Reserved Files

All of the data describing the layout of files on a disk is contained on the disk itself. When a disk is first accessed (after a reboot or the mounting of a new disk), there must be some way to locate enough information to get started. In order to accomplish this, Files-11 places certain key information in known locations.

A certain number of reserved files exist on disk, some portions at specific locations, which allows Files-11 to learn the state of all information on a disk. During system operation, the contents of these files are kept updated with the latest file system information ^[2].

The main reserved files holding disk structure information are named in the following paragraphs.

INDEXF.SYS This is the central file containing the names, attributes, and physical locations of files on disk. In addition, it contains much of the information needed to begin accessing a disk on system startup. Its contents are described the next section.

BITMAP.SYS This file contains a map representing free space on the volume. Each bit in the file represents one cluster (not block) of disk storage, indicating at a glance whether or not it is free. While it is possible to determine the same information from INDEXF.SYS, it would be an unreasonably slow process.

000000.DIR This file is the top-level directory for the volume and represents the starting point for directory operations. Please note that INDEXF.SYS contains complete information about all files on the volume and that directories are merely a means to represent a hierarchical relationship among files. From a technical perspective, directories are not strictly necessary for using a Files-11 disk volume.

^[2]For performance reasons, certain information may be allowed to be temporarily out-of-date. None of the data are unrecoverable; should the system fail while some file system data are obsolete, all of this information is automatically checked and, if necessary, reconstructed the next time the disk is mounted.