.sys is a filename extension in DOS and Windows operating systems.

Most DOS .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 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.

Applying .sys files

  • In MS-DOS and operating systems based on DOS, such as Windows 98, these files can be used as configuration files for the operating system. The MSDOS.SYS and CONFIG.SYS files contain various configuration parameters.

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