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As a kid a favorite game was Space Invaders on Atari (I'm guessing it was a 2600, it was the late 70s to early 80s). In addition to fast reflexes I had a card up my sleeve that secured me high scores: when starting the game I would quickly and forcefully shift the reset or power lever up and down causing a "glitch" in behavior, namely making the cannon fire twice at each round. Is this documented and/or expected behavior? How was this possible?

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2 Answers 2

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It is found and investigated by other people as well.

Apparently the switch/button states are read when game ROM code starts after powerup during initializing the game variables and also the reset switch state affects the game mode or state variables.

Based on the variables the game code ends up working so that the projectiles of both players are being controlled by one player.

Now, whether that is an accidental bug or an intentional easter egg is still being discussed.

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  • Thanks! Looking around I came across atariage, condensing the comments there suggests something along these lines. The evidence there is that this must be intentional behavior. I thought however that it was due to my "abuse", not the default every time I switched the reset lever.
    – Buck Thorn
    Apr 14 at 11:50
  • Do you have any links to the investigations/discussions? Apr 14 at 17:26
  • @Greenonline I know this was partially reverse-engineered on Atariage some 20 years ago.
    – Justme
    Apr 14 at 18:38
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An interesting quirk of Space Invaders (2600) is that in two-player simultaneous mode, there is a minimum separation between the two players' shots. Another interesting design aspect is that there isn't a fixed association between player entities and controllers, since some modes e.g. switch controllers after after each shot, or have one controller operate left movement while the other operates right movement, one player controls movement while the other controls firing, etc. The game is designed with two player-ship entities and two shot entities, and configurable associations between those and the controllers.

As it happens, in the storage locations that keep track of the current configuration and state, all-bits-zero represents a state in which one ship, with the player 1 color, is visible, its left and right motion are controlled by the left controller, and both shot 1 firing and shot 2 firing are attached to the left controller and the first ship position.

If it weren't for the fact that the two players' shots can't be fired simultaneously, pushing the fire button in this state would seem to only fire one shot that would behave oddly when it hits an alien. As it is, however, the fact that firing one player's shot prevents the other player's shot from firing briefly means that the first player ends up with the ability to have two independent shots on screen at once.

I would guess that if the programmer had realized that the game could support this state, it would have been included in the table of game variations, thus allowing it to be combined with other options such as moving shields or invisible aliens. I don't know when the behavior was first discovered, but it does offer some insight into the mechanics behind games' operation.

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