Files with the .SYS extension contain settings, variables, and system functions used during system launch – for the DOS and Windows systems. They usually contain device drivers and other basic functions of the Windows operating system, without which it would not be able to work.

By default, the files are stored in the Windows catalog, and, for later versions, also in the Windowswinsxs catalog, in the part of the drive on which the system is installed. In the case of DOS, the most used SYS files are IO.SYS, which contain default device drivers and a program that initializes system launch, also used for Windows 9x-family systems. MSDOS.SYS contains the source code of the system core, namely the DOS kernel, whereas CONFIG.SYS contains the basic system configuration.