Key Board

In computing, a keyboard is a peripheral partially modeled after the typewriter keyboard.

Physically, keyboards are an arrangement of rectangular buttons, or keys. Keyboards typically have characters engraved or printed on the keys; in most cases, each press of a key corresponds to a single written symbol. However, to produce some symbols requires pressing and holding several keys simultaneously or in sequence; other keys do not produce any symbol, but instead affect the operation of the computer or the keyboard itself. See input method editor.

A majority of all keyboard keys produce letters, numbers or signs (characters) that are appropriate for the operator's language. Other keys can produce actions when pressed, and other actions are available by the simultaneous pressing of more than one action key.

Contents [hide]
1 Designs
1.1 Connection types
1.2 Wireless types
1.3 Buckling spring vs. dome switch
1.4 Alternatives
1.5 Standards
1.6 Historical
2 Usage
2.1 Keystroke
2.2 Commands
2.3 Games
2.4 Buying considerations
2.5 Safety Precautions
3 How it works
4 Customization
5 Keys on a computer keyboard
6 See also
7 References
8 External links



[edit] Designs

Most keyboards are rigid, but this foldable keyboard demonstrates one of many variations from the usual.There exist a large number of different arrangements of symbols on keys. These different keyboard layouts arise mainly because different people need easy access to different symbols; typically, this is because they are writing in different languages, but specialized keyboard layouts for mathematics, accounting, and computer programming also exist.

Most of the more common keyboard layouts (QWERTY-based and similar) were designed in the era of the mechanical typewriters, so their ergonomics had to be slightly compromised in order to tackle some of the technical limitations of the typewriters. The letters were attached to levers that needed to move freely; jamming would result if commonly-used letters were placed too close to one another. The QWERTY layout is an invention of Christopher Sholes. With the advent of modern electronics, this is no longer an issue. QWERTY layouts and their brethren had been a de facto standard for decades prior to the introduction of the very first computer keyboard, and were primarily adopted for electronic keyboards for this reason. Alternative layouts do exist, the best known of which is the Dvorak Simplified Keyboard; however, these layouts are not in widespread use.

The number of keys on a keyboard varies from the standard of 101 keys introduced in the late 1980s to the 104-key windows keyboards and all the way up to 130 keys or more, with many of the additional keys being symbol-less programmable keys that can simulate multiple functions such as starting a web browser or e-mail client. There also were "Internet keyboards," sold in the late 1990s, that replaced the function keys with pre-programmed internet shortcuts. Pressing the shortcut keys would launch a browser to go to that website.


[edit] Connection types
There are several different ways of connecting a keyboard which have evolved over the years. These include the standard AT (DIN-5) connector commonly found on pre-80486 motherboards, which was eventually replaced by the PS/2 and USB connection. Prior to the iMac line of systems, Apple Computer used ADB, a proprietary system, for its keyboard connector.


[edit] Wireless types
Wireless keyboards have become popular for their increased user freedom. However, wireless keyboards need batteries to work, and may pose a security problem due to the risk of eavesdropping.[1]

A wireless keyboard often includes a required combination transmitter and receiver unit that attaches to the computer's keyboard port (see Connection types above). The wireless aspect is achieved either by radio frequency (RF) or by infrared (IR) signals sent and received from both the keyboard and the unit attached to the computer. A wireless keyboard may use an industry standard RF, called Bluetooth.


[edit] Buckling spring vs. dome switch
Keys on older IBM keyboards were made with a "buckling spring" mechanism, in which a coil spring under the key buckles under pressure from the user's finger, pressing a rubber dome, whose inside is coated with conductive graphite, which connects two leads below, completing a circuit. This produces a clicking sound, and a "positive" feel of feedback, so that the typist knows when the key is fully pressed. Keys on most modern keyboards are made with a so-called "dome switch" mechanism, without the buckling spring. Many typists prefer the buckling spring board, which is still manufactured.[2][3]


[edit] Alternatives

A multimedia keyboard like this one offers special keys for accessing music, web, and other oft-used programs.A standard keyboard is physically quite large, as each key must remain large enough to be easily pressed by fingers. Other types of keyboards have been proposed for small portable equipment where a standard keyboard is too large. One way to reduce the size of the keyboard is to reduce the number of keys and use chording keyer, i.e. pressing several keys simultaneously. For example, the GKOS keyboard has been designed for small wireless devices. Other two-handed alternatives more akin to a game controller, such as the AlphaGrip, are also used as a way to input data and text.

Another way to reduce the size of a keyboard is to use smaller buttons and pack them closer together. Such keyboards, often called a "thumbboard" (thumbing) are used in some personal digital assistants such as the Treo and BlackBerry and some Ultra-Mobile PCs such as the OQO.

A relatively new type of keyboard, the I-Tech Virtual Laser Keyboard, works by projecting an image of a full size keyboard onto a surface. Sensors in the projection unit identify which key is being "pressed" and relay the signals to a computer or PDA.

It is possible to limit or eliminate the use of computer keyboards with the introduction of speech recognition and optical character recognition. Speech recognition however, while is already implemented in various commercial products, is far away from the horizons where it can fully replace typing and represents a very difficult scientific research task being too dependent on voice characteristics.

Some keyboards are specifically designed for speed. The most common is the Dvorak keyboard layout. The fastest so far is the stenotype -- some people who use a stenotype type faster than 300 words per minute.


[edit] Standards
In principle, computer keyboard designs are governed by the ISO/IEC 9995 international standard.


[edit] Historical
While the IBM PC keyboard was hardly the first electronic keyboard, it does merit particular mention, if only for its ubiquity. The original IBM PC/XT had 83 keys, the AT keyboard had 84 (adding a SysRq key and separating keys into sections, also changing the communication protocol), next the "Enhanced" 101 keys (duplicating the cursor movement keys from the numeric pad, adding the function key row along the top and increasing their number from 10 to 12, other minor changes, and of course the often maligned control-key/caps-lock switch. The above-mentioned 104 keys were obtained by adding three "windows" keys. The internationally common 102/105 key keyboards have a smaller 'left shift' key and an additional key with some more symbols between that and the letter to its right (usually Z or Y). [4]


[edit] Usage

An on-screen keyboard controlled with the mouse can be used by users with limited mobility.In normal usage, the keyboard is used to type text into a word processor, text editor, or any other textbox.

In modern computers the interpretation of keypresses is generally left to the software. Modern keyboards distinguish each physical key from every other and report all keypresses to the controlling software. This flexibility is not often taken advantage of and it usually does not matter, for example, whether the left or right shift key is held down in conjunction with another character, even though they are coded as completely separate keys.


[edit] Keystroke

Laptop keyboards such as on this Sony Vaio have a shorter travel distance for the keystroke and usually have a reduced set of keys to make the keyboard (and laptop) more convenient to carry.A keystroke refers to the simple act of pressing a button on a keyboard that is connected to some form of digital computer. Nefarious programs may log keystrokes and thereby capture such sensitive information as operating system passwords and credit card numbers.


[edit] Commands
A keyboard is also used to type commands in a computer. One famous example on the PC is the Control-Alt-Delete combination. On most versions of Microsoft Windows, this command brings up a window (such as the Task Manager on Windows NT based versions of Windows) which allows users to manage currently-running processes, shut down the machine, and other functions. Under Linux, MS-DOS and some older versions of Windows, the command performs either a 'cold' or 'warm' reboot.


[edit] Games
A keyboard is one of the primary methods of control in computer games. For instance, the arrow keys or a group of letters resembling the pattern of the arrow keys, like WASD, can be used for movement of a game character. In many games, keys can be configured to the user's preferences. Alphabet keys are also sometimes used to perform actions starting with that letter. (e.g. pressing j to jump, r to reload or c to crouch).


[edit] Buying considerations
Some low-quality keyboards suffer problems when multiple keys are pressed in quick succession; some types of keyboard circuitry will register a maximum number of keys at one time. This is undesirable for games (designed for multiple keypresses, e.g. casting a spell while holding down keys to run) and undesirable for extremely fast typing (hitting new keys before the fingers can release previous keys). A common side effect of this shortcoming is called "phantom key blocking": on some keyboards, pressing three keys simultaneously sometimes resulted in a 4th keypress being registered. Modern keyboards prevent this from happening by blocking the 3rd key in certain key combinations, but while this prevents phantom input, it also means that when two keys are depressed simultaneously, many of the other keys on the keyboard will not respond until one of the two depressed keys is lifted. With better keyboards designs, this seldom happens in office programs, but it remains a problem in games even on expensive keyboards, due to wildly different and/or configurable key/command layouts in different games.


[edit] Safety Precautions
Some experts believe that the use of any keyboard may cause serious injury to hands, wrists, arms, neck or back. Ways to reduce the risks of injuries can be done by:

Take frequent short breaks. Get up and walk around at least a couple of times every hour.
Vary your tasks throughout the day.
Keep your shoulders relaxed with your elbows at your side. Postition your keyboard and mouse so you don't have to reach.
Adjust your chair and keyboard so your wrists are straight.
Avoid resting your wrists on sharp edges. If you use a wrist or palm rest don't use it while typing.

[edit] How it works
The following briefly describes a "dome-switch" keyboard (sometimes incorrectly referred to as a membrane keyboard), the most common type in use today:

When a key is pressed, it pushes down on a rubber dome sitting beneath the key. A conductive contact on the underside of the dome touches (and hence connects) a pair of conductive lines on the circuit below.
This bridges the gap between them and allows electric current to flow (the open circuit is closed).
A scanning signal is emitted by the chip along the pairs of lines to all the keys. When the signal in one pair becomes different, the chip generates a "make code" corresponding to the key connected to that pair of lines.
The code generated is sent to the computer either via a keyboard cable (using on-off electrical pulses to represent bits) or over a wireless connection. It may be repeated.
A chip inside the computer receives the signal bits and decodes them into the appropriate keypress. The computer then decides what to do on the basis of the key pressed (e.g. display a character on the screen, or perform some action).
When the key is released, a break code (different than the make code) is sent to indicate the key is no longer pressed. If the break code is missed (e.g. due to a keyboard switch) it is possible for the keyboard controller to believe the key is pressed down when it is not, which is why pressing then releasing the key again will release the key (since another break code is sent).
Other types of keyboards function in a similar manner, the main differences being how the individual key-switches work. For more on this subject refer to the article on keyboard technology.

Certain key presses are special, namely Ctrl-Alt-Delete and SysRq, but what makes them special is a function of software. In the PC architecture, the keyboard controller (the component in the computer that receives the make and break codes) sends the computer's CPU a hardware interrupt whenever a key is pressed or released. The CPU's interrupt routine which handles these interrupts usually just places the key's code in a queue, to be handled later by other code when it gets around to it, then returns to whatever the computer was doing before. The special keys cause the interrupt routine to take a different "emergency" exit instead. This more trusted route is much harder to intercept.


[edit] Customization
Sometimes, it is desired to customize the layout of a keyboard or remap the keys. Keyboard remapping is supported at a driver level configurable within the operating system, or as add-ons to the existing programs.

For Windows, Microsoft provides a free downloadable tool called Microsoft Keyboard Layout Creator, and there are several other software programs for this purpose:

SharpKeys: free
KeyTweak: free
Under systems running X11 (e.g. GNU/Linux) this can be done with xmodmap.


[edit] Keys on a computer keyboard
Modifier key
Control key
Shift key
Alt key / Option key (Macintosh)
AltGr key
Command key / Meta key (MIT computer keyboards)
Windows key
Fn key (compact keyboard layout)
Dead key
Compose key
Lock key
Scroll lock
Num lock
Caps lock
Navigation keys
Arrow keys
Page scroll keys (Page up key / Page down key)
Home / End key
Edit keys
Return key / Enter key
Backspace
Insert key
Delete key
Tab key
SysRq / Print screen
Break / Pause key
Escape key
Menu key
Space bar
Numeric keypad
Function key
Language input keys (on Japanese/Korean keyboards)
Kanji key
Conversion key
Non-conversion key
Half-width/Full-width key
Hiragana/Katakana key
Alphanumeric key
Hancha key
Han/Yong key
Power management keys
Power key
Sleep key
Wake key
Internet keys:
Internet (web browser) key.
E-mail
Multimedia keys:
Volume keys or wheel (+/-/mute).

[edit] See also
Wikimedia Commons has media related to:
KeyboardErgonomics
Repetitive strain injury
Keyboard styles
Alphanumeric keyboard
Enhanced keyboard
AT keyboard
Velotype (chorded keyboard)
Virtual keyboard
Optimus Maximus keyboard
Das Keyboard (blank keyboard)
"fixed split keyboard" and "adjustable split keyboard" with a larger gap between the "left hand keys" and the "right hand keys"
EZ-Reach (keyboard)
Microsoft Natural keyboard
Maltron keyboard
Kinesis keyboard
Datahand (featuring joystick-like buttons to reduce finger movement)
Keyboard layout
Dvorak keyboard
British and American keyboards
Keyboard technology
Membrane keyboard
Chiclet keyboard
Buckling spring
Apple Keyboard
ASCII
ASDF (sequence of letters)
British and American keyboards
Chinese input methods for computers
Chord keyset
F-Lock
GKOS keyboard (chorded keyboard)
IBM PC keyboard
Lighted Program Function Keyboard
Model M Keyboard
Overlay keyboard
Space-cadet keyboard
Touch typing
Typing
Hunt and peck typing
Home row
Key jamming
Rollover (key)
QWERTY and accents
Table of keyboard shortcuts
Happy Hacking Keyboard
FrogPad
References regarding problems with keypresses in short succession:

http://forums.logitech.com/logitech/board/message?board.id=hardware&thread.id=991
http://ask.metafilter.com/51940/Whats-a-brand-of-keyboard-that-allow-multiple-keys-to-be-pressed

[edit] References
^ Brandt, Andrew. "Privacy Watch: Wireless Keyboards That Blab", PC World, 2003-01-29.
^ A Passion for the Keys: Particular About What You Type On? Relax -- You're Not Alone. LOOSE WIRE, By JEREMY WAGSTAFF, Wall Street Journal, November 23, 2007
^ Dan's Data Review: IBM 42H1292 and 1391401 keyboards, Review date: 15 August 1999, updated 13-Nov-2007]
^ "Standard Keyboard Layouts".

[edit] External links
Large searchable database of keyboard shortcuts at Keyxl.com
Keyboard Help — Typing world language accent marks and other diacritics with your keyboard.
Online Translit - Free keyboard layout conversion web service
Keyboard Utility - Keyboard Lock Software



IBM/Wintel Keyboard: v • d • e
Esc F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 PtSc ScLk Brk

Ins Home PgUp Num / * -
Del End PgDn 7 8 9 +
4 5 6
↑ 1 2 3 Ent
← ↓ → 0 .
[hide]v • d • eKeyboard keys
Dead keys Compose
Modifier keys Control · Shift · Alt/Option (Apple) · AltGr · Command/Meta (Apple/MIT/Sun keyboards) · Windows/Super · Fn (compact keyboards)
Lock keys Scroll lock · Num lock · Caps lock
Navigation Arrow · Page Up/Page Down · Home/End
Editing Return/Enter · Backspace · Insert · Delete · Tab · Space bar
Misc. System request/Print screen · Break/Pause · Escape · Menu · Numeric keypad · Function · Power management (Power, Sleep, Wake) · Language input · Any key

Video monitor

A video monitor is a device similar to a television, used to monitor the output of a video generating device, such as a video camera, VCR, or DVD player. It may or may not have audio monitoring capability.

Unlike a television, a video monitor has no tuner and, as such, is unable to independently tune into an over-the-air broadcast.

One common use of video monitors in is Television stations and Outside broadcast vechicles, where broadcast engineers use them for confidence checking of signals throughout the system.

Video monitors are also used extensively in the security industry with Closed-circuit television cameras and recording devices.


[edit] Common display types for video monitors
Cathode ray tube
Liquid crystal display
Plasma display

[edit] Common monitoring formats for broadcasters
Serial Digital Interface (SDI, as SD-SDI or HD-SDI)
Composite video
Component video

[edit] Common monitoring formats for security
Composite video
S-Video

Hard disk drive

A hard disk drive (HDD), commonly referred to as a hard drive, hard disk or fixed disk drive,[1] is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit with fixed media.[2]

HDDs were originally developed for use with computers. In the 21st century, applications for HDDs have expanded beyond computers to include digital video recorders, digital audio players, personal digital assistants, digital cameras and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia.[3] The need for large-scale, reliable storage, independent of a particular device, led to the introduction of configurations such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.

Contents [hide]
1 Technology
2 Capacity and access speed
2.1 Capacity measurements
3 Hard disk drive characteristics
4 Access and interfaces
4.1 Disk interface families used in personal computers
5 Integrity
5.1 Landing zones and load/unload technology
5.2 Disk failures and their metrics
6 Manufacturers
7 History
8 See also
9 Notes and References
10 External links



[edit] Technology
HDDs record data by magnetizing a ferromagnetic material directionally, to represent either a 0 or a 1 binary digit. They read the data back by detecting the magnetization of the material. A typical 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 glass or aluminum, and are coated with a thin layer of magnetic material. Older disks used iron(III) oxide as the magnetic material, but current disks use a cobalt-based alloy.


A hard disk drive with the disks and spindle motor hub removed. In the center, the internal structure of the spindle motor can be seen. To the left of center is the actuator arm with a read-write head under the tip of its very end (near center); the orange wires along the side of the arm are part of the path the signals take to and from the read-write head. The flexible, somewhat 'U'-shaped, ribbon cable barely visible below and to the left of the actuator arm is another part of its path connecting the head to the controller board on the opposite side.
A cross section of the magnetic surface in action. In this case the binary data encoded using frequency modulation:The platters are spun at very high speeds. Information is written to a platter as it rotates past mechanisms called read-and-write heads that operate very close over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it. There is one head for each magnetic platter surface on the spindle, mounted on a common arm. An actuator arm (or access arm) moves the heads on an arc (roughly radially) across the platters as they spin, allowing each head to access almost the entire surface of the platter as it spins. The arm is moved using a voice coil actuator or (in older designs) a stepper motor.

The magnetic surface of each platter is divided into many small sub-micrometre-sized magnetic regions, each of which is used to encode a single binary unit of information. In today's HDDs each of these magnetic regions is composed of a few hundred magnetic grains. Each magnetic region forms a magnetic dipole which generates a highly localized magnetic field nearby. The write head magnetizes a magnetic region by generating a strong local magnetic field nearby. Early HDDs used an electromagnet both to generate this field and to read the data by using electromagnetic induction. Later versions of inductive heads included metal in Gap (MIG) heads and thin film heads. In today's heads, the read and write elements are separate but in close proximity on the head portion of an actuator arm. The read element is typically magneto-resistive while the write element is typically thin-film inductive.[4]

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state be lost because of thermal effects. To counter this, the platters are coated with two parallel magnetic layers, separated by a 3-atom-thick layer of the non-magnetic element ruthenium, and the two layers are magnetized in opposite orientation, thus reinforcing each other.[5] Another technology used to overcome thermal effects to allow greater recording densities is perpendicular recording, which has been used in some hard drives as of 2006.

Hard disk drives are sealed to prevent dust and other sources of contamination from interfering with the operation of the hard disks heads. The hard drives are not air tight, but rather utilize an extremely fine air filter, to allow for air inside the hard drive enclosure. The spinning of the disks causes the air to circulate forcing any particulates to become trapped on the filter. The same air currents also act as a gas bearing which enables the heads to float on a cushion of air above the surfaces of the disks.

Hard drives are precise devices, moving at very high speed, and a number of analogies have been made to try to describe this. One states:

“ As an analogy, a magnetic head slider flying over a disk surface with a flying height of 25 nm with a relative speed of 20 meters/second is equivalent to an aircraft flying at a physical spacing of 0.2 µm at 900 kilometers/hour. This is what a disk drive experiences during its operation. ”
—Magnetic Storage Systems Beyond 2000, George C. Hadjipanayis, p. 487



[edit] Capacity and access speed

PC hard disk drive capacity (in GB). The plot is logarithmic, so the fit line corresponds to exponential growth.Using rigid disks and sealing the unit allows much tighter tolerances than in a floppy disk drive. Consequently, hard disk drives can store much more data than floppy disk drives and can access and transmit it faster. In 2007, a typical enterprise, i.e. workstation HDD, might store between 160 GB and 1 TB of data (as of local US market by July 2007), rotate at 7,200 or 10,000 revolutions per minute (RPM) and have a media transfer rate of over 1 Gbit/sec or higher[6] The fastest enterprise HDDs spin at 15,000 rpm, and can achieve sequential media transfer speeds above 1.6 Gbit/sec.[7] Mobile, i.e., Laptop HDDs, which are physically smaller than their desktop and enterprise counterparts, tend to be slower and have less capacity. In the 1990s, most spun at 4,200 rpm.[8] In 2007, a typical mobile HDD spins at 5,400 rpm, with 7,200 rpm models available for a slight price premium.

The exponential increases in disk space and data access speeds of HDDs have enabled the commercial viability of consumer products that require large storage capacities, such as the TiVo personal video recorder and digital music players.[9] In addition, the availability of vast amounts of cheap storage has made viable a variety of web-based systems with extraordinary capacity requirements, such as the search and email systems offered by companies like Google.

The main way to decrease access time is to increase rotational speed, while the main way to increase throughput and storage capacity is to increase areal density. A vice president of Seagate Technology projects a future growth in disk density of 40% per year.[10] Access times have not kept up with throughput increases, which themselves have not kept up with growth in storage capacity.

As of 2006, disk drives include perpendicular recording technology, in an attempt to increase recording density and throughput.[11]

The first 3.5" HDD marketed as able to store 1 TB is the Hitachi Deskstar 7K1000. It contains five platters at approximately 200 GB each, providing 935.5 GiB of usable space.[12] Hitachi has since been joined by Samsung (Samsung SpinPoint F1, which has 3x334GB platters) , Seagate and Western Digital in the 1 TB drive market.[13][14]

Standard Name Width Largest capacity to date (2007) Platters (Max)
5.25" FH 146 mm 47 GB[15] 14
5.25" HH 146 mm 19.3 GB[16] 4[17]
3.5" 102 mm 1 TB[12] 5
2.5" 69.9 mm 320 GB[18] 3
1.8" (PCMCIA) 54 mm 160 GB[19]
1.8" (ATA-7 LIF) 53.8 mm


[edit] Capacity measurements

A disassembled and labeled 1997 hard drive.The capacity of an HDD can be calculated by multiplying the number of cylinders by the number of heads by the number of sectors by the number of bytes/sector (most commonly 512). On ATA drives bigger than eight gigabytes, the values are set to 16383 cylinder, 16 heads, 63 sectors for compatibility with older operating systems. It should be noted that the values for cylinder, head & sector reported by a modern drive are not the actual physical parameters since, amongst other things, with zone bit recording the number of sectors varies by zone.

Hard disk drive manufacturers specify disk capacity using the SI prefixes mega, giga and tera, and their abbreviations M, G and T. Byte is typically abbreviated B.

Operating systems frequently report capacity using the same abbreviations but in reference to binary-based units. For instance, the prefix mega in the context of data storage can mean 220 (1,048,576), which is approximately equal to the actual value of the SI prefix mega, 106 (1,000,000). Similar usage has been applied to prefixes of greater magnitude. This results in a discrepancy between the disk manufacturer's stated capacity and the apparent capacity of the drive when examined through the operating system. The difference becomes much more noticeable in the multi-gigabyte range. For example, Microsoft Windows reports disk capacity both in decimal-based units to 12 or more significant digits and with binary-based units to three significant digits. Thus a disk specified by a disk manufacturer as a 30 GB disk might have its capacity reported by Windows 2000 both as "30,065,098,568 bytes" and "28.0 GB". The disk manufacturer used the SI definition of "giga", 109 to arrive at 30 GB; however, because the utilities provided by Windows define a gigabyte as 1,073,741,824 bytes (230 bytes, often referred to as a gibibyte, or GiB), the operating system reports capacity of the disk drive as (only) 28.0 GB.


[edit] Hard disk drive characteristics

5.25" MFM 110 MB HDD (2.5" ATA 6495 MB HDD, US & UK pennies for comparison)Capacity of a hard disk drive is usually quoted in gigabytes. Older HDDs quoted their smaller capacities in megabytes.

The data transfer rate at the inner zone ranges from 44.2 MB/s to 74.5 MB/s, while the transfer rate at the outer zone ranges from 74.0 MB/s to 111.4 MB/s. An HDD's random access time ranges from 5 ms to 15 ms.

The physical size of a hard disk drive is quoted in inches. The majority of HDDs used in desktops today are 3.5 inches (9 cm) wide, while the majority of those used in laptops are 2.5 inches (6 cm) wide. As of early 2007, manufacturers have started selling SATA and SAS 2.5 inch drives for use in servers and desktops.

An increasingly common form factor is the 1.8-inch (5 cm) ATA-7 LIF form factor used inside digital audio players and subnotebooks, which provide up to 160GB storage capacity at low power consumption and are highly shock-resistant. A previous 1.8-inch (5 cm) HDD standard exists, for 2–5 GB sized disks that fit directly into a PC card expansion slot. From these, the smaller 1-inch (3 cm) form factor was evolved, which is designed to fit the dimensions of CF Type II, which is also usually used as storage for portable devices including digital cameras. 1 inch was a de facto form factor led by IBM's Microdrive, but is now generically called 1 inch due to other manufacturers producing similar products. There is also a 0.85-inch (2 cm) form factor produced by Toshiba for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets.

The size designations are more nomenclature than descriptive. The names refer to the width of the disk inserted into the drive rather than the actual width of the entire drive. A 5.25 inches (13 cm) drive has an actual width of 5.75 inches (15 cm), a 3.5 inches (9 cm) drive 4 inches (10 cm), a 2.5 inches (6 cm) drive 2.75 inches (7 cm). A 1.8-inch (5 cm) drive can have different widths, depending on its form factor. A PCMCIA drive has a width of 54 mm, while an ATA-7 LIF form factor drive has a width of 53.85 mm.

A hard disk is defined to be at "full height" if its height is 3.25 inches (8 cm). It is "half height" at a height of 1.625 inches (4 cm). A "slim height" or "low profile" HDD has a height of 1 inch (3 cm). "Ultra low profile" drives can have heights of 0.75 inches (19 mm), 0.67 inches (17 mm), 0.49 inches (12 mm) or 0.37 inches (9 mm).[citation needed]


[edit] Access and interfaces
This section may require cleanup to meet Wikipedia's quality standards.
Please improve this article if you can (October 2007).

Hard disk drives are accessed over one of a number of bus types, including parallel ATA (also called IDE or EIDE), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect hard disk drives to buses that they cannot communicate with natively, such as IEEE 1394 and USB.

Back in the days of the ST-506 interface, the data encoding scheme was also important. The first ST-506 disks used Modified Frequency Modulation (MFM) encoding, and transferred data at a rate of 5 megabits per second. Later on, controllers using 2,7 RLL (or just "RLL") encoding increased the transfer rate by 50%, to 7.5 megabits per second; this also increased disk capacity by fifty percent.

Many ST-506 interface disk drives were only specified by the manufacturer to run at the lower MFM data rate, while other models (usually more expensive versions of the same basic disk drive) were specified to run at the higher RLL data rate. In some cases, a disk drive had sufficient margin to allow the MFM specified model to run at the faster RLL data rate; however, this was often unreliable and was not recommended. (An RLL-certified disk drive could run on a MFM controller, but with 1/3 less data capacity and speed.)

Enhanced Small Disk Interface (ESDI) also supported multiple data rates (ESDI disks always used 2,7 RLL, but at 10, 15 or 20 megabits per second), but this was usually negotiated automatically by the disk drive and controller; most of the time, however, 15 or 20 megabit ESDI disk drives weren't downward compatible (i.e. a 15 or 20 megabit disk drive wouldn't run on a 10 megabit controller). ESDI disk drives typically also had jumpers to set the number of sectors per track and (in some cases) sector size.

SCSI originally had just one speed, 5 MHz (for a maximum data rate of five megabytes per second), but later this was increased dramatically. The SCSI bus speed had no bearing on the disk's internal speed because of buffering between the SCSI bus and the disk drive's internal data bus; however, many early disk drives had very small buffers, and thus had to be reformatted to a different interleave (just like ST-506 disks) when used on slow computers, such as early IBM PC compatibles and early Apple Macintoshes.

ATA disks have typically had no problems with interleave or data rate, due to their controller design, but many early models were incompatible with each other and couldn't run in a master/slave setup (two disks on the same cable). This was mostly remedied by the mid-1990s, when ATA's specification was standardised and the details began to be cleaned up, but still causes problems occasionally (especially with CD-ROM and DVD-ROM disks, and when mixing Ultra DMA and non-UDMA devices).

Serial ATA does away with master/slave setups entirely, placing each disk on its own channel (with its own set of I/O ports) instead.

FireWire/IEEE 1394 and USB(1.0/2.0) HDDs are external units containing generally ATA or SCSI disks with ports on the back allowing very simple and effective expansion and mobility. Most FireWire/IEEE 1394 models are able to daisy-chain in order to continue adding peripherals without requiring additional ports on the computer itself.


[edit] Disk interface families used in personal computers
Notable families of disk interfaces include:

Historical bit serial interfaces — connected to a hard disk drive controller with three cables, one for data, one for control and one for power. The HDD controller provided significant functions such as serial to parallel conversion, data separation and track formatting, and required matching to the drive in order to assure reliability.
ST506 used MFM (Modified Frequency Modulation) for the data encoding method.
ST412 was available in either MFM or RLL (Run Length Limited) variants.
Enhanced Small Disk Interface (ESDI) was an interface developed by Maxtor to allow faster communication between the PC and the disk than MFM or RLL.
Word serial interfaces — connect to a host bus adapter (today typically integrated into the "south bridge") with two cables, one for data/control and one for power. The earliest versions of these interfaces typically had a 16 bit parallel data transfer to/from the drive and there are 8 and 32 bit variants. Modern versions have serial data transfer. The word nature of data transfer makes the design of a host bus adapter significantly simpler than that of the precursor HDD controller.
Integrated Drive Electronics (IDE), later renamed to ATA, and then later to PATA ("parallel ATA", to distinguish it from the new Serial ATA). The original name reflected the innovative integration of HDD controller with HDD itself, which was not found in earlier disks. Moving the HDD controller from the interface card to the disk drive helped to standardize interfaces, including reducing the cost and complexity. The 40 pin IDE/ATA connection of PATA transfers 16 bits of data at a time on the data cable. The data cable was originally 40 conductor, but later higher speed requirements for data transfer to and from the hard drive led to an "ultra DMA" mode, known as UDMA, which required an 80 conductor variant of the same cable; the other conductors provided the grounding necessary for enhanced high-speed signal quality. The interface for 80 pin only has 39 pins, the missing pin acting as a key to prevent incorrect insertion of the connector to an incompatible socket, a common cause of disk and controller damage.
EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA does not require the CPU/program/operating system to leave other tasks idle while the data transfer occurs.
Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, was an early competitor of ESDI. SCSI disks were standard on servers, workstations, and Apple Macintosh computers through the mid-90s, by which time most models had been transitioned to IDE (and later, SATA) family disks. Only in 2005 did the capacity of SCSI disks fall behind IDE disk technology, though the highest-performance disks are still available in SCSI and Fibre Channel only. The length limitations of the data cable allows for external SCSI devices. Originally SCSI data cables used single ended data transmission, but server class SCSI could use differential transmission, either low voltage differential (LVD) or high voltage differential (HVD).
Fibre Channel (FC), is a successor to parallel SCSI interface on enterprise market. It is a serial protocol. In disk drives usually the Fibre Channel Arbitrated Loop (FC-AL) connection topology is used. FC has much broader usage than mere disk interfaces, it is the cornerstone of storage area networks (SANs). Recently other protocols for this field, like iSCSI and ATA over Ethernet have been developed as well. Confusingly, drives usually use copper twisted-pair cables for Fibre Channel, not fibre optics. The latter are traditionally reserved for larger devices, such as servers or disk array controllers.
Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. The same differential signaling system is used in RS485, LocalTalk, USB, Firewire, and differential SCSI.
Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.
Acronym Meaning Description
SASI Shugart Associates System Interface Historical predecessor to SCSI.
SCSI Small Computer System Interface Bus oriented that handles concurrent operations.
SAS Serial Attached SCSI Improvement of SCSI, uses serial communication instead of parallel.
ST-506 Historical Seagate interface.
ST-412 Historical Seagate interface (minor improvement over ST-506).
ESDI Enhanced Small Disk Interface Historical; backwards compatible with ST-412/506, but faster and more integrated.
ATA Advanced Technology Attachment Successor to ST-412/506/ESDI by integrating the disk controller completely onto the device. Incapable of concurrent operations.
SATA Serial ATA Improvement of ATA, uses serial communication instead of parallel.


[edit] Integrity

An IBM HDD head resting on a disk platter. Since the drive is not in operation, the head is simply pressed against the disk by the suspension.
Close-up of a hard disk head resting on a disk platter, and its suspension. A reflection of the head and suspension are visible beneath on the mirror-like disk.Due to the extremely close spacing between the heads and the disk surface, any contamination of the read-write heads or platters can lead to a head crash — a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, wear and tear, corrosion, or poorly manufactured platters and heads.

The HDD's spindle system relies on air pressure inside the enclosure to support the heads at their proper flying height while the disk rotates. An HDD requires a certain range of air pressures in order to operate properly. The connection to the external environment and pressure occurs through a small hole in the enclosure (about 0.5 mm in diameter), usually with a carbon filter on the inside (the breather filter, see below). If the air pressure is too low, then there is not enough lift for the flying head, so the head gets too close to the disk, and there is a risk of head crashes and data loss. Specially manufactured sealed and pressurized disks are needed for reliable high-altitude operation, above about 10,000 feet (3,000 m). Note that modern commercial aircraft have a pressurized cabin, whose pressure altitude does not normally exceed 8,500 feet - thus, ordinary hard drives can safely be used in flight. Modern disks include temperature sensors and adjust their operation to the operating environment. Breather holes can be seen on all disks — they usually have a sticker next to them, warning the user not to cover the holes. The air inside the operating disk is constantly moving too, being swept in motion by friction with the spinning platters. This air passes through an internal recirculation (or "recirc") filter to remove any leftover contaminants from manufacture, any particles or chemicals that may have somehow entered the enclosure, and any particles or outgassing generated internally in normal operation. Very high humidity for extended periods can corrode the heads and platters.

For giant magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes (so called "thermal asperity," a problem which can partially be dealt with by proper electronic filtering of the read signal).

The hard disk's electronics control the movement of the actuator and the rotation of the disk, and perform reads and writes on demand from the disk controller. Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.


[edit] Landing zones and load/unload technology

Microphotograph of a hard disk head. The size of the front face (which is the "trailing face" of the slider) is about 0.3 mm × 1.0 mm. The (not visible) bottom face of the slider is about 1.0 mm × 1.25 mm (so called "nano" size) and faces the platter. One functional part of the head is the round, orange structure in the middle - the lithographically defined copper coil of the write transducer. Also note the electric connections by wires bonded to gold-plated pads.In old disk models, power interruptions could result in the device shutting down with the heads in the data zone, which greatly increased the risk of data loss. To minimise this risk, a manual procedure existed for parking the hard disk heads before shutting down the computer.

To prevent such situation, most modern HDDs move the heads to a landing zone when powering down: this is an area of the platter usually near its inner diameter (ID), where no data is stored. This area is called the Contact Start/Stop (CSS) zone. Disks are designed such that either a spring or, more recently, rotational inertia in the platters is used to park the heads in the case of unexpected power loss.

Spring tension from the head mounting constantly pushes the heads towards the platter. While the disk is spinning, the heads are supported by an air bearing and experience no physical contact or wear. In CSS drives the sliders carrying the head sensors (often also just called heads) are designed to survive a number of landings and takeoffs from the media surface, though wear and tear on these microscopic components eventually takes its toll. Most manufacturers design the sliders to survive 50,000 contact cycles before the chance of damage on startup rises above 50%. However, the decay rate is not linear: when a disk is younger and has had fewer start-stop cycles, it has a better chance of surviving the next startup than an older, higher-mileage disk (as the head literally drags along the disk's surface until the air bearing is established). For example, the Seagate Barracuda 7200.10 series of desktop hard disks are rated to 50,000 start-stop cycles.[20] This means that no failures attributed to the head-platter interface were seen before at least 50,000 start-stop cycles during testing.

Around 1995 IBM pioneered a technology where a landing zone on the disk is made by a precision laser process (Laser Zone Texture = LZT) producing an array of smooth nanometer-scale "bumps" in a landing zone, thus vastly improving stiction and wear performance. This technology is still largely in use today (2007), predominantly in desktop and enterprise (3.5 inch) drives. In general, CSS technology can be prone to increased stiction (the tendency for the heads to stick to the platter surface), e.g. as a consequence of increased humidity. Excessive stiction can cause physical damage to the platter and slider or spindle motor. This drawback, combined with the need of higher non-operating shock robustness in mobile applications has led to the introduction of load/unload technology for drives used in laptops, MP3 players, and other portable devices. Load/unload technology, also introduced by IBM around the same time frame as LZT, relies on the heads being lifted off the platters onto plastic "ramps" near the outer disk edge, thus eliminating the risks of wear and stiction altogether. All HDDs today still use one of these two technologies. Each has a list of advantages and drawbacks in terms of loss of storage area on the disk, relative difficulty of mechanical tolerance control, cost of implementation, etc.

Addressing shock robustness, IBM also created a technology for their ThinkPad line of laptop computers called the Active Protection System. When a sudden, sharp movement is detected by the built-in accelerometer in the Thinkpad, internal hard disk heads automatically unload themselves to reduce the risk of any potential data loss or scratch defects. Apple later also utilized this technology in their PowerBook, iBook, MacBook Pro, and MacBook line, known as the Sudden Motion Sensor. Toshiba has released similar technology in their laptops.[21]


[edit] Disk failures and their metrics
Most major hard disk and motherboard vendors now support self-monitoring, analysis and reporting technology (S.M.A.R.T.), which attempts to alert users to impending failures.

However, not all failures are predictable. Normal use eventually can lead to a breakdown in the inherently fragile device, which makes it essential for the user to periodically back up the data onto a separate storage device. Failure to do so can lead to the loss of data. While it may be possible to recover lost information, it is normally an extremely costly procedure, and it is not possible to guarantee success. A 2007 study published by Google suggested very little correlation between failure rates and either high temperature or activity level.[22] While several S.M.A.R.T. parameters have an impact on failure probability, a large fraction of failed drives do not produce predictive S.M.A.R.T. parameters.[22] S.M.A.R.T. parameters alone may not be useful for predicting individual drive failures.[22]

SCSI, SAS and FC drives are typically more expensive and are traditionally used in servers and disk arrays, whereas inexpensive ATA and SATA drives evolved in the home computer market and were perceived to be less reliable. This distinction is now becoming blurred.

The mean time between failures (MTBF) of SATA drives is usually about 600,000 hours (some drives such as Western Digital Raptor have rated 1.2 million hours MTBF), while SCSI drives are rated for upwards of 1.5 million hours.[citation needed] However, independent research indicates that MTBF is not a reliable estimate of a drive's longevity.[23] MTBF is conducted in laboratory environments in test chambers and is an important metric to determine the quality of a disk drive before it enters high volume production. Once the drive product is in production, the more valid[citation needed] metric is annualized failure rate (AFR). AFR is the percentage of real-world drive failures after shipping.

SAS drives are comparable to SCSI drives, with high MTBF and high[citation needed] reliability.

Enterprise SATA drives designed and produced for enterprise markets, unlike standard SATA drives, have reliability comparable other enterprise class drives.[citation needed]

Typically enterprise drives (all enterprise drives, including SCSI, SAS, enterprise SATA and FC) experience between .70%-.78% annual failure rates from the total installed drives.[citation needed]


[edit] Manufacturers
This section does not cite any references or sources.
Please improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed. (June 2006)

A Western Digital 3.5 inch 250 GB SATA HDD.The technological resources and know-how required for modern drive development and production mean that as of 2007, over 98% of the world's HDDs are manufactured by just a handful of large firms: Seagate (which now owns Maxtor), Western Digital, Samsung, and Hitachi (which owns the former disk manufacturing division of IBM). Fujitsu continues to make mobile- and server-class disks but exited the desktop-class market in 2001. Toshiba is a major manufacturer of 2.5-inch and 1.8-inch notebook disks. ExcelStor is a small HDD manufacturer.

Dozens of former HDD manufacturers have gone out of business, merged, or closed their HDD divisions; as capacities and demand for products increased, profits became hard to find, and the market underwent significant consolidation in the late 1980s and late 1990s. The first notable casualty of the business in the PC era was Computer Memories Inc. or CMI; after an incident with faulty 20 MB AT disks in 1985,[24] CMI's reputation never recovered, and they exited the HDD business in 1987. Another notable failure was MiniScribe, who went bankrupt in 1990 after it was found that they had engaged in accounting fraud and inflated sales numbers for several years. Many other smaller companies (like Kalok, Microscience, LaPine, Areal, Priam and PrairieTek) also did not survive the shakeout, and had disappeared by 1993; Micropolis was able to hold on until 1997, and JTS, a relative latecomer to the scene, lasted only a few years and was gone by 1999, after attempting to manufacture HDDs in India. Their claim to fame was creating a new 3" form factor drive for use in laptops. Quantum and Integral also invested in the 3" form factor; but eventually gave up as this form factor failed to catch on.[citation needed] Rodime was also an important manufacturer during the 1980s, but stopped making disks in the early 1990s amid the shakeout and now concentrates on technology licensing; they hold a number of patents related to 3.5-inch form factor HDDs.

This list is incomplete; you can help by expanding it.
1988: Tandem Computers sold its disk manufacturing division to Western Digital (WDC), which was then a well-known controller designer.
1989: Seagate Technology bought Control Data's high-end disk business, as part of CDC's exit from hardware manufacturing.
1990: Maxtor buys MiniScribe out of bankruptcy, making it the core of its low-end disk division.
1994: Quantum bought DEC's storage division, giving it a high-end disk range to go with its more consumer-oriented ProDrive range, as well as the DLT tape drive range.
1995: Conner Peripherals, which was founded by one of Seagate Technology's co-founders along with personnel from MiniScribe, announces a merger with Seagate, which was completed in early 1996.
1996: JTS merges with Atari, allowing JTS to bring its disk range into production. Atari was sold to Hasbro in 1998, while JTS itself went bankrupt in 1999.
2000: Quantum sells its disk division to Maxtor to concentrate on tape drives and backup equipment.
2003: Following the controversy over mass failures of its Deskstar 75GXP range, HDD pioneer IBM sold the majority of its disk division to Hitachi, who renamed it Hitachi Global Storage Technologies (HGST).
December 21, 2005: Seagate and Maxtor announced an agreement under which Seagate would acquire Maxtor in an all stock transaction valued at $1.9 billion. The acquisition was approved by the appropriate regulatory bodies, and closed on May 19, 2006.
2007
April: Hitachi releases the 1 TB Hitachi Deskstar 7K1000 (1TB = 1 trillion bytes, roughly 931.5 GiB).[25][26][27]
July: Western Digital (WDC) acquires Komag U.S.A, a thin-film media manufacturer, for USD 1 Billion.[28]
September: Hitachi releases 2.5-inch 320 GB hard disk.

[edit] History
Main article: History of hard disk drives

IBM 62PC "Piccolo" HDD, circa 1979 - an early 8" diskFor many years, HDDs were large, cumbersome devices, more suited to use in the protected environment of a data center or large office than in a harsh industrial environment (due to their delicacy), or small office or home (due to their size and power consumption). Before the early 1980s, most HDDs had 8-inch (20 cm) or 14-inch (35 cm) platters, required an equipment rack or a large amount of floor space (especially the large removable-media disks, which were often referred to as "washing machines"), and in many cases needed high-current or even three-phase power hookups due to the large motors they used. Because of this, HDDs were not commonly used with microcomputers until after 1980, when Seagate Technology introduced the ST-506, the first 5.25-inch HDD, with a capacity of 5 megabytes. In fact, in its factory configuration, the original IBM PC (IBM 5150) was not equipped with a hard disk drive.[citation needed]

Most microcomputer HDDs in the early 1980s were not sold under their manufacturer's names, but by OEMs as part of larger peripherals (such as the Corvus Disk System and the Apple ProFile). The IBM PC/XT had an internal HDD, however, and this started a trend toward buying "bare" disks (often by mail order) and installing them directly into a system. Hard disk drive makers started marketing to end users as well as OEMs, and by the mid-1990s, HDDs had become available on retail store shelves.

While internal disks became the system of choice on PCs, external HDDs remained popular for much longer on the Apple Macintosh and other platforms. The first Apple Macintosh built between 1984 and 1986 had a closed architecture that did not support an external or internal hard drive. In 1986, Apple added a SCSI port on the back, making external expansion easy. External SCSI drives were also popular with older microcomputers such as the Apple II series, and were also used extensively in servers, a usage which is still popular today. The appearance in the late 1990s of high-speed external interfaces such as USB and FireWire has made external disk systems popular among PC users once again, especially for laptop users, users that install Linux in the additional external unit and users who move large amounts of data between two or more areas. Most HDD makers now make their disks available in external cases.


[edit] See also
Click of death
Disk Usage
Disk formatting
Hybrid drive

[edit] Notes and References
^ Other terms used to describe hard disk drives include disk drive, disk file, DASD (Direct Access Storage Device), fixed disk, CKD disk and Winchester Disk Drive (after the IBM 3340).
^ How Hard Disks Work, howstuffworks.com
^ Finally! The Samsung SPH-V5400, world's first cellphone with a hard drive, engadget.com, 6 September 2004
^ http://www.hitachigst.com/hdd/technolo/gmr/gmr.htm
^ Brian Hayes, Terabyte Territory, American Scientist, Vol 90 No 3 (May-June 2002) p. 212
^ HGST Deskstar 7K1000[1]
^ Seagate Cheetah 15K.5[2]
^ The 1999 Disk/Trend Hard Disk Drive Report lists 81 mobile HDDs having rotational speeds ranging from 3,634 to 4,900 rpm with 44 models operating at 4200 rpm
^ Walter, Chip. "Kryder's Law", Scientific American, Verlagsgruppe Georg von Holtzbrinck GmbH, 25 July 2005. Retrieved on 2006-10-29.
^ http://www.hardwarezone.com/articles/view.php?cid=1&id=1805&pg=2
^ 500GB SATA drives reviews
^ a b Hitachi's 7K1000 Terabyte Hard Drive
^ Seagate, Samsung Begin to Ship 1 TB Desktop Hard Drives
^ WD Caviar GP: The "Green" 1 TB Drive
^ Seagate Elite 47, shipped 12/97 per 1998 Disk/Trend Report - Rigid Disk Drives
^ Quantum Bigfoot TS, shipped 10/98 per 1999 Disk/Trend Report - Rigid Disk Drives
^ The Quantum Bigfoot TS used a maximum of 3 platters, other earlier and lower capacity product used up to 4 platters in a 5.25" HH form factor, e.g. Microscience HH1090 circa 1989.
^ Toshiba's 320GB 2.5-inch hard drive: a world's best for laptops. 070824 http://www.engadget.com
^ Samsung unveils 160GB iPod-sized drive. 070808 macnn.com
^ http://www.seagate.com/support/disc/manuals/sata/100402371a.pdf
^ Toshiba HDD Protection measures.
^ a b c Barroso, L.A., et al. Failure Trends in a Large Disk Drive Population. February 2007.
^ Everything You Know About Disks Is Wrong. StorageMojo (February 20, 2007). Retrieved on 2007-08-29.
^ Apparently the CMI disks suffered from a higher soft error rate than IBM's other suppliers (Seagate and MiniScribe) but the bugs in Microsoft's DOS Operating system may have turned these recoverable errors into hard failures. At some point, possibly MS-DOS 3.0, soft errors were reported as disk hard errors and a subsequent Microsoft patch turned soft errors into corrupted memory with unpredictable results ("crashes"). MS-DOS 3.3 apparently resolved this series of problems but by that time it was too late for CMI. See also, "IBM and CMI in Joint Effort to Rehab AT Hard-Disk Rejects," PC Week, v.2 n.11, p.1, March 19, 1985
^ Hitachi ships first 1TB hard drive. Retrieved on 2007-12-12.
^ First hands-on with the only 1 TB drive. Retrieved on 2007-08-13.
^ Hitachi Deskstar 7K1000 Terabyte Hard Drive Review. Retrieved on 2007-08-13.
^ Western Digital buys Komag for $1 Billion. Retrieved on 2007-08-18.

RAM

Random access memory

Random access memory (usually known by its acronym, RAM) is a type of computer data storage. It takes the form of integrated circuits that allow the stored data to be accessed in any order, i.e. at random. The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.[1]

This contrasts with storage mechanisms such as tapes, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.

The word RAM is mostly associated with volatile types of memory, where the information is lost when power is switched off. However, many other types of memory are RAM as well (i.e. Random Access Memory), including most types of ROM and a kind of flash memory called NOR-Flash.

Contents [hide]
1 Overview
1.1 Types of RAM
1.2 Memory hierarchy
1.2.1 Swapping
1.3 Other uses of the term
1.3.1 "RAM disks"
2 Recent developments
3 Memory wall
4 See also
5 Notes and references
6 External links



overview

Types of RAM
Modern types of writable RAM generally stores a bit of data as either the state of a flip-flop, as in SRAM (static RAM), or as an charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM), EPROM, EEPROM and Flash. Some types have circuitry to detect and/or correct random faults called memory errors in the stored data, using parity bits or error correction codes. RAM of the read-only type, ROM, instead uses a metal mask to permanetly enable/disable selected transistors, instead of storing a charge in them.

As both SRAM and DRAM are volatile, other forms of computer storage, such as disks and magentic tapes, have been used as "permanent" storage in traditional computers. Newer products such as PDAs and small music players (up to 8 GB in Jan 2007) may not have hard disks however, but often rely on flash memory to maintain data between sessions of use; the same can be said about products such as mobile phones, advanced calculators, synthesizers etc; even certain categories of personal computers have begun replacing magnetic disk with so called flash drives. There are two basic types of flash memory: the NOR type, which is capable of true random access, and the NAND type, which is not; the former is therefore often used in place of ROM, while the latter is used in most memory cards and solid-state drives, due to a lower price.


Memory hierarchy

1 Module of 128Mb NEC SD-RAMMany computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external caches, DRAM, paging systems, and virtual memory or swap space on a hard drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different access times, violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or interleave organization of the components make the access time variable, although not to the extent that rotating storage media or a tape is variable.

In most modern personal computers, the RAM comes in easily upgraded form of modules called memory modules or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or too small for current purposes. As suggested above, smaller amounts of RAM (mostly SRAM) is integrated also in the CPU and other ICs on the motherboard, as well as in hard-drives, CD-ROMs, and several other parts of the computer system.


Swapping
If a computer becomes low on RAM during intensive application cycles, the computer can resort to swapping. In this case, the computer temporarily uses hard drive space as additional memory. Constantly relying on this type of backup memory is called thrashing, which is generally undesirable because it lowers overall system performance. In order to reduce the dependency on swapping, more RAM can be installed.


Other uses of the term
Other physical devices with read/write capablity can have "RAM" in their names: for example, DVD-RAM. "Random access" is also the name of an indexing method: hence, disk storage is often called "random access" because the reading head can move relatively quickly from one piece of data to another, and does not have to read all the data in between. However the final "M" is crucial: "RAM" (provided there is no additional term as in "DVD-RAM") always refers to a solid-state device.


"RAM disks"
Software can "partition" a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a RAM disk. Unless the memory used is non-volatile, a RAM disk loses the stored data when the computer is shut down. However, volatile memory can retain its data when the computer is shut down if it has a separate power source, usually a battery.


Recent developments
Several new types of non-volatile RAM, which will preserve data while powered down, are under development. The technologies used include carbon nanotubes and the magnetic tunnel effect. In summer 2003, a 128 KB magnetic RAM chip manufactured with 0.18 µm technology was introduced. The core technology of MRAM is based on the magnetic tunnel effect. In June 2004, Infineon Technologies unveiled a 16 MB prototype again based on 0.18 µm technology. Nantero built a functioning carbon nanotube memory prototype 10 GB array in 2004. Whether some of these technologies will be able to eventually take a significant market share from either DRAM, SRAM, or flash-memory technology, remains to be seen however.

In 2006, "Solid-state drive" (based on flash memory) with capacities exceeding 150 gigabytes and speeds far exceeding traditional disks have become available. This development has started to blur the definition between traditional random access memory and "disks", dramatically reducing the difference in performance


Memory wall
The "memory wall" is the growing disparity of speed between CPU and memory outside the CPU chip. An important reason for this disparity is the limited communication bandwidth beyond chip boundaries. From 1986 to 2000, CPU speed improved at an annual rate of 55% while memory speed only improved at 10%. Given these trends, it was expected that memory latency would become an overwhelming bottleneck in computer performance. [2]

Currently, CPU speed improvements have slowed significantly partly due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Intel summarized these causes in their Platform 2015 documentation (PDF):

“First of all, as chip geometries shrink and clock frequencies rise, the transistor leakage current increases, leading to excess power consumption and heat (more on power consumption below). Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called Von Neumann bottleneck), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in the means of producing inductance within solid state devices, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address.”

The RC delays in signal transmission were also noted in Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures which projects a maximum of 12.5% average annual CPU performance improvement between 2000 and 2014. The data on Intel Processors clearly shows a slowdown in performance improvements in recent processors. However, Intel's new processors, Core 2 Duo (codenamed Conroe) show a significant improvement over previous Pentium 4 processors; due to a more efficient architecture, performance increased while clock rate actually decreased.


RAMCAS latency (CL)
DIMM
DVD-RAM
Dual-channel architecture
Error-correcting code (ECC)
Registered/Buffered memory
Compact Flash
PC card
Static RAM (SRAM)
STT RAM (Spin Torque Transfer RAM)
Non-Volatile RAM (NVRAM)
Dynamic RAM (DRAM)
Fast Page Mode DRAM
EDO RAM or Extended Data Out DRAM
XDR DRAM
SDRAM or Synchronous DRAM
DDR SDRAM or Double Data Rate Synchronous DRAM; now being replaced by DDR2 SDRAM
RDRAM or Rambus DRAM

TFT MONITOR

TFT LCD

TFT-LCD (thin film transistor liquid crystal display) is a variant of liquid crystal display (LCD) which uses thin film transistor (TFT) technology to improve image quality. TFT LCD is one type of active matrix LCD, though it is usually synonymous with LCD. It is used in televisions, flat panel displays and projectors.