INT 61 - Banyan VINES - "Soint" - SET USER COMPLETION FUNCTION
        AX = 0001h
        DS:DX -> communications control block (function 000Bh)
Return: AX = status
            0000h  successful
            0001h  service not installed
            0002h  invalid service ID
            0003h-000Ah reserved for BANV interface errors
            00A2h  no more buffer space available
Notes:  BANYAN can use any interrupt from 60h through 66h.  The Banyan
          interrupt handler is identified by the string "BANV" in the four
          bytes immediately preceding the interrupt handler
        FAR user function is invoked with SS,DS, and ES set to segment of
          control block, and with the stack containing
                DWORD   return address
                WORD    argument pointer (sosend or sorec argument block)
                WORD    error return code
                        0000h argument pointer is valid
                        00A3h timeout

Format of control block:
Offset  Size    Description
 00h    WORD    000Bh
 02h    WORD    pointer to argument block (see below)
 04h    WORD    error return code
 06h  2 BYTEs   reserved
 08h    WORD    user CS register

Format of argument block:
Offset  Size    Description
 00h    WORD    pointer to user interrupt function
 02h    WORD    pointer to user stack
 04h    WORD    initial timeout value in multiples of 200ms, FFFFh = infinite