INT 21 - PC/TCP IPCUST.SYS - WRITE CONFIGURATION DATA
        AH = 40h
        BX = handle for character device "$IPCUST"
        CX = number of bytes to write
        DS:DX -> buffer for configuration data (AH=3Fh"IPCUST")
Return: CF clear if successful
            AX = number of bytes actually written
        CF set on error
            AX = error code (05h,06h) (see AH=59h)
Notes:  if less than the entire data is read or written, the next read/write
          continues where the previous one ended; IOCTL calls AX=4402h and
          AX=4403h both reset the location at which the next operation starts
          to zero
        the data pointer is also reset to zero if the previous read or write
          reached or exceeded the end of the data, when the current function
          is read and the previous was write, or vice versa
SeeAlso: AX=4402h"IPCUST"