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The archive bit is a small piece of information attached to a computer file that tells a backup program whether or not the file has been changed since the last backup. It allows such software to perform an incremental backup, i.e., only copying files that have modified. The feature is only supported on computers running Microsoft Windows® and can be unreliable when used with multiple backup programs.
A bit is the simplest possible unit of information in digital computer systems. It has only two possible values: 1 or 0. In the case of an archive bit, a 1 or “set” value indicates that a file or directory has changed since the last backup, while a 0 or “clear” value means no changes have occurred. This allows backup software to perform something called an incremental backup. Unlike a full backup, an incremental backup only makes copies of files that have been changed, which can save time and disk space.
The archive bit is an example of an extra piece of information, called a file attribute, that helps a computer operating system figure out what to do with a given file or folder. File attributes are implemented by the file system, i.e., the overall method of organizing data on a disk. Different file systems support separate types of file attributes. In computers running DOS or Microsoft Windows®, every file and folder on a disk contains an archive bit, while computers running other operating systems do not support the feature.
A Windows® user can easily check the status of the archive bit for a given file by right-clicking on the file and selecting "Properties." A checked box next to “Archive” or “Ready for Archiving” means the bit has a value of 1 and has been set. Many backup programs also have the ability to reset the bit to a value of 0 after the operation is complete. This prevents the software from making another identical copy of the file if it hasn’t changed by the time of the next backup. The bit is only reset when a file is modified; merely opening or reading a file has no impact on the backup procedure, although changing other file attributes can reset the bit.
When used with multiple backup programs on the same computer, the archive bit can be unreliable. If one program clears the bit, other programs will not back up the file. Ill-behaved programs may also make changes to a file without setting the bit, thus preventing the modified file from being archived. For these reasons, modern backup software can often keep track of file changes on its own and perform backups without relying on the archive bit.