Zanac (also known as Zanac A.I.) is a sci-fi vertical-scrolling shoot-'em-up developed by Compile and published by Ponyca for MSX computers in both Japan and Europe (co-published with Eaglesoft) sometime in 1986.
Playing similar to other vertical-scrolling shoot-'em-ups of the era (such as Xevious and TwinBee), Zanac has players controlling starfighter AFX-6502 "ZANAC" as they try to fend off an alien invasion by the "System". In addition to an upgradable rapid-fire cannon, the game features eight different types of secondary weapons (which are switched by collecting numbered power-ups, with collecting the same power-up upgrading it).
The game also features an elaborate dynamic difficulty system known as "Automatic Level of Difficulty Control", or "ALC". One of the game's main selling points, the ALC measures the player's performance and adjusts the enemy ship spawns accordingly. Some actions (such as collecting power-ups and constantly using the special weapon), increase the amount of tougher enemies while other actions (such as losing ships or destroying reconnaissance ships) reduce them.
The game received an enhanced re-release for MSX2 computers in the same year as Zanac EX, featuring enhanced graphics (with smoother scrolling and a revamped user interface) and a larger playfield. The original MSX version also received an updated "2nd Version" re-release in 1987. It also received a port to both the Famicom Disk System in Japan on November 28, 1986 and the Nintendo Entertainment System in North America (co-published by FCI) on October 1987. The NES version of the game is notable for its unique hardware-based cheat code, where players can access the Level Select feature by consecutively reset their console 13 times.
Both the FDS and NES versions of the game were later included in the 2001 Japanese PlayStation release Zanac X Zenac, along with an enhanced "Special Version" and a new modern sequel (known as Zanac Neo). The FDS version was later digitally re-released for the Wii (as a Virtual Console title) in Japan on October 9, 2007, while the NES version received a similar digital re-release by D4 in North America on December 3, 2007. The MSX version was later ported to iOS devices (as Zanac MSX) in Japan on August 2, 2014.
The game later received a mobile port for Java-based feature phones. This version was later ported to the Nintendo Switch on January 28, 2021 as the 29th entry in G-Mode's G-Mode Archives series. It is one of the few games in the G-Mode Archives series to receive a release outside of Japan.