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Brief description of the design of modern HDDs

In this article we discuss the most popular storage devices with the ATA interface (AT Attachments), also called as IDE (Integrated Drive Electronic) used in most PCs. So, a storage device with hard magnetic disks consists of magnetic disks themselves (1, 2, 3, sometimes 4 of them), stacked and mounted on the axis of a spindle motor, and a read-write head assembly, all contained in a dust-resistant case (Head-Disk Assembly — HDA), and a controller board.

Since 1989-90, all manufactured HDDs have a liner actuator (rotating around its axis by magnetic field) of the head assembly. Such layout reaches a higher seeking speed than a discrete mechanism with a step actuator, but requires a servo-system with a feedback to determine the actual rotation angle of the actuator. That is why the head assembly is positioned in HDD using a special servo-data written on the disk. A ready-to-use disk surface contains servo-marks used to position the heads, the so-called low-level format including sector marks with their addresses and identifiers, and a logical format of the operating system. The operating system has access only to the data fields of the sectors where it creates its own logical structure. Several thousands «physical» cylinders are appeared on the HDD’s working surfaces, divided into several zones with different numbers of sectors per track. (A cylinder is a combination of tracks similarly positioned on all surfaces of the disk stack). For example, Quantum FB EX has 11,550 physical cylinders. While working in a usual user mode, the controller transforms the physical format into a standard logical one with the computer BIOS and OS work with.

All the HDD electronics, a head-assembly amplifier-switch chip excluded, are on the controller board. The controller is a special microcomputer consisting of a controlling processor, ROM (often built in the processor), RAM, and chips controlling the motor and actuator coil, coding/decoding/transforming the data, and interfacing the HDD with external circuitry. This microcomputer works under its own software (a specialized operating system) consisting of several modules. Working programs of the controllers are stored partially in ROM, partially (in many HDD models) on disks in an area specifically allocated for that purpose and inaccessible for the user. A storage device cannot work if those programs are not loaded. Other data necessary for an HDD to work are stored on service cylinders (a bad block table is an example). The data is read out from the service area when the storage device starts up or when it is necessary during its work.

When the power is turned off, the controller automatically parks the heads, that is, moves them to a special parking zone (not used to store data) where the heads are landed on the disk surface. Automatic parking is made from the power of the rotating disk stack, the spindle motor used as a generator.