EmuTOS is partially derived from Digital Research's original GEMDOS 1.1 operating system and GEM GUI. EmuTOS is basically made up of eight parts:

• The BIOS, which is the basic input output system
• The XBIOS, which provides the interface to the hardware
• The GEMDOS (also known as BDOS), which contains the high level OS routines
• EmuCON, which is a minimalist command line interpreter
• The VDI, which provides an abstracted device interface
• The Line-A interface, which provides an alternative low-level screen interface
• The AES, which does all the higher level graphics stuff and
• EmuDesk, which provides the 'desktop' GUI you see immediately after boot

GEMDOS was first used in Apple's Lisa computer, then later, as home computers became popular, in the Atari ST series. In both systems, GEMDOS was the base for Digital Research's GEM graphical user interface, which became very popular with the introduction of the Atari ST.

In 1999 Caldera bought all the GEM and GEMDOS stuff from Digital Research with the intention of using it as a base for some thin clients. But this never happened and they decided to release GEMDOS together with GEM under the GPL (GNU General Public License). In the spring of 2001 we began to work on these old sources and have continued to fix bugs and add features up to the present.

The BIOS and XBIOS code is our own development. It is written from scratch and implements all of the TOS 3.0x BIOS/XBIOS functionality, and many TOS4.0x functions (excluding 16-bit video support).

The Line-A code is also our own development. It is written from scratch and implements all of the TOS Line-A functions.

The GEMDOS (BDOS) part is based on Digital Research's GEMDOS sources, which were made available under GPL license in 1999 by Caldera. The same is true of the graphical parts, the VDI, AES and desktop. However, we have rewritten major parts of all these subsystems to fix bugs and to make them conform to the Atari TOS implementation.

Of course, being a single-user and single-tasking OS, EmuTOS cannot compete with other modern operating systems that are available today. In the TOS world, all of GEMDOS has been replaced by a superior operating system called FreeMiNT (MiNT is Now TOS) which has a wide community among Atari users today. FreeMiNT is more POSIX-like and provides most modern concepts, like a multi-user environment, symmetric multitasking, memory protection and so on.

But since EmuTOS provides the same functionality as the original TOS and the code is lean enough to fit within the same space as the original Atari ROMs, it is a perfect replacement for the original TOS. When talking about EmuTOS, you may wish to consider it as a cool firmware replacement for TOS-compatible machines rather than a full-blown operating system.