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  Function 06H (decimal 6) is a complex function that combines the
  operations of keyboard input and display output into one untidy package.
  As with everything else in DOS versions 2.0 and later, the I/O is not
  connected to the keyboard and display, but rather to the standard input
  and output devices (which default to the keyboard and display).

  Here is how this function works: The AL register is used for input and the
  DL register for output. If you call function 06H with DL = FFH (decimal
  255), the function performs input:

  .  If a key was pressed, function 06H returns the corresponding ASCII code
     in AL and clears the zero flag.

  .  If no key was pressed, function 06H sets the zero flag.

  If you call function 06H with any other value in DL, the function performs
  output: The character in DL is copied to the standard output device.

  Function 06H does not wait for keyboard input, and it does not echo input
  to the display screen. In addition, function 06H does not interpret Ctrl-C
  as a keyboard break; instead, it returns the value 03H (the ASCII value of
  Ctrl-C) in AL.

  Compare this function with functions 01H, 07H, and 08H. See function
  0CH for a variation of this service.