Retrospace-invadersarcadegame-designtech-history
Space Invaders Speeds Up as You Win Because Its 1978 Code Moved One Alien at a Time, and Tomohiro Nishikado Kept It
Fewer aliens meant each one came around sooner, so the march and its four-note thump sped up on their own; Nishikado kept the effect, while also saying he always wanted the game to get harder and regrets what the hardware couldn't do.
Shoot the second-to-last alien in Space Invaders and the last one takes off. It zips across the screen at a speed the formation never reached, and the thump of the march speeds up with it.
That speed-up wasn’t a line of code anyone wrote. It fell out of how the 1978 game draws its aliens.
The mechanism is right there in the code. A routine “called from the mid-screen ISR” sets “the cursor for the next alien to draw.” Each interrupt handles one alien. “When the cursor moves over all aliens then it is reset to the beginning,” and the formation’s position is tracked by one “reference alien,” the bottom left, with “all other aliens drawn relative to this.”
Do the math. With 55 aliens alive, a full step of the formation takes 55 turns. With ten left, it takes ten. With one, the alien moves every turn, and the code gives it a bigger stride too: “If there is one alien it will move right at 3” pixels.
Wikipedia’s account of the design puts it this way: “rather than design a compensation for the speed increase, he decided to keep this undocumented feature as a gameplay mechanism.” The march music speeds up alongside it.
Here’s where it gets disputed. Nishikado wrote the game himself. “For Space Invaders, that was the first one to use code, and I wrote that in assembly,” he told Time Extension, on “a CPU called 8080.” He also told the site, “Due to the hardware limitations, I wanted to move the aliens faster. Something like what Galaxian eventually did.” And: “That’s why, as an engineer, I feel regret.”
In a 2024 interview with GamesBeat he framed difficulty as the plan from the start: “Making a game that gets harder and harder is something I always kept in my mind.” Development took “about one and a half years, including the period where I had to create the tools for development.”
In the older Shmuplations translation, he credits the microprocessor with “having the speed of the invaders increase” and adds, “You couldn’t do things like that in games without a computer.”
Both can be true. He wanted a game that got harder, and his hardware handed him one, at a speed lower than he wanted.
What none of the sources settle is the exact order of events: whether Nishikado noticed the speed-up first and built the difficulty around it, or planned a harder game and found the hardware had already done it for him.