Intel 8080 bottleneck made classic 47-year-old Space Invaders arcade game run faster as enemies died — expert coder asserts 'hardware accident' to blame

One of the most charming bug = feature tales is the story behind the thrilling crescendo of pacing gamers experienced when playing the original Space Invaders arcade machine. This weekend, self-proclaimed C/C++ expert Zuhaitz reminded us that the adrenaline-pumping rising intensity of Taito’s arcade classic was not due to genius-level coding. Rather, it was simply the fact that the underlying Intel 8080 could run the game code faster as aliens were wiped from the screen one by one, by the player dishing out laser missile death.

Intel’s 8080 processor, launched in 1974, was the successor to the legendary 8008, enhanced with a more powerful and flexible architecture. However, with its ~5,000 transistors, 8/16-bit bus, and ~2.0 MHz clock speed, powering the original Space Invaders with 224 x 256 mono pixels was probably near the limits of its gaming capabilities. Hence, the impact on gameplay ‘feel’ as the alien horde was eliminated by the player.

The processor-bottlenecked performance slowdown persisted, even though the CPU was aided by a dedicated Fujitsu MB14241 video shifter in this cabinet design. Also, a Texas Instruments SN76477 sound chip was used for the iconic audio.

As Zuhaitz states in the above-linked Tweet, checking the original arcade game’s source code confirms there is no code to adjust the game speed as aliens are wiped from the game.

At the beginning of the game, “changing the position, redrawing the sprites and checking for collisions for each one of the 55 aliens was really expensive,” notes the computer engineering prodigy. In a way this isn’t just a bug becoming a feature, but for players of the original game this was a killer hook, and helped arcade machine owners liberate gamers of many shiny silvery quarters.

Space Invaders was launched by Taito in Japan in June 1978.

Later versions of the game, and emulators running on more modern, powerful platforms, would need mitigations to implement the ‘arcade perfect gameplay’ the original was renowned for. Rewrites and reboots could achieve the correct balance of game pacing using code. However, as emulators like MAME became popular, allowing the running of original code ROMs, they would benefit from custom configurations that limit CPU/timing parameters to period- and architecture-correct levels.

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