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