Click above to get retro games delivered to your door ever month!

  Notice that bits 4 through 7 in the bytes at 0040:0017H and 0040:0018H
  refer to the same keys. In the first byte, the bits show the current state
  of the toggle keys; in the second byte, they show whether or not the
  corresponding toggle key is pressed.

  You can read the status of any of these bits to your heart's content, but
  few, if any, are likely to be useful in your programs. With the partial
  exception of controlling the Caps Lock state, we don't think it's wise to
  change any of the shift-state bits (bits 4 through 6 of byte 0040:0017H).
  And it is potentially very disruptive to change any of the key-is-pressed
  bits (bits 0 through 3 of byte 0040:0017H or any bits in byte 0040:0018H).