What is a hard disk?

Dec 3

History, Ideas, Internet, Laptops and Pc, PSP, Portable Player, Software, Tech News, Tweaks and Tutorials | Viewed 3584 times, 4 so far today

HARDDISK

What Is a Hard Disk?
A hard disk or drive is the part of your computer responsible for long-term storage of information. Unlike volatile memory (often referred to as RAM) which loses its stored information once its power supply is shut off, a hard disk stores information permanently, allowing you to save programs, files, and other data. Hard disks also have much greater storage capacities than RAM.

Basic components of a Hard Disk
A hard disk is comprised of four basic parts: platters, a spindle, read/write heads, and integrated electronics.
Basic components of a hard disk
• Platters are rigid disks made of metal or plastic. Both sides of each platter are
covered with a thin layer of iron oxide or other magnetizable material.
• The platters are mounted on a central axle or spindle, which rotates all the platters at the same speed.
• Read/write heads are mounted on arms that extend over both top and bottom
surfaces of each disk. There is at least one read/write head for each side of each platter. The arms jointly move back and forth between the platters’ centers and outside edges; this movement, along with the platters’ rotation, allow the read/write heads to access all areas of the platters.
• The integrated electronics translate commands from the computer and move the read/write heads to specific areas of the platters, thus reading and/or writing the needed data.

How Is Data Stored and Retrieved?
Computers record data on hard disks as a series of binary bits. Each bit is stored as a magnetic charge (positive or negative) on the oxide coating of a disk platter. When a computer saves data, it sends the data to the hard disk as a series of bits. As the disk receives the bits, it uses the read/write heads to magnetically record or “write” the bits on the platters. Data bits are not necessarily stored in succession; for example, the data in one file may be written to several different areas on different platters. When the computer requests data stored on the disk, the platters rotate and the read/write heads move back and forth to the specified data areas. The read/write heads read the data by determining the magnetic field of each bit, positive or negative, and then relay that information back to the computer. The read/write heads can access any area of the platters at any time, allowing data to be accessed randomly (rather than sequentially, as with a magnetic tape). Because hard disks are capable of random access, they can typically access any data within a few millionths of a second.

What Is Disk Formatting?
Computers must be able to access needed information on command; however, even the smallest hard disk can store millions and millions of bits. How does the computer know where to look for the information it needs? To solve this problem, hard disks are organized into discrete, identifiable divisions, thus
allowing the computer to easily find any particular sequence of bits. The most basic form of disk organization is called formatting. Formatting prepares the
hard disk so that files can be written to the platters and then quickly retrieved when needed. Hard disks must be formatted in two ways: physically and logically.

Physical Formatting
A hard disk must be physically formatted before it can be logically formatted. A hard disk’s physical formatting (also called low-level formatting) is usually performed by the manufacturer.
Physical formatting divides the hard disk’s platters into their basic physical elements: tracks, sectors, and cylinders. These elements define the way in which data is physically recorded on and read from the disk.
• Tracks are concentric circular paths written on each side of a platter, like those on a record or compact disc. The tracks are identified by number, starting with track zero at the outer edge.
• Tracks are divided into smaller areas or sectors, which are used to store a fixed amount of data. Sectors are usually formatted to contain 512 bytes of data (there are 8 bits in a byte).
• A cylinder is comprised of a set of tracks that lie at the same distance from the
spindle on all sides of all the platters. For example, track three on every side of every platter is located at the same distance from the spindle. If you imagine these tracks vertically connected, the set forms the shape of a cylinder.
Computer hardware and software frequently work using cylinders. When data is
written to a disk in cylinders, it can be fully accessed without having to move the
read/write heads. Because head movement is slow compared to disk rotation and switching between heads, cylinders greatly reduce data access time.
After a hard disk is physically formatted, the magnetic properties of the platter coating may gradually deteriorate. Consequently, it becomes more and more difficult for the read/write heads to read data from or write data to the affected platter sectors. The sectors that can no longer be used to hold data are called bad sectors. Fortunately, the quality of modern disks is such that bad sectors are rare. Furthermore, most modern computers can determine when a sector is bad; if this happens, the computer simply marks the sector as bad (so it will never be used) and then uses an alternate sector.

Logical Formatting
After a hard disk has been physically formatted, it must also be logically formatted. Logical formatting places a file system on the disk, allowing an operating system (such as DOS, OS/2, Windows, or Linux) to use the available disk space to store and retrieve files. Different operating systems (OS) use different file systems, so the type of logical formatting you apply depends on the OS you plan to install. Formatting your entire hard disk with one file system necessarily limits the number and types of OSs you can install on the disk. Luckily, there is a solution to this problem. Before a disk is logically formatted, it can be divided into partitions. Each partition can then be formatted with a different file system, allowing you to install multiple OSs. Dividing your hard disk into partitions also allows you to use disk space more efficiently.