.sys

.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.^[1] Certain files using this extension are not, however:

MSDOS.SYS and IO.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.^[2]^[3]
COUNTRY.SYS^[4] is a binary database containing country and codepage related information for use with the CONFIG.SYS COUNTRY directive and the NLSFUNC driver.^[4]
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.^[5]^[6]

^ Cite error: The named reference Driver was invoked but never defined (see the help page).
^ Cite error: The named reference MSDOS.SYS was invoked but never defined (see the help page).
^ Cite error: The named reference CONFIG.SYS was invoked but never defined (see the help page).
^ ^a ^b Cite error: The named reference Paul_2001_COUNTRY was invoked but never defined (see the help page).
^ Cite error: The named reference Paul_2001_KEYBOARD was invoked but never defined (see the help page).
^ Cite error: The named reference Mendelson_2001_KEYBOARD was invoked but never defined (see the help page).