Atari TOS
TOS (The Operating System) is the operating system of the Atari ST range of computers. This range includes the 520ST and 1040ST, their STF/M/FM and STE variants and the Mega ST/STE. Later, 32-bit machines (TT, Falcon030) were developed using a new version of "TOS", called MultiTOS, which allowed multitasking. More recently, users have further developed TOS into FreeMiNT. Details. The Atari TOS (The Operating System) debuted with the Atari 520ST in 1985. TOS combines Digital Research's GEM GUI running on top of the DOS-like GEMDOS. Features include a flat memory model, DOS -compatible disk format (starting with TOS 1.04), support for MIDI, and a variant of SCSI called ACSI in later versions. Atari's TOS is usually run from ROM chips contained in the computer: Thus, before local hard drives were available in home computers, it was an almost instant-running OS. TOS booted off floppy disks in the very first STs, but only about half a year after the ST was introduced, all ST models started shipping with the latest version of TOS in ROM. TOS consisted of the following: The following were extensions to TOS (loaded separately): True multitasking was  not directly supported, but TOS allowed up to six desk accessories to be loaded into the system, which are similar to TSRs (Terminate and Stay Resident programs) on PCs. MultiTOS was developed to allow TOS to preemptively multitask. Desktop. The TOS desktop uses icons to represent files and devices, windows and dialog boxes to display info. The desktop file "DESKTOP.INF" was read to determine window settings, icon placements and drive icons, otherwise the standard default desktop of two floppy icons and the trash icon was used. Later versions use "NEWDESK.INF" for saving and reading the desktop configuration. Executable files are  identified by their extensions: TOS programs (but not GEM programs) can auto boot by placing them in a folder named "AUTO". TOS 1.4 allows GEM programs to be set to load automatically from the "Install Application" dialog. Programs with *.TTP extensions and environments can not be used for auto boot. Desktop accessories were placed in the root directory of the default drive and loaded automatically. File system. Atari TOS is based on GEMDOS which uses a modified FAT12 (or, on hard disks, FAT16) file system. The major differences are the fact that the boot sector does not need to contain  the IBM compatible jump sequence at the beginning (typically or ), the lack (before TOS 1.04) of an OEM identifier compatible with PC-based systems, and the fact that a checksum is used to mark the boot sector as executable (the PC format uses the signature word instead). Executable boot sectors for the Atari platform typically start with an MC68K jump opcode (e.g. , and the last two byte word must sum with the rest of the boot sector (in big-endian word form) to in order to be bootable. Unlike MS-DOS, GEMDOS would typically allow disks with unusual sector  and track counts, so disks with 10 or even 11 sectors per track and over 80 formatted tracks were not uncommon in the Atari community. Typically a safe combination, such as 10 sectors per track by 80 tracks, was used, yielding an unformatted capacity of 800KB, but many users pushed the capacity of their double-density disks beyond 900KB using custom formats. GEMDOS disc file systems can be read using DOS or Windows 9x. TOS 4.92 was a version of MultiTOS, the multitasking version of TOS, in a format (.IMG) designed to be written to a ROM chip . Versions. TOS 4. TOS 4 ROM contains five user-selectable language versions.