
.sys

.sys is a filename extension used in MS-DOS applications and Microsoft Windows operating systems. They often contain device drivers or hardware configurations for the system.
Most DOS 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 text file 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, whereas 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.
File location.
In particular in Windows Vista and its successors, the .sys files are mainly found under the following paths: