This one’s for the hardcore techies out there. Computer Archeology is a terrific site with information on the inner workings of several prominent games. One of their most complete and detailed is on the code and hardware of arcade Space Invaders, including a disassembly. While it is not, as the page says, “one of the first” arcade video games, not unless you count everything manufactured between 1972 and 1978 as unimportant, there’s a vast amount of good information there.
Some of the info revealed:
- The game tracks five objects every frame. The player’s base, their shot, and three invader shots. The invaders don’t “count” as moving objects; the Mystery Ship temporarily replaces one of the shots, meaning, while it’s on-screen the aliens can only have two shots on-screen instead of three.
- Only one of the shots is “aimed” to fall on or near the player. The others are just dropped wherever.
- There is a bug where the player’s shot hitting the rightmost shield on the right edge of the screen can be misinterpreted as hitting the last invader on the very left edge.
- The reason the invaders speed up as their numbers are diminished is because the game draws the new position of exactly one invader into the framebuffer per frame. Fewer invaders means each gets to move more often, until the last one gets to move every frame.
