Translations:Tetris (NES, Nintendo)/10/en

Leftover Demo Recording Code

The game has an unused subroutine that records the player's input for use in the gameplay demo. This subroutine is activated when $00D0 in RAM is set to $FF. However, since the game tries to write the captured inputs directly to ROM ($DD00-$DEFF), it doesn't work correctly on the actual NES hardware or unmodified emulators.

(Source: Applying Artificial Intelligence to Nintendo Tetris)