INT 61 - PC/TCP kernel v2.05+ - "net_asynch" - SET UP ASYNCHRONOUS CALL-BACK
        AH = 1Fh
        BX = network descriptor
        CX = event type (see below)
        DS:SI -> event handler routine
        ES:DI = 32-bit hint passed to handler
Return: CF clear if successful
            DS:DX -> previous handler
        CF set on error
            AX = error code (see INT 61"PC/TCP")
Note:   ICMP messages do not trigger events on stream connections
SeeAlso: INT 61"PC/TCP",INT 61/AH=00h"PC/TCP"

Values for event type:
 00h    alarm
 01h    open (successfully opened stream connection)
 02h    receive (data available)
 03h    transmit (ACK received on stream connection)
 04h    transmit flush???
 05h    foreign close (remote host closed data connection)
 06h    close (local host closed connection and protocol is complete)
 07h    error (error code passed to handler as arg)

Event handler called with:
        BX = network descriptor
        CX = event type (see above)
        DS:DX -> arg
        ES:DI = 32-bit hint value
        STACK:  small stack, possibly the DOS stack

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