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  The keyboard-controller circuitry on the computer's system board monitors
  the keyboard for input. The keyboard controller generates interrupt 09H
  each time it receives a byte of data from the keyboard. The ROM BIOS
  contains an interrupt 09H handler that reads the byte from the keyboard
  controller and processes it. (I/O port 60H contains the keyboard data
  byte.) The interrupt 09H handler translates scan codes into 2-byte values
  that are generally more useful to a program than the original scan codes.

  The low-order byte of each 2-byte keyboard value contains the ASCII value
  corresponding to each key pressed. The high-order byte usually contains
  the corresponding keyboard scan code.

  Special keys, such as the function keys and the numeric-keypad keys, have
  a 0 in the low-order byte, with the keyboard scan code in the high-order
  byte. (More about this later, on page 134.)

  The ROM BIOS routines place the translated byte-pairs in a queue, which is
  kept in low memory in location 0040:001EH. The byte-pairs are stored there
  until they are requested by a program, such as DOS or interpreted BASIC,
  that expects to read keyboard input.