The Disk Operating System, short DOS, is a file management system for the Atari. It's main function is the management of storage such as floppy disk or harddisks. It contains the necessary driver functions for the computer to communicate with the disk-drives or other peripheral devices.

Some DOS Systems consist of two parts, In der Regel besteht das DOS aus zwei Teilen:

DOS.SYS

this is the resident part, also know as the file management system (FMS). it contains all the low level functions for disk access

DUP.SYS

Disk Utility Package - exposes the low level functions in a user friendly interface to the user. It also contains some high level functions not available in the FMS, such as disk copy.