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Tuesday, February 12, 2008


When the power to a PC is switched off, the contents of memory are lost. It is the PC's hard disk that serves as a non-volatile, bulk storage medium and as the repository for a user's documents, files and applications. It's astonishing to recall that back in 1954, when IBM first invented the hard disk, capacity was a mere 5MB stored across fifty 24in platters. 25 years later Seagate Technology introduced the first hard disk drive for personal computers, boasting a capacity of up to 40MB and data transfer rate of 625 KBps using the MFM encoding method. A later version of the company's ST506 interface increased both capacity and speed and switched to the RLL encoding method. It's equally hard to believe that as recently as the late 1980s 100MB of hard disk space was considered generous. Today, this would be totally inadequate, hardly enough to install the operating system alone, let alone a huge application such as Microsoft Office.

There's a read/write head for each side of each platter, mounted on arms which can move them towards the central spindle or towards the edge. The arms are moved by the head actuator, which contains a voice-coil - an electromagnetic coil that can move a magnet very rapidly. Loudspeaker cones are vibrated using a similar mechanism.

The PC's upgradeability has led software companies to believe that it doesn't matter how large their applications are. As a result, the average size of the hard disk rose from 100MB to 1.2GB in just a few years and by the start of the new millennium a typical desktop hard drive stored 18GB across three 3.5in platters. Thankfully, as capacity has gone up prices have come down, improved areal density levels being the dominant reason for the reduction in price per megabyte.


Hard disks are rigid platters, composed of a substrate and a magnetic medium. The substrate - the platter's base material - must be non-magnetic and capable of being machined to a smooth finish. It is made either of aluminium alloy or a mixture of glass and ceramic. To allow data storage, both sides of each platter are coated with a magnetic medium - formerly magnetic oxide, but now, almost exclusively, a layer of metal called a thin-film medium. This stores data in magnetic patterns, with each platter capable of storing a billion or so bits per square inch (bpsi) of platter surface.

Platters vary in size and hard disk drives come in two form factors, 5.25in or 3.5in. The trend is towards glass technology since this has the better heat resistance properties and allows platters to be made thinner than aluminium ones. The inside of a hard disk drive must be kept as dust-free as the factory where it was built. To eliminate internal contamination, air pressure is equalised via special filters and the platters are hermetically sealed in a case with the interior kept in a partial vacuum. This sealed chamber is often referred to as the head disk assembly (HDA).
Typically two or three or more platters are stacked on top of each other with a common spindle that turns the whole assembly at several thousand revolutions per minute. There's a gap between the platters, making room for magnetic read/write head, mounted on the end of an actuator arm. This is so close to the platters that it's only the rush of air pulled round by the rotation of the platters that keeps the head away from the surface of the disk - it flies a fraction of a millimetre above the disk. On early hard disk drives this distance was around 0.2mm. In modern-day drives this has been reduced to 0.07mm or less. A small particle of dirt could cause a head to "crash", touching the disk and scraping off the magnetic coating. On IDE and SCSI drives the disk controller is part of the drive itself.

There's a read/write head for each side of each platter, mounted on arms which can move them towards the central spindle or towards the edge. The arms are moved by the head actuator, which contains a voice-coil - an electromagnetic coil that can move a magnet very rapidly. Loudspeaker cones are vibrated using a similar mechanism.
The heads are designed to touch the platters when the disk stops spinning - that is, when the drive is powered off. During the spin-down period, the airflow diminishes until it stops completely, when the head lands gently on the platter surface - to a dedicated spot called the landing zone (LZ). The LZ is dedicated to providing a parking spot for the read/write heads, and never contains data.
When a disk undergoes a low-level format, it is divided it into tracks and sectors. The tracks are concentric circles around the central spindle on either side of each platter. Tracks physically above each other on the platters are grouped together into cylinders which are then further subdivided into sectors of 512 bytes apiece. The concept of cylinders is important, since cross-platter information in the same cylinder can be accessed without having to move the heads. The sector is a disk's smallest accessible unit. Drives use a technique called zoned-bit recording in which tracks on the outside of the disk contain more sectors than those on the inside.
Data is recorded onto the magnetic surface of the disk in exactly the same way as it is on floppies or digital tapes. Essentially, the surface is treated as an array of dot positions, with each "domain' of magnetic polarisation being set to a binary "1" or "0". The position of each array element is not identifiable in an "absolute" sense, and so a scheme of guidance marks helps the read/write head find positions on the disk. The need for these guidance markings explains why disks must be formatted before they can be used.
When it comes to accessing data already stored, the disk spins round very fast so that any part of its circumference can be quickly identified. The drive translates a read request from the computer into reality. There was a time when the cylinder/head/sector location that the computer worked out really was the data's location, but today's drives are more complicated than the BIOS can handle, and they translate BIOS requests by using their own mapping.

In the past it was also the case that a disk's controller did not have sufficient processing capacity to be able to read physically adjacent sectors quickly enough, thus requiring that the platter complete another full revolution before the next logical sector could be read. To combat this problem, older drives would stagger the way in which sectors were physically arranged, so as to reduce this waiting time. With an interleave factor of 3, for instance, two sectors would be skipped after each sector read. An interleave factor was expressed as a ratio, "N:1", where "N" represented the distance between one logical sector and the next. The speed of a modern hard disk drive with an integrated controller and its own data buffer renders the technique obsolete

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