INT 21,6 - Direct Console I/O

	AH = 06
	DL = (0-FE) character to output
	   = FF if console input request

	on return:
	AL = input character if console input request (DL=FF)
	ZF = 0	if console request character available (in AL)
	   = 1	if no character is ready, and function request
	     was console input

	- reads from or writes to the console device depending on
	  the value of DL
	- cannot output character FF  (DL=FF indicates read function)
	- for console read, no echo is produced
	- returns 0 for extended keystroke, then function must be
	  called again to return scan code
	- ignores Ctrl-Break and Ctrl-PrtSc