Windows System File

.sys is a filename extension used in MS-DOS applications and Microsoft Windows operating systems. They are system files that contain device drivers or hardware configurations for the system.

Most DOS .sys files are real mode device drivers. Certain files using this extension are not, however:

• MSDOS.SYS and IO.SYSIO.SYS are core operating system files in MS-DOS and Windows 9x.
• CONFIG.SYS is a text file that contains various configuration options and specifies what device drivers will be loaded.
• COUNTRY.SYS is a binary database containing country and codepage related information for use with the CONFIG.SYS COUNTRY directive and the NLSFUNC driver.
• KEYBOARD.SYS is a binary database containing keyboard layout related information including short P-code sequences to be executed by an interpreter inside the KEYB keyboard driver.

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.