PaCifiST was the first real attempt at a true Atari ST emulator for the PC. Others came before it (Gemulator and StonX) but neither could handle any of the things which made the ST so much fun (i.e. games, demos and music).
SainT is an ATARI-ST emulator running on Windows (9x,NT,2000,XP) based machines. SainT was one of the first "cycle accurate" ATARI emulator. Cycle accuracy is very suitable to emulate ATARI demos.
It hadn't reach the 0.1 version, but when it appeared, it was a little revolution in the Atari ST emulation world.
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Don't you know what it is an emulator? It's simple: an emulator is a program that let you run games and programs of your old computers into the modern PCs, Macs, mobile phones and new generation consoles. You could load the images of those games, whose are available for thousands in Internet.