Posted: February 11, 2011 in Embedded System

A hard disk drive (HDD) is a non-volatile, random access device for digital data. Hard disk drives have been the dominant device for secondary storage.

  1. Introduced by IBM in 1956
  2. Hard disk drives have fallen in cost and physical size over the years while dramatically increasing in capacity.
  3. Average access time decreasing from greater than 0.1 second to a few thousandths of a second

Seek time is a measure of the speed with which the drive can position its read/write heads over any particular data track.

Access time is simply the sum of the seek time and the latency.

Once over the data track, the heads must not write the data until the selected free sectors on that track pass beneath the head. This time is related to the rotation speed of the disk: the faster the speed, the shorter this “latency” period.

A HDD design consists of a spindle which holds one or more flat circular disks called platters, onto which the data is recorded. The platters are made from a non-magnetic material, usually aluminum alloy or glass, and are coated with a thin layer of magnetic material. The platters are spun at very high speeds. The platters rotate at up to 10,000 revolutions per minute (rpm) so the read-write heads can access any part of them. Information is written to a platter as it rotates past devices called read-and-write heads that operate very close (tens of nanometers in new drives) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. The head is mounted on an arm. An actuator arm (or access arm) moves the heads on an arc across the platters as they spin, allowing head to access almost the entire surface of the platter as it spins. The arm is moved using a stepper motor.


The data is stored digitally as tiny magnetized regions, called bits, on the disk. A magnetic orientation in one direction on the disk could represent a “1”; an orientation in the opposite direction could represent a “0”. Magnetism is used in computer storage because it goes on storing information even when the power is switched off.

The science of magnetism is complex. But if you’ve ever fooled around with a magnet and some nails, you’ll know that the technology. Iron nails start off unmagnetized but, if you rub a magnet back and forth over them, you can make them magnetic so they stick to one another.

If your computer has a 20 gigabyte (GB) hard drive, it’s a bit like a box containing 1.6 million microscopically small irons nails, each of which can store one tiny piece of information called a bit. Suppose you want to store the number 1000001 in your computer in that big box of iron nails. You need to find a row of seven unused nails. You magnetize the first one (to store a 1), the next five demagnetized (to store five zeros), and magnetize the last one (to store a 1).

In your computer’s hard drive, there aren’t really any iron nails. There’s just a circular “plate called a platter. The data is stored in a very orderly pattern on each platter. Bits of data are arranged in concentric, circular paths called tracks. Each track is broken up into smaller areas called sectors. Part of the hard drive stores a map of sectors that have already been used up and others that are still free. (In Windows, this map is called the File Allocation Table or FAT)

All magnetic storage devices read and write data by using electromagnetism. This basic principle of physics states that as electric current flows through a conductor (wire), a magnetic field is generated around the conductor. The magnetic field generated by a wire conductor can exert an influence on magnetic material in that field. When the direction of the flow of electric current or polarity is reversed, the magnetic field’s polarity also reversed. Another effect of electromagnetism was that if a conductor is passed through a moving magnetic field, an electrical current is generated. As the polarity of the magnetic field changes, so does the direction of the electric current’s flow.

Each of the individual magnetic particles on the storage medium has its own magnetic field. When the medium is blank, the polarities of those magnetic fields are normally in a state of random disarray. Because the fields of the individual particles point in random directions, each tiny magnetic field is canceled out by one that points in the opposite direction; the cumulative effect of this is a surface with no observable field polarity. With many randomly oriented fields, the net effect is no observable unified field or polarity.

The read/write heads in a magnetic storage device are conductors, with the ends directly above the surface of the actual data storage medium (Platter). The head is wrapped with coils or windings of conductive wire, through which an electric current can flow. When a current flows through these coils, it generates a magnetic field in the drive head. As this field passes through the medium directly under the head, it polarizes the magnetic particles it passes through so they are aligned with the field. When the individual magnetic domains of the particles are in alignment, they no longer cancel out one another, and an observable magnetic field exists in that region of the medium. Reversing the polarity of the electric current causes the polarity of the generated field to change also. So the field of the magnetic medium also changes.

The polarity or direction of the field induced in the magnetic medium is based on the direction of the flow of electric current through the coils.

When a command is made to read some data on a disk, a similar process occurs in reverse. After consulting the table of stored data locations in the drive’s electronics, the actuator moves the head over the track where the chosen data is located. When the correct sectors pass beneath the head, the magnetic fields from the bits (domain) cause currents to flow through the head. The current variations are then detected and decoded to reveal the data that had been stored on the disk.


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