A system file in computing is a critical computer file without which a computer system may not operate correctly. These files may come as part of the operating system, a third-partydevice driver or other sources. Microsoft Windows and MS-DOS mark their more valuable system files with a "system" attribute to protect them against accidental deletion. (Although the system attribute can be manually put on any arbitrary file, these files do not become system files.)

Specific example of system files include the files with .sys filename extension in MS-DOS and Windows, the System suitcase on Mac OS and the files located in sys, the root folderof the Linux file system, sysfs.

File Systems (FAT, HPFS, NTFS)

At the BIOS level, a disk partition contains sectors numbered 0, 1, etc. Without additional support, each partition would be one large dataset. Operating systems add a directory structure to break the partition up into smaller files, assign names to each file, and manage the free space available to create new files.
The directory structure and methods for organizing a partition is called a File System. Different File Systems reflect different operating system requirements or different performance assumptions. Unix, for example, has the convention that lowercase and uppercase are different in file names, so "sample.txt" and "Sample.txt" are two different files. DOS and the systems that descend from it (Windows 95, OS/2, and Windows NT) ignore case differences when finding file names. Some File Systems work better on small machines, others work better on large servers.
Each partition is assigned a type (in the MBR for primary partitions, in the Extended Partition directory for logical volumes). When the partition is formatted with a particular File System, the partition type will be updated to reflect this choice.
The same hard disk can have partitions with File Systems belonging to DOS, OS/2, NT, and Linux (or other Unix clones). Generally, an operating system will ignore partitions whose type ID represents an unknown file system type. It is fairly easy (given a big enough disk) to install all of the different operating systems and all of the File System types. There are a few rules to make things simple.
Each File System is described in detail in a separate section.

• FAT File System
  The FAT File system is used by DOS and is supported by all the other operating systems. It is simple, reliable, and uses little storage.
• VFAT
  VFAT is an alternate use of the FAT file system available in Windows 95 and Windows NT 3.5. It allows files to have longer names than the "8.3" convention adopted by DOS. VFAT stores extra information in the directory that older DOS and OS/2 systems can ignore.
• HPFS
  HPFS is used by OS/2 and is supported by Windows NT. It provides better performance than FAT on larger disk volumes and supports long file names. However, it requires more memory than FAT and may not be a reasonable choice on systems with only 8 megs of RAM.
• NTFS
  NTFS provides everything. It supports long file names, large volumes, data security, and universal file sharing. A departmental NT file server will probably have all its partitions formatted for NTFS. Because the other operating systems cannot use it, NTFS is less attractive on personal desktop workstations or portables.