Compatibility: 1.0+
Expects: AH    06H
DL    0 to 0feH   character to send to the Standard Output
0ffH        request for input from the Standard Input
Returns: ZF    clear (NZ) if character is ready       on input requests
AL    character read, if ZF is clear         (when DL=0ffH)
Info: If DL is 0ffH, this performs a "no wait" console input, returning
the Zero Flag (ZF) set (ZR) if there is no character ready.  If a
character is ready, returns ZF cleared (NZ) with the character
that was read in AL.

If DL is anything but 0ffH, DL is sent to the Standard Output.

Notes:   Does not check for Ctrl-Break.
  Call twice for Extended ASCII.

- -

DOS Fn 06H: Console I/O