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Introduction to the Hard Disk Drive| Category | | | Summary | | | Body | ::-=Introduction to the Hard Disk Drive =-::
The Hard Disk Drive (HDD or simply hard drive) is the main storage location on your computer. It is a part of the computer that takes a lot of abuse from day to day. If your hard drive fails. . . well, you've got a problem on your hand. Because of its important role in the user's relationship with his or her computer, the hard drive should be a part of the system that everyone should be at least basically acquainted with.
The goal of this tutorial is to acquaint the reader with the basic parts of hard drives and how they work together. After reading this article you may want to gain a deeper knowledge of hard drives. I would suggest two sources to learn about hard drives. The first is Charles M Kozierok's PC Guide . His section on hard disks is located at http://www.pcguide.com/ref/hdd/index.htm. Another good resource for all sorts of information on all aspects of hard drives is StorageReview.com, a website and forum for hard drive lovers. The information that can be gained from this site keeps pace with technology because most of it comes from visitor submission in the forums. With that said, lets introduce you to the hard drive.
Parts of Hard Drive
Case
The most obvious part of the hard drive is the case. This is the box that keeps all the hard drive's internal parts protected and in place. Even though it may look air tight, the case is not actually sealed. The read/write head needs to be able to float above the platters (more on this later), so there must be air inside the case at the same pressure as outside the case. Most hard drives employ some type of filter to make sure that any dust that may get into the drive stays off of the spinning platters.
Spindle and Spindle Motor
If you look inside the hard drive you will see an assortment of objects. In the middle of the drive is a circle that looks like a plate. This is the platter. In the middle of the platter is the spindle. The spindle serves as the axis of the platters' rotation. The spindle is driven by the spindle motor. The speed rating (7,200 RPM or 15K RPM) of the hard drive tells you how fast (in revolutions per minute, or RPM) the platters go round and round inside the drive.
Platters
A platter is a little metal circle inside of the hard drive. Many, if not most, hard drives have more than one platter. The platter is where all your data is stored. The basic way that the platter stores data is similar to the way that cassette tapes or other magnetic media store data. The information is stored as 1s and 0s encoded as changes in charge in the magnetic arrangement of crystals on the surface of the platter. Some of the earliest hard drives used a coating called metal oxide to store the data; this was nothing more than fancy rust. Today's hard drives use methods of data storage that press more and more information in smaller and smaller areas. To get better storage capacity, hardware engineers must combine new coatings on platters as well as other considerations such as read/write head and spacing between the units of storage.
Actuator Arm and Read/write head
The actuator arm moves the read/write head from the outside of the platter to the very center of the platter. There is one level of actuator and read/write head for each used side of the platters. If the hard drive has 2 platters and 2 sides of one platter are used, but one side of the 2 nd is not used for storage, there would be three actuator arms. The actuator is controlled by a motor, which is in turn controlled by the complex circuitry of the hard drive. In the old days, actuators were driven by stepper motors just as modern floppy drives are. The problem with this setup was that with the spinning of the platters and the heat generated, the platters would expand. This caused the inconvenienced of the data not always being where it was supposed to be. Modern actuators move in smooth arcs and have built in systems to counteract the effect of heat and other changes in the platters.
The read/write head is what picks up the charged particles on the platter and converts them into electric signals, and vice versa. To write to the disk, the write head magnetizes the particles that will contain the data. To read, the reading head detects the magnetic fields of the particles and converts it to an electric signal. The reading head and the writing head are both located at the end of the actuator arm. As hard drives get more advanced the particles storing the data get smaller and closer together. This is a problem for engineers designing the read/write heads. The reading heads must be able to detect the small charges and distinguish one bit from another. The writing heads must be able to impart a small charge and not change the states of neighboring particles. All this must be done at ever increasing speeds of rotation too!
If the power to the hard drive is cut off suddenly, a head crash may occur. This is every computer owner's worst nightmare. The actuator and the read/write head come crashing down on the spinning platters. Needless to say, the scratching that can result is not a pretty picture. To counter this, hard drive makers have data-free areas on each platter specially designed to be landing strips for the actuators.
Data Structures
Information is stored on a hard drive encoded into binary. Binary uses series of 1s and 0s to carry information. Each binary number (either a single 0 or a single 1) is a bit eight bits makes a byte and so on and so on. These bits are represented on the platters in a variety of ways. No system used today on hard drives is a simple as + charge is 1 and charge is 0. Most systems use some variation on the change in the charge carried by the particles on the platter to represent a 0 or a 1.
In most hard drives there are multiple platters. The information from a single file may be spread out over several, or even all of these platters. To address the issue of locating there are set ways of giving the location of a piece of data.
The simplest data structure on the hard drive to understand is a track. Tracks on hard drives are like tracks on a CD or record. They go around the spindle like the rings of a tree. Because all of the actuator arms move at the same time, each read/write head is positioned over the same track no matter what platter is being read from or written to.
A cylinder is another term that is frequently used to describe the location of a piece of data. Cylinders are basically tracks with depth. One cylinder includes the same track on all platters. This makes since, because, since all the actuator heads are on the same track any way, there is really no need to name the track on an individual platter.
Sectors are frequently encountered by the user when a diagnostic program reports that there is a lost sector. Sectors are the most basic unit of data storage on the hard drive. The sector is a sub unit of each track. Each sector usually stores 512 bytes of user data (Kozierok). This comes along with other data for the use of the drive itself (such as information about the sector and Error Correcting Code) to make up the sector.
Because the platter is a circle, the outer tracks are longer than the center tracks. Because the tracks differ in length, hard drives us a system called zoned bit recording. This system essentially squeezes more sectors on the outside tracks than on the inside. While this is simple in theory, the actual implementation is much more complex.
Defragmenting
Because hard drives cant predict all the files you will save in the future, they save bits and pieces of a file wherever there is room. Room may be made by deleted files. (Which have been removed from the directory listing, but really are still present until over written by more data.) File parts may be scattered all over the hard drives. This arrangement, while speeding up saving the file and making the most efficient use of free space, can eat into the time that it takes to read the file. Because the reading process requires mechanical parts to do the searching, any improvement in this process is very good. The smart people who design hard drives figured out a way to do this: defragmenting. At the simplest level, defragmenting takes all the file pieces and puts them all right next to each other.
Hard Drive Interfaces: SCSI and IDE
There are two common HDD interfaces (how they talk with other computer components and the motherboard, etc) SCSI and IDE/ATA. SCSI (pronounced by most as Skuzzy) is a high performance interface. SCSI stands for Small Computer Systems Interface. It is made up of several flavors, which I will not discuss here. SCSI is truly a system bus. SCSI is the way that hard drives, CD-drives, and other devices talk to each other. SCSI is commonly used in high-end systems and is the more expensive interface. Because of these facts it is less common than its younger counterpart: IDE/ATA.
Chances are if you are like most computer users, you will have an IDE hard drive. Despite a few disadvantages to this interface, IDE/ATA drives are the most common because almost all mother boards support them without special connectors and the drives are much cheaper than their SCSI equivalents.
Conclusion
I hope this tutorial has given you some basic insight into the workings of your hard drive. I encourage you to take this knowledge and expand on it by checking out the following sources.
Works Cited and Hard Drive Resources:
General Information
Kozierok, Charles M. Hard Disk Drives. The PC Guide . 17 Apr. 2001 . 28 July 2003 http://www.pcguide.com/ref/hdd/index.htm
Storage Review.com 25 June 2003 . 18 July 2003 http://storagereview.com/
SCSI and IDE
Octane. SCSI Technology Primer. ARS Technica 18 July 2003 http://arstechnica.com/paedia/s/scsi-1.html
Westergren, Markus and Mattias Sandgren. SCSI and IDE: Overview and Comparison . Umeå University . 31 Jan. 2001 . 18 July 2003 http://www.acc.umu.se/~sagge/scsi_ide/
Error Correction
White, Phil. ECC FAQs. ECC Technologies, Inc .18 July 2003 http://members.aol.com/mnecctek/faqs.html
All copyrights are the property of their respective owners. Neither the author nor anyone else is responsible for any results of your actions resulting from the reading of this article. The author believes that all the above information is accurate, however he is not perfect.
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Written by Darkwater685 (March 11 2004)
Member of CAU Knowledge-Bank Tutorial Writers
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This article was imported from the CyberArmy University site. (original author: PixieLuv)
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