Karbosguide.com - Module 4b2.

Hard disks - the physical aspect


The contents:

  • The construction of a hard disk
  • Writing in domains
  • The magnetic disks
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  • Physical aspects of the hard disk

    Let us look at the construction of the hard disk.

    Read/write heads

    All hard disks consist of thin platters with a magnetic coating. They rotate quite fast inside a metal container. Data are written and read by read/write heads, which are designed to ride on a microscopic cushion of air, without touching the platter. They register bits from the magnetic coating, which races past them. On the illustration below, you see a hard disk with three platters. It has 6 read/write heads, which move synchronously.

    The arms, which guide the movement of the read/write heads, move in and out almost like the pick-up arm in an old fashioned phonograph. As illustrated below, there will typically be 6 arms, each with read/write heads. The synchronous movement of these arms is performed by an electro-mechanical system called the head actuator. The hard disk data can only be attained via one head at a time.

    The read/write head consists of a tiny electromagnet. The shape of the head end acts like an air foil, lifting the read/write head slightly above the spinning disk below.

    When the disk rotates under the read/write head, it can either read existing data or write new ones:

  • If a current is applied to the coil, the head will become magnetic. This magnetism will orient the micro magnets in the track. This is write mode.

  • If the head moves along the track without current applied to the coil, it will sense the micro magnets in the track. This magnetism will induce a current in the coil. These flashes of current represent the data on the disk. This is read mode.

    The read/write heads are by far the most expensive parts of the hard disk. They are incredibly tiny. In modern hard disks they float between 5 and 12 micro inches (millionths of an inch) above the disk. When the PC is shut down, they are auto parked on a designated area of the disk, so they will not be damaged during transport.


    Domains

    The bits are stored in microscopic magnets (called domains) on the disk. They are written in this manner: Before recording data, the drive uses the read/write heads to orient the domains in a small region so that the magnetic poles all point in the same direction. Then:

  • A reversal of polarity is interpreted as a digit one.

  • Unchanged polarity is interpreted as a digit zero. If we read the magnets from right to left, we might see the following example:


    The magnetic disks

    The magnetic disks are typically made of aluminum. There are also experiments with disks made of glass. The disks are covered with an ultra thin magnetic coating. With improved coating technologies, an increasing number of micro magnets can be placed on the disk. Currently, there are more than 2000 tracks per inch disk radius. There are only 135 on a floppy disk.

    The narrower the tracks are, the bigger the disk capacity gets. At the same time the magnetic signals get weaker and weaker. Therefore, the read/write heads must get closer to the disk. This requires even smoother platters, etc.

    Another improvement in modern disks is the employment of a technology called Multiple Zone Recording. This allows for about twice as many sectors (120) in the outermost track as in the innermost. Thus, outer tracks, which are much longer, can hold much more data than inner tracks. Previously, all tracks had the same number of sectors, which was not very efficient.

    Writing in layers

    Since a hard disk typically contains three platters with a total of 6 read/write heads, the concept cylinders is employed. Read/write heads move synchronously. Therefore, data are written up and down from platter to platter. Thus, one file can easily be spread over all 6 platter sides. Let us say the writing starts on track 112 on the first platter. That is completed and writing continues on track 112 - only from read/write head number 2. Then it continues to numbers 3, 4, 5 and 6. Only then does writing move to track 113.

    In this case, a cylinder consists of 6 tracks. For example, cylinder number 114 is made up of track number 114 on all 6 platter sides.



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    Learn more

    Also see Module 4c about optical media (CDROM and DVD).

    Also read about EIDE and UDMA

    And about the most advanced and elegant controller principle of all: SCSI.

    Read module 7a about monitors, and 7b on graphics card.

    Read module 7c about sound cards, and 7d on digital sound and music.

    HD Tach - a fine program to test harddisks.

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