Released in 1985 the Atari TOS was an operating system for Atari ST computers. Like the contemporary Apple systems, it was based on the desktop metaphor which had already become a standard.
GEM (Graphics Environment Manager) was the actual graphical interface system that ran on top of the TOS. An interesting detail about the Atari TOS is the use of three dimensional view points in the icon design. The Y-axis of the icons is set in a 45 degree angle.