.sys

Stores system settings and variables used by the DOS and Windows operating systems; also contains functions that are used to run the operating system; SYS files typically should not be edited.

IO.SYS is used by DOS and Windows 9x; it contains the default DOS device drivers, the DOS initialization program and processes MSDOS.SYS and CONFIG.SYS files.

MSDOS.SYS contains the core operating system code for DOS, known as the DOS kernel; CONFIG.SYS is the primary configuration file used for the DOS operating system.