First appearing in early 1985, TOS stands for The Operating System, with the distinction of being one of the few mainstream operating systems burned into ROM. GEM stands for Graphic Environment Manager, Digital Research's graphical user interface that Atari licensed and burned into ROM as well (collectively known as TOS). There were GEM applications for DOS, the best known being Ventura Publisher, but Windows would takeoff and now very little is heard of the GEM interface. (Legend has it that TOS really stands for "Tramiel Operating System", named after Atari's boss at the time, but this was probably more of an inside joke than anything else: the early Atari manuals make direct references to "The Operating System".)

TOS itself originated from the landmark CP/M operating system -- created by Gary Kildall who would later form Digital Research -- by modifying CP/M68, a version for the Motorola 68000 processor which Atari was to use in its new 16-bit computer. Competing with MS-DOS at the time, Digital Research's Intel-based operating system was known as DR-DOS, with their own GEM being the counterpart to Microsoft's Windows. Apple took legal action on Digital Research for the look and feel of its desktop, forcing GEM for the PC to be somewhat crippled. Indeed, GEM was written by someone who had just left the Xerox PARC lab where the concept of the GUI was born and, from Steve Jobs' visits there, subsequently "borrowed" by Apple. Atari's use of GEM, however, was not restricted by these early legal wranglings.

Atari abandoned the personal computer market in 1994, concentrating instead on its ill-fated gaming products. Purchased in mid-1996 by JTS, a maker of hard disk drives, the Atari corporate namesake continued on as a threadbare division of JTS until early 1998 when Hasbro Interactive purchased all of their Atari assets (including TOS itself, although more interested in the classic gaming properties). "Atari" clones, however, are available from Europe, and TOS software development including the licensed OS continues to this day.

At one time you could buy WordPerfect and Microsoft Word for TOS. Those were the early days, when the Atari ST (pictured) was the first 1MB computer for under $1000. Macs were very expensive at the time and Windows was in its infancy. People loved GEM's easy-to-use interface and the then-snappy performance of the Motorola 68000 running at 8MHz.

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With TOS/GEM being a ROM-based OS/GUI, many optimized software enhancements are available that replace the original code. These replacements improve the functionality and look and feel to the point where almost everything has been redefined: Thing, for instance, replaces the standard GEM desktop; NVDI replaces GEM code that handles screen management and, through VDI's loadable extension called GDOS, adds a TrueType and PostScript font engine; N.AES replaces GEM code that handles windows, menus, dialogs and adds a preemptive multitasking environment drawn from Atari's 1993 disk-based MultiTOS extension, most of whose new features were adopted from the Unix-like MiNT kernel.

Now a free project developed by volunteers, MiNT -- a recursive acronym for "MiNT is Not TOS" -- creates a bridge to the Unix (BSD) world by extending TOS's GEMDOS subsystem, allowing simultaneous access to both TOS and Unix applications (e.g. loadable file systems, device drivers, and networking support). MiNT on its own does not include GEM, whose components (the Desktop, VDI and AES) can be loaded from disk during bootup.

By far, though, the most popular and complete ("all-in-one") operating system replacement is MagiC, a highly optimized and somewhat proprietary version of MultiTOS but without the Unix support. Because Germany (and Europe in general) took TOS more seriously than North America, these enhancements are usually written by German programmers.