A file extension is the set of letters or numbers at the end of any computer file name, preceded by a period (usually called "dot" when referring to file extensions). The .sys file extension, used in the DOS and Windows operating systems, usually indicates files that are vital to the operation of your computer.
The .sys file extension is short for "system."
Files with the .sys extension store settings and variables that help Windows and DOS function properly.
Some common .sys files are io.sys, config.sys, and msdos.sys (Windows 98 and before), and win32k.sys, hiberfil.sys, and pagefile.sys (later versions of Windows). You may have other important files with the .sys file extension on your computer.
If your computer displays an error message about a .sys file, consult an expert before doing anything with the file. Some .sys files, if altered incorrectly or deleted, can cause your computer to stop working properly or altogether.
Files with other extensions, such as .dll or .exe, may also be vital system files. Do not assume that a file can be deleted safely if is not a .sys file.
Some computer viruses masquerade as .sys files; run an anti-virus program regularly to keep your computer safe.
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