other than 11h will cause
          unpredictable results

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