Overview

Before learning about files and file structures, it is necessary to understand the physical structure of the common secondary ^[1] storage medium, the magnetic disk. In large measure, the disk's physical structure and its limitations dictate how it is used to store data files. To be specific, a disk is the magnetic medium on which the data are stored. A disk drive is a larger hardware system that includes the disk and its supporting machinery record-playback (or read-write) heads, the arm that holds them, the positioner (or actuator) that moves them, the motor that spins the disk, and the electronics that control these components. To this end, the characteristics of disks and disk drives are presented first.

After describing the hardware, we will turn to the logical organization of the data on the disk. The logical file structure is maintained by the file subsystem, a subset of the operating system. No general purpose operating system is functional unless it has file system capabilities, no matter how primitive. The only exception to this observation is an embedded real-time operating system in which the entire application, its data, and the operating system are permanently memory-resident, for instance, a weapons control system on a tactical fighter plane or the software in an automobile. Typical of these real-time systems are RSX-11S for the PDP-11 and some operating systems for microcomputers, like the 68000 family.