Notes:
Memory location $fe contains a new random byte on every instruction.
Memory location $ff contains the ascii code of the last key pressed.
Memory locations $200 to $5ff map to the screen pixels. Different values will
draw different colour pixels. The colours are:
$0: Black
$1: White
$2: Red
$3: Cyan
$4: Purple
$5: Green
$6: Blue
$7: Yellow
$8: Orange
$9: Brown
$a: Light red
$b: Dark grey
$c: Grey
$d: Light green
$e: Light blue
$f: Light grey