Test if keystrokes are waiting.

If a keystroke is waiting, the Carry Flag will be set. Also when a parameter is provided, it will jump to that location.

If no keystroke is waiting, the Carry flag will be clear.

Examples:

    Keypressed              ; test for keystroke
    Keypressed  READ_KEY    ; if a key pressed jump to READ_KEY

See also: KeyNotPressed, KeyWait and KeyRead.