ACX file format is used by Atari ST computer system. ACX is an executable file and can be opened on modern PC using dedicated emulation software.

ACX file stores program or game data for Atari ST computer, which debuted in 1984 and was produced until 1993, and by that time gained much popularity among users. Emulators running Atari ST games have low system requirements as CPU of Atari ST was clocked at modest 8 MHz and had only 512 kB of cache memory.