Int 21 Fn 40  - PC/tcp Ipcust.sys - Write Configuration Data               [N]

   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
   v2.1+ uses a new configuration method, but allows the installation
     of IPCUST.SYS for backward compatibility with other software which
     must read the PC/TCP configuration

See Also: AH=3Fh"IPCUST",AX=4402h"IPCUST"