)
Call driver "general request" function with:
	AH = 08h
	AL = port number (00h-0Fh)
	DX = requested operations
	    bit 0: flush transmit buffers
	    bit 1: flush receive buffers
	    bit 4: define XON/XOFF characters
	ES:BX -> XON/XOFF characters (see #02925) if DX bit 4 set
	interrupts disabled
Return: CF clear if successful
	CF set on error
	interrupts disabled