Series

Galaxian

8 games, 1979–2011.

Games

Mechanics

Mechanic	Galaxian 1979	Galaga 1981
Bonus stage		Scheduled
Difficulty ramp	Level and time
Enemy formation	Break-off
Extra life	Once	Repeating
Remaining-enemy speed-up	Threshold

A marked cell changed from the game before. “Uses” means the game has the mechanic, with no variant recorded yet.

Every claim, by mechanic and game

Concept	Galaxian 1979	Galaga 1981
Bonus stage		Every fourth stage of Galaga is a challenging stage: at each stage's start `stg_init_splash` adds 1 to the stage counter and stores (counter + 1) mod 4, and a 0 there marks a challenge, so the counter values 3, 7, 11, … bring one. Code
Difficulty ramp	Galaxian's difficulty has two parts. A base value rises by 1 with each level cleared, up to 7. An extra value starts each level at 0 and rises by 1 each time two nested counters run out, up to 7. The counters reload to 60 and 20, so once running they step it every 1,200 passes of the game logic, but the cited code doesn't reset them when a level starts, so a level's first step can come sooner. They hold while the player is not in play or after a flagship in flight is shot. The two values set how many single attackers may fly at once and how many attack counters tick. Code
Enemy formation	Galaxian's swarm holds its formation while aliens break off from it to attack. An array of 8 in-flight slots holds them: slot 1 for a flagship, slots 2 and 3 for its escorts, of which there are at most 2, and slots 4 to 7 for single attackers, so at most 7 aliens fly at once. No attack uses slot 0, which the disassembly's comment says is kept for other uses such as explosions. A single attack scans only $\min (⌊ (b + e) / 2 ⌋, 3) + 1$ of the last four slots, where $b$ and $e$ are the two difficulty values, so 1 to 4 single attackers can fly at once. Code
Extra life	Galaxian awards one extra ship, a "bonus galixip", when the score's top digits reach the DIP setting: 7,000, 10,000, 12,000, or 20,000 points. A per-player flag then blocks any further award. Code	Galaga awards extra fighters at a first and a second score set by DIP switches, and with some settings again at every interval after the second, the interval being the second score: one setting gives 20,000, 70,000, and every 70,000 points. The three switches index one of two 8-entry tables, picked by bit 2 of the starting-fighter setting. In the first, five settings repeat, two award only the first two scores, and one awards none; in the second, every first score is 30,000, three settings repeat, four award at most the first two, and one awards none. The source reassembles MAME's `galaga` set, Namco rev. B. Code
Remaining-enemy speed-up	Galaxian counts the aliens still in the swarm, leaving out those in flight, and sets `HAVE_AGGRESSIVE_ALIENS` when 3 or fewer remain. With the flag set, an attacking alien other than a flagship that reaches the bottom of the screen reappears at the top and attacks again instead of rejoining the swarm, once the player has spawned; before that, it rejoins. Flagships take a separate routine. The disassembly reads the Namco ROM, MAME's `galaxian` set. Code