Int 21 Fn 5F9B  - Lantastic - Set Message Processing Flags                 [N]

   AX = 5F9Bh
   DL = bits describing processing for received unsolicited messages
        (see #1420)

Return: CF clear if successful
   CF set on error
       AX = error code

See Also: AX=5F9Ah,AX=5F9Eh