INT 6B - NPC NCSI EXTENDED SERIAL I/O - READ DATA ON A VIRTUAL CIRCUIT          
	AH = 19h
	AL = virtual circuit number
	CX = number of characters to read
	ES:BX -> buffer for received characters
Return: CX = 0000h if failed
	CX = nonzero (possibly number of characters received) if successful
Note:	this function is also supported by TelAPI
SeeAlso: AX=0100h,AH=12h,AH=18h