INT 21 - PCIBM PC/TCP(Transmission Control Protocol) A higher level (session layer) of the TCP/IP protocol suite.  See also IP, TCP/IP. 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 #01680 at AH=59h/BX=0000h)
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 PCIBM PC/TCP(Transmission Control Protocol) A higher level (session layer) of the TCP/IP protocol suite.  See also IP, TCP/IP. configuration
SeeAlso: AH=3Fh"IPCUST",AX=4402h"IPCUST"