SYS files are better known as system files. They are commonly used by the DOS and Microsoft Windows operating systems, and are used to store required system settings and variables.

It is not recommended that you edit a system file, as doing so can have unexpected results.

Two of the most common system files are IO.SYS which is used to initialize the environment for some versions of DOS/Windows and CONFIG.SYS, which is the initial configuration file for the DOS operating system.