A SYS file is a system file used by Microsoft DOS and Windows operating systems. It contains system settings and variables, as well as functions that are used to run the operating system. SYS files are commonly used to store device drivers and other core Windows functions.

Some examples of SYS files include the following: IO.SYS: stores the default DOS device drivers and the DOS initialization program; used by DOS and Windows 9x. MSDOS.SYS: contains the core operating system code for DOS, known as the DOS kernel. CONFIG.SYS: contains the primary configuration information used by DOS. System files are most commonly found within the Windows directory of a Windows installation. In particular, many SYS files are stored in the Windows\winsxs directory in later versions of Windows.