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Hard Disc Fundamentals

By Muthu Senthil Kumar M

This article describes the main technical fundamentals of Hard Disc Drive and its operation. I focus my attraction on the particular perspective of describing Physical and Logical view of HDD, Reading and Writing in the HDD, How does data get stored in that and touched some information about HDD interfaces. This article also brings out the technology wise company working on this hard disc drives and this related technology.

Starting of HDD:

Hard disc drive, it is the magnetic device used to store soft copy of data in the form of 0's and 1's. Specific drivers are used to retrieve back the data in a specified format. Magnetic hard disks are used from 1956 onwards. On that day, the size of the hard disks is very huge for a small amount of storage and it also takes more time to store and retrieve. But today we are using very compact, high capacity, high latency hard disks for our systems.

Physical representation of HDD:

The user data / information are stored on a disc's surface media. The media is a specially coated surface. Each media surface is divided into tracks and sectors. These sectors are often called "blocks". Tracks are often referred to as cylinders. The Track, Sector separations of the hard disk are shown below. The Cylinder, Head, Sector (CHS) gives a physical location.

Logical representation of HDD:

Hard disks use circular hard platters to store data on. Each platter has two surfaces. Both sides of the media are used to store information. If each surface had 4 tracks and 4 sectors means there would be 32 sectors, or blocks. Each surface requires a "head" to read/write the information. Surfaces are often referred to as the "head". A cylinder is the area that a head can write to or read from without being positioned to another track.

Take a hard disk has two surfaces/head; four sectors/blocks then the logical representation of that disk will be like this figure 3. In that figure, the two heads of that disk represented as 0 and 1. The four cylinders/tracks and sectors are represented by 0 to 3. So for, each surface has 16 logical sectors/blocks that is named as Logical block Addressing (LBA).

The Host and the file system using this LBA only for HDD operation like reading, writing the data, searching and seek etc., But for heads and media, the hard disk driver circuits convert the LBA to CHS (Cylinder, Head, Sector).

Reading and Writing in HDD:

Hard Disk Drive is a digital data storage device that reads and writes data via magnetization changes of a magnetic storage disk. This includes one or more randomly accessible rotatable storage media, or disks. In a magnetic disk drive, the data is encoded as bits of information using magnetic field reversals grouped in tracks on the magnetic hard surface of rotating disks. It also includes a transducer supported by a hydrodynamic bearing which flies above each magnetic disk. The transducer head supported by an actuator arm is used to read data from or write data to the disks. Hard disk drives are an indispensable component in most modern computing/data handling systems.