.sys

.sys is a filename extension in Microsoft Windows and DOS-type operating system.

Most .sys files are real-mode device drivers. Certain files using this extension are not, however: MSDOS.SYS and IO.SYS are core operating system files in MS-DOS and Windows 9x. The CONFIG.SYS file contains various configuration options (as well as specifying what device drivers will be loaded). .sys files can be found in the C:/Windows directory of Windows computers. It is found in the C:/Windows/winsxs subdirectory in later versions of Windows.