Saturday, April 30, 2011

The Culture of Tea

Tea is the agricultural product of the leaves, leaf buds, and internodes of various cultivars and sub-varieties of the Camellia sinensis plant, processed and cured using various methods(SONY Vaio VGN-NW Battery). "Tea" also refers to the aromatic beverage prepared from the cured leaves by combination with hot or boiling water, and is the common name for theCamellia sinensis plant itself. After water, tea is the most widely consumed beverage in the world. It has a cooling, slightly bitter, astringent flavour which many enjoy(SONY Vaio VGN-NW21EF/S Battery).

There are at least six varieties of tea: white, yellow, green, oolong, black, and post-fermented teas of which the most commonly found on the market are white, green,oolong, and black. Some varieties, such as traditional oolong tea and Pu-erh tea(SONY Vaio VGN-NW21JF Battery), a post-fermented tea, can be used medicinally.

The term "tea" is sometimes loosely used to refer to "herbal teas", which are an infusion or tisane of leaves, flowers, fruit, herbs, or other plant material that contains noCamellia sinensis(SONY Vaio VGN-NW21MF Battery). In East Asian culture, the term "red tea" has always been used to represent what the West understands as "black tea". This can be confusing in the English speaking world because the same term is now also used to represent the drink made with the South African rooibos plant which contains no Camellia sinensis(SONY Vaio VGN-NW21MF/W Battery).

Cultivation and harvesting

Camellia sinensis is an evergreen plant that grows mainly in tropical and sub-tropical climates. Nevertheless, some varieties can also tolerate marine climates and are cultivated as far north as Pembrokeshire in the British mainland and Washington in the United States(SONY Vaio VGN-NW31EF/W Battery).

Tea plants are propagated from seed or by cutting; it takes approximately 4 to 12 years for a tea plant to bear seed, and about 3 years before a new plant is ready for harvesting. In addition to a zone 8 climate or warmer, tea plants require at least 127 cm. (50 inches) of rainfall a year and prefer acidic soils(SONY Vaio VGN-NW21ZF Battery). Traditional Chinese Tea Cultivation and Studies believes that high-quality tea plants are cultivated at elevations of up to 1,500 metres (4,900 ft): at these heights, the plants grow more slowly and acquire a better flavour.

Only the top 1-2 inches of the mature plant are picked. These buds and leaves are called flushes(SONY Vaio VGN-NW31JF Battery). A plant will grow a new flush every seven to fifteen days during the growing season, and leaves that are slow in development always produce better flavored teas.

A tea plant will grow into a tree of up to 16 metres (52 ft) if left undisturbed, but cultivated plants are pruned to waist height for ease of plucking(SONY Vaio VGN-NW320F/B Battery).

Two principal varieties are used: the China plant (C. sinensis sinensis), used for most Chinese, Formosan and Japanese teas (but not Pu-erh); and the clonal Assam plant (C. sinensis assamica), used in most Indian and other teas (but not Darjeeling). Within these botanical varieties(SONY Vaio VGN-NW320F/TC Battery), there are many strains and modern Indian clonal varieties. Leaf size is the chief criterion for the classification of tea plants, with three primary classifications being: Assam type, characterized by the largest leaves; China type, characterized by the smallest leaves; and Cambod, characterized by leaves of intermediate size(SONY Vaio VGN-NW35E Battery).

Processing and classification

A tea's type is determined by the processing which it undergoes. Leaves of Camellia sinensis soon begin to wilt and oxidize, if not dried quickly after picking. The leaves turn progressively darker as their chlorophyll breaks down and tannins are released. This enzymatic oxidation process(SONY Vaio VGN-NW11S/S Battery), known as fermentation in the tea industry, and is caused by the plant's intracellular enzymes and causes the tea to darken. In tea processing, the darkening is stopped at a predetermined stage by heating, which deactivates the enzymes responsible. In the production of black teas, the halting of oxidization by heating is carried out simultaneously with drying(SONY Vaio VGN-NW11Z/S Battery).

Without careful moisture and temperature control during manufacture and packaging, the tea may become unfit for consumption, due to the growth of undesired molds and bacteria. At minimum it may alter the taste and make it undesirable.

Tea is traditionally classified based on the techniques with which it is produced and processed(SONY Vaio VGN-NW11S/T Battery).

White tea: Wilted and unoxidized

Yellow tea: Unwilted and unoxidized, but allowed to yellow

Green tea: Unwilted and unoxidized

Oolong: Wilted, bruised, and partially oxidized

Black tea: Wilted, sometimes crushed, and fully oxidized

Post-fermented tea: Green tea that has been allowed to ferment/compost

Blending and additives(SONY Vaio VGN-NW11Z/T Battery)

Although single estate teas are available, almost all teas in bags and most other teas sold in the West are blends. Blending may occur in the tea-planting area (as in the case of Assam), or teas from many areas may be blended. The aim of blending is to obtain better taste, higher price, or both, as a more expensive, better-tasting tea may cover the inferior taste of cheaper varieties(Dell Latitude E4300 battery).

Some teas are not pure varieties, but have been enhanced through additives or special processing. Tea is highly receptive to inclusion of various aromas; this may cause problems in processing, transportation and storage, but also allows for the design of an almost endless range of scented and flavored variants, such as bergamot (Earl Grey), vanilla, caramel, and many others(Dell Latitude 2110 battery).

Content

Tea contains catechins, a type of antioxidant. In a freshly picked tea leaf, catechins can compose up to 30% of the dry weight. Catechins are highest in concentration in white and green teas, while black tea has substantially fewer due to its oxidative preparation(Dell Latitude 2100 battery). Research by the U.S. Department of Agriculture has suggested that levels of antioxidants in green and black tea do not differ greatly, with green tea having an oxygen radical absorbance capacity (ORAC) of 1253 and black tea an ORAC of 1128 (measured in μmolTE/100g) (Dell Latitude D830 battery). Tea also contains theanine and the stimulant caffeine at about 3% of its dry weight, translating to between 30 mg and 90 mg per 8 oz (250 ml) cup depending on type, brand and brewing method. Tea also contains small amounts of theobromine and theophylline. Due to modern day environmental pollution fluoride and aluminum have also been found to occur in tea(Dell LATITUDE D820 battery), with certain types of brick tea made from old leaves and stems having the highest levels. This occurs due to the tea plant's high sensitivity to and absorption of environmental pollutants.

Dry tea has more caffeine by weight than coffee; nevertheless, more dried coffee is used than dry tea in preparing the beverage, which means that a cup of brewed tea contains significantly less caffeine than a cup of coffee of the same size(Dell Latitude D810 battery).

Tea has negligible carbohydrates, fat, and protein.

Although tea contains various types of polyphenols and tannin, tea does not contain tannic acid. Tannic acid is not an appropriate standard for any type of tannin analysis because of its poorly defined composition(Dell LATITUDE D800 battery).

Origin and history

Tea plants are native to East and South Asia and probably originated around the point of confluence of the lands of northeast India, north Burma, southwest China, and Tibet. Although tales exist in regards to the beginnings of tea being used as a beverage, no one is sure of its exact origins(Dell Latitude D630 battery). The usage of tea as a beverage was first recorded in China, with the earliest records of tea consumption with records dating back to the 10th century BC. It was already a common drink during Qin Dynasty(around 200 BC) and became widely popular during Tang Dynasty, when it was spread to Korea and Japan. Trade of tea by the Chinese to Western nations in the 19th century spread tea and the tea plant to numerous locations around the world(Dell Latitude D620 battery).

Tea was imported to Europe during the Portuguese expansion trips in 16th century and called it chá. In 1750 some experts in tea travelled from China to the AzoresIslands, where tea has grown since that time, and planted jasmines and mallows to give aroma and distinction(Dell Latitude D610 battery). Actually green tea and black tea grows in these Islands that are the supplier for continental Portugal. Catherine of Braganza, wife of Charles II took the tea habit to Great Britain around 1660 but it was not until the 19th century that tea managed to have in England the interest that it has nowadays(Dell Latitude D600 battery).

Health effects

The possible beneficial effects of tea consumption in the prevention of cancer and cardiovascular diseases have been demonstrated in animal models and suggested by studies in vitro. Similar beneficial effects, however, have not been convincingly demonstrated in humans(Dell Latitude D520 battery): beneficial effects have been demonstrated in some studies but not in others. If such beneficial effects do exist in humans, they are likely to be mild, depending on many other lifestyle-related factors, and could be masked by confounding factors in certain populations(Dell Latitude D510 battery). Another concern is that the amounts of tea consumed by humans are lower than the doses required for demonstrating the disease-prevention effects in animal models. Caution should be applied, however, in the use of high concentrations of tea for disease prevention(Dell Latitude D505 battery). Ingestion of large amounts of tea may cause nutritional and other problems because of the caffeine content and the strong binding activities of tea polyphenols, although there are no solid data on the harmful effects of tea consumption. More research is needed to elucidate the biologic activities of green and black tea and to determine the optimal amount of tea consumption for possible health-beneficial effects(Dell Latitude D500 battery).

Tea leaves contain more than 700 chemicals, among which the compounds closely related to human health are flavanoides, amino acids, vitamins (C, E and K), caffeine and polysaccharides. Moreover, tea drinking has recently proven to be associated with cell-mediated immune function of the human body(Dell Latitude D430 battery). Tea plays an important role in improving beneficial intestinal microflora, as well as providing immunity against intestinal disorders and in protecting cell membranes from oxidative damage. Tea also prevents dental caries due to the presence of fluorine(Dell Latitude D420 battery). The role of tea is well established in normalizing blood pressure, lipid depressing activity, prevention of coronary heart diseases and diabetes by reducing the blood-glucose activity. Tea also possesses germicidal and germistatic activities against various gram-positive and gram negativehuman pathogenic bacteria(Dell Latitude D410 battery). Both green and black tea infusions contain a number of antioxidants, mainly catechins that have anti-carcinogenic, anti-mutagenic and anti-tumoric properties.

In a large study of over 11,000 Scottish men and women completed in 1993 and published in the 1999 Journal of Epidemiology and Community Health (1999, pp. 481–487) (Dell Latitude CPX battery), there was an increase in the risk of coronary disease with the regular consumption of tea, although it disappeared after adjustment for confounding factors (age and occupational status).

The IARC list teas as under Group 3 carcinogens since injection of black tea concentrates under the skins of mice showed some cancerous growths. However, it has not been possible to prove that tea affects humans in similar ways through consumption(Dell Latitude CPI battery).

Consumption of some forms of tea has the potential to result in acute liver damage in some individuals. Several herbal & dietary supplements have been linked to liver damage, caused in part or completely by the presence of green tea extract in these supplements; the most notable cases include Hydroxycut (Dell Latitude C840 battery) (415 mg of a mixture of green, white, and oolong tea extracts, and several other herbal extracts, per dose); Exolise (360 mg of green tea extract per dose); and Tealine (250 mg of green tea extract per dose). These concerns resulted in withdrawals of the first two products and a class action lawsuit against the manufacturer of Hydroxycut(Dell Latitude C810 battery). The risk is thought to be quite small: in case of Hydroxycut, 9 million bottles were sold in the U.S. over the lifetime of the product, resulting in 23 known severe cases, however, these included at least one fatality and at least three cases of liver failure resulting in a liver transplantation(Dell Latitude C800 battery). In case of Exolise, the risk of an adverse effect was estimated as less than 1 per 100,000.

Etymology and cognates in other languages

The Chinese character for tea is 茶, but it is pronounced differently in the various Chinese dialects. Two pronunciations have made their way into other languages around the world(Dell Latitude C640 battery).

One common pronunciation is , which comes from the Hokkien dialect, spoken in Fujian Province, Taiwan and by expatriate Chinese in Philippines, Indonesia, Malaysia and Singapore. It reached the West particularly from the Amoy Min Nan dialect, spoken around the portof Xiamen (Amoy) (Dell Latitude C610 battery), once a major point of contact with Western European traders. This pronunciation is believed to come from the old words for tea梌 (tú) or 荼 (tú).

The other common pronunciation is chá, used by the Cantonese dialect spoken around the ports of Guangzhou (Canton), Hong Kong, Macau, and in overseas Chinese communities, as well as in the Mandarin dialect of northern China(Dell Latitude C600 battery). This term was used in ancient times to describe the first flush harvest of tea. Yet another different pronunciation is zu, used in the Wu dialect spoken around Shanghai. The words for tea in Korea and Japan are 차 and 茶 (ちゃ), respectively. Both are transliterated as cha. (In Japanese, it is sometimes お茶 (おちゃ) or ocha, which is more polite.) (Dell Latitude C540 battery)

(5) They are both direct derivatives of the Chinese 茶; the latter term is used mainly in the north and describes a tea made with freshly picked leaves.

The colloquial Greek word for tea is tsáï, from chai. Its formal equivalent, used in earlier centuries, is téïon, from (Dell Latitude C510 battery).

The Polish word for a tea-kettle is czajnik, which could be derived directly from cha or from the cognate Russian word. However, tea in Polish is herbata, which, as well as Lithuanian arbata, was derived from the Latin herba thea, meaning "tea herb." (Dell Latitude C500 battery)

The normal word for tea in Finnish is tee, which is a Swedish loan. However, it is often colloquially referred, especially in Eastern Finland and in Helsinki as tsai, tsaiju, saiju or saikka, which is cognate to Russian word chai. The latter word refers always to black tea, while green tea is always tee(Dell Latitude C400 battery).

It is tempting to correlate these names with the route that was used to deliver tea to these cultures, although the relation is far from simple at times. As an example, the first tea to reach Britain was traded by the Dutch from Fujian, which uses te, and although later most British trade went through Canton, which uses cha, the Fujianese pronunciation continued to be the more popular(Dell Latitude 131L battery).

In Ireland, or at least in Dublin, the term cha is sometimes used for "tea," as is pre-vowel-shift pronunciation "tay" (from which the Irish Gaelicword "tae" is derived). Char was a common slang term for tea throughout British Empire and Commonwealth military forces in the 19th and 20th centuries, crossing over into civilian usage(Dell Latitude D400 battery).

The British English slang word "char" for "tea" arose from its Cantonese Chinese pronunciation "cha" with its spelling affected by the fact that ar is a more common way of representing the phoneme /ɑː/ in British English.

In North America, the word chai is used to refer almost exclusively to the Indian masala chai (spiced tea) beverage(Dell XPS 1647 battery).

The original pronunciation "cha" in the Cantonese and Mandarin languages has no [j] ending. The forms with this ending in many Eurasian languages come from the Chinese compound word denoting "tea leaves" (simplified Chinese: 茶叶; traditional Chinese: 茶葉; pinyin: chá yè) (Dell XPS 1645 battery). The different articulations of the word for tea into the two main groups: "teh-derived" (Min Chinese dialects) and "cha-derived" (Mandarin, Cantonese and other non-Min Chinese dialects) reveals the particular Chinese local cultures where non-Chinese nations acquired their tea and "tea cultures." (Dell XPS 1640 battery)India and the Arab world most likely got their tea cultures from the Cantonese or the Southwestern Mandarin speakers, whereas the Russians got theirs from the northern Mandarin speakers. The Portuguese, the first Europeans to import the herb in large amounts, took the Cantonese form "chá," as used in their trading posts in the south of China, especially Macau(Dell XPS 16 battery). Conversely, other Western Europeans who copied the Min articulation "teh" probably traded with the Hokkienese while in Southeast Asia.

Recently, no earlier than 1980, "chai" entered North American English with a particular meaning: Indian masala black tea. Of course this is not the case in other languages, where "chai" usually just means black tea (as people traditionally drink more black tea than green outside of East Asia) (Dell XPS 13 battery). English is thus one of the few languages that allow for the dual articulations of "tea" into a "teh-derived" word and a "cha-derived" one (though this was already the case - see mention of 'char' above), such as Moroccan colloquial Arabic (Darija): in the case of Moroccan Arabic(Dell XPS M140 battery), "ash-shay" means "generic, or black Middle Eastern tea" whereas "atay" refers particularly to Zhejiang or Fujian green tea with fresh mint leaves. The Moroccans are said to have acquired a unique penchant in the Arab world for East Chinese green tea after the ruler Mulay Hassan exchanged some European hostages captured by the Barbary Pirates for a whole ship of Chinese tea. They have thus acquired a word for this special tea different from the generic "ash-shay." See Moroccan tea culture(Dell Inspiron Mini 12 battery)

One place in which a word unrelated to tea is used to describe the beverage is South America (particularly Andean countries), because a similar stimulant beverage, yerba mate, was consumed there long before tea arrived(Dell XPS M2010 battery).

Tea culture

In many cultures, tea is often had at high class social events, such as afternoon tea and the tea party. It may be consumed early in the day to heighten alertness; it contains theophylline and bound caffeine (sometimes called "theine"), although there are also decaffeinated teas(Dell XPS M1730 battery).

Tea is prevalent in most cultures in the Middle East. In Arab culture, tea is a focal point for social gatherings. In Iranian (Persian) culture, tea is so widely consumed that it is generally the first thing offered to a household guest(Dell XPS M1710 battery).

There are tea ceremonies which have arisen in different cultures, Japan's complex, formal and serene one being one of the most well known. Other examples are the Chinese tea ceremony which uses some traditional ways of brewing tea. One form of Chinese tea ceremony is the Gongfu tea ceremony, which typically uses small Yixing clay teapots and oolong tea(Dell XPS M170 battery).

The American poet Wallace Stevens, a tea-fancier, is credited by Eleanor Cook with a "delicately implicit trope of drinking tea as a metaphor for reading (ingesting a drink from leaves)." See for instance his "Tea".

In the United States, 80% of tea is consumed cold, as iced tea(Dell XPS M1530 battery).

Preparation

The traditional method of making a cup of tea is to place loose tea leaves, either directly or in a tea infuser, into a tea pot or teacup and pour hot water over the leaves. After a couple of minutes the leaves are usually removed again, either by removing the infuser, or by straining the tea while serving(Dell XPS 1340 battery).

Most green teas should be allowed to steep for about two minutes, although some types of tea require as much as ten minutes, and others as little as thirty seconds. The strength of the tea should be varied by changing the amount of tea leaves used, not by changing the steeping time(Dell XPS M1330 battery). The amount of tea to be used per amount of water differs from tea to tea but one basic recipe may be one slightly heaped teaspoon of tea (about 5 ml) for each teacup of water (200 ml) (8 oz) prepared as above. Stronger teas, such as Assam, to be drunk with milk are often prepared with more leaves, and more delicate high grown teas such as a Darjeeling are prepared with a little less (Dell XPS M1210 battery) (as the stronger mid-flavors can overwhelm the champagne notes).

The best temperature for brewing tea depends on its type. Teas that have little or no oxidation period, such as a green or white tea, are best brewed at lower temperatures, between 65 and 85 °C (149 and 185 °F) (Dell INSPIRON MINI 9 battery), while teas with longer oxidation periods should be brewed at higher temperatures around 100 °C (212 °F). The higher temperatures are required to extract the large, complex, flavorful phenolic molecules found in fermented tea, although boiling the water reduces the amount of dissolved oxygen in the water(Dell STUDIO 1450 battery).

Some tea sorts are often brewed several times using the same tea leaves. Historically, in China, tea is divided into a number of infusions. The first infusion is immediately poured out to wash the tea, and then the second and further infusions are drunk. The third through fifth are nearly always considered the best infusions of tea(Dell VOSTRO 1400 battery), although different teas open up differently and may require more infusions of hot water to produce the best flavor.

One way to taste a tea, throughout its entire process, is to add hot water to a cup containing the leaves and after about 30 seconds to taste the tea. As the tea leaves unfold (known as "The Agony of the Leaves")(Dell VOSTRO 1500 battery) they give up various parts of themselves to the water and thus the taste evolves. Continuing this from the very first flavours to the time beyond which the tea is quite stewed will allow an appreciation of the tea throughout its entire length.

Black tea

In the West, water for black tea is usually added near boiling point 99 °C (210 °F). Many of the active substances in black tea do not develop at temperatures lower than 90 °C (194 °F). Lower temperatures are used for some more delicate teas(Dell XPS M1210 battery). The temperature will have as large an effect on the final flavor as the type of tea used. The most common fault when making black tea is to use water at too low a temperature. Since boiling point drops with increasing altitude, it is difficult to brew black tea properly in mountainous areas(Dell XPS M1330 battery). It is also recommended that the teapot be warmed before preparing tea, easily done by adding a small amount of boiling water to the pot, swirling briefly, before discarding. In the West, black teas are usually brewed for about 4 minutes and are usually not allowed to steep for less than 30 seconds or more than about five minutes (Dell XPS 1340 battery) (a process known as brewing or mashing in Britain). In many regions of the world, however, boiling water is used and the tea is often stewed. For example, in India black tea is often boiled for fifteen minutes or longer as a strong brew is preferred for making Masala chai. When the tea has brewed long enough to suit the tastes of the drinker(Dell XPS M1530 battery), it should be strained while serving. The popular varieties of black (red) tea include Assam tea, Nepal tea, Darjeeling tea, Nilgiri tea, Turkish tea and Ceylon tea.

Green tea

Water for green tea, according to regions of the world that prefer mild tea, should be around 80 to 85 °C (176 to 185 °F); the higher the quality of the leaves, the lower the temperature. Hotter water will produce a bitter taste(Dell XPS M170 battery) . However, this is the method used in many regions of the world, such as North Africa or Central Asia where bitter tea is appreciated. For example, in Morocco green tea is steeped in boiling water for fifteen minutes(Dell XPS M1710 battery). In the West and Far East a milder tea is appreciated. The container in which the tea is steeped, the mug, or teapot is often warmed beforehand so that the tea does not immediately cool down. High-quality green and white teas can have new water added as many as five or more times, depending on variety, at increasingly high temperatures(Dell XPS M1730 battery).

Oolong tea

Oolong teas should be brewed around 90 to 100 °C (194 to 212 °F), and again the brewing vessel should be warmed before pouring in the water. Yixing purple clay teapots are the traditional brewing vessel for oolong tea. For best results use spring water, as the minerals in spring water tend to bring out more flavor in the tea(Dell XPS M2010 battery). High quality oolong can be brewed multiple times from the same leaves, and unlike green tea it improves with reuse. It is common to brew the same leaves three to five times, the third steeping usually being the best.

Premium or delicate tea

Some teas, especially green teas and delicate Oolong teas, are steeped for shorter periods, sometimes less than 30 seconds(Dell Inspiron Mini 12 battery). Using a tea strainerseparates the leaves from the water at the end of the brewing time if a tea bag is not being used. However, black Darjeeling tea, the premium Indian tea, needs a longer than average steeping time. Elevation and time of harvest offer varying taste profiles; proper storage and water quality also have a large impact on taste(SONY VGP-BPS9A/S battery).

Pu-erh tea

Pu-erh tea is also called Pu'er tea. Pu-erh teas require boiling water for infusion. Some prefer to quickly rinse pu-erh for several seconds with boiling water to remove tea dust which accumulates from the aging process. Infuse pu-erh at the boiling point (100 °C or 212 °F), and allow to steep for 30 seconds or up to five minutes(SONY VGP-BPS9/B battery).

Serving

In order to preserve the pre-tannin tea without requiring it all to be poured into cups, a second teapot may be used. The steeping pot is best unglazed earthenware; Yixing pots are the best known of these, famed for the high quality clay from which they are made. The serving pot is generally porcelain, which retains the heat better(SONY VGP-BPS9/S battery). Larger teapots are a post-19th century invention, as tea before this time was very rare and very expensive. Experienced tea-drinkers often insist that the tea should not be stirred around while it is steeping (sometimes called winding in the UK). This, they say, will do little to strengthen the tea, but is likely to bring the tannins out in the same way that brewing too long will do(SONY VGP-BPS9A battery). For the same reason one should not squeeze the last drops out of a teabag; if stronger tea is desired, more tea leaves should be used.

Additives

The addition of milk to tea in Europe was first mentioned in 1680 by the epistolist Madame de Sévigné. Many teas are traditionally drunk with milk in cultures where dairy products are consumed(SONY VGP-BPS9A/B battery). These include Indian masala chai, and British tea blends. These teas tend to be very hearty varieties of black tea which can be tasted through the milk, such as Assams, or the East Friesian blend. Milk is thought to neutralize remaining tannins and reduce acidity(HP Pavilion DV6-1210SA battery). The Chinese (Hans) do not usually drink milk with tea (or indeed use milk at all) but the Manchurians do, and the elite of the Qing Dynasty of the Chinese Empire continued to do so. Hong Kong-style milk tea is based on British colonial habits. Tibetans and other Himalayan peoples traditionally drink tea with milk or yak butter and salt. Hungarians almost invariably have their tea with lemon juice(SONY VGP-BPS11 battery).

The order of steps in preparing a cup of tea is a much-debated topic. Some say that it is preferable to add the milk before the tea, as the high temperature of freshly brewed tea can denature the proteins found in fresh milk, similar to the change in taste of UHT milk, resulting in an inferior tasting beverage(SONY VGP-BPL11 battery). Others insist that it is better to add the milk after brewing the tea, as most teas need to be brewed as close to boiling as possible. The addition of milk chills the beverage during the crucial brewing phase, if brewing in a cup rather than using a pot, meaning that the delicate flavor of a good tea cannot be fully appreciated(SONY VGP-BPL12 battery). By adding the milk afterwards, it is easier to dissolve sugar in the tea and also to ensure that the desired amount of milk is added, as the color of the tea can be observed.

A 2007 study published in the European Heart Journal found that certain beneficial effects of tea may be lost through the addition of milk(SONY VGP-BPS12 battery).

Many flavourings are added to varieties of tea during processing. Among the best known are Chinese Jasmine tea, with jasmine oil or flowers, the spices in Indian Masala chai and Earl Grey tea, which contains oil of bergamot. A great range of modern flavours have been added to these traditional ones(SONY VGP-BPS14 Battery).In eastern India people also drink lemon tea or lemon masala tea. Lemon tea simply contains hot tea with lemon juice and sugar. Masala lemon tea contains hot tea with roasted cumin seed powder,lemon juice,black salt and sugar which gives it a tangy, spicy taste(SONY VGP-BPS14/B Battery).

Other popular additives to tea by the tea-brewer or drinker include sugar, liquid honey or a solid Honey Drop, agave nectar, lemon (traditional in Russia and Italy), fruitjams, and mint. In China sweetening tea was traditionally regarded as a feminine practice. In colder regions such as Mongolia(SONY VGP-BPS14/S Battery), Tibet and Nepal, butter is added to provide necessary calories. Tibetan butter tea contains rock salt and dre (yak) butter, which is then churned vigorously in a cylindrical vessel closely resembling a butter churn. The same may be said for salt tea, which is consumed in some cultures in the Hindu Kush region of northern Pakistan(SONY VGP-BPS14B Battery).

Alcohol may also be added to tea, such as whisky or brandy.

The flavor of the tea can also be altered by pouring it from different heights, resulting in varying degrees of oxidization. The art of high-altitude pouring is used principally by people in Northern Africa (e.g. Morocco and Libya), but also in West Africa (e.g. Guinea, Mali, Senegal) and can positively alter the flavor of the tea(SONY VGP-BPS22 battery), but it is more likely a technique to cool the beverage destined to be consumed immediately. In certain cultures the tea is given different names depending on the height it is poured from. In Mali,gunpowder tea is served in series of three, starting with the highest oxidization or strongest(SONY VGP-BPS18 battery), unsweetened tea (cooked from fresh leaves), locally referred to as "bitter as death," followed by a second serving, where the same tea leaves are boiled again with some sugar added ("pleasant as life"), and a third one, where the same tea leaves are boiled for the third time with yet more sugar added ("sweet as love")(SONY VGP-BPS22/A battery). Green tea is the central ingredient of a distinctly Malian custom, the "Grin," an informal social gathering that cuts across social and economic lines, starting in front of family compound gates in the afternoons and extending late into the night, and is widely popular inBamako and other large urban areas(SONY VGP-BPS22A battery).

In Southeast Asia, particularly in Malaysia, the practice of pouring tea from a height has been refined further using black tea to which condensed milk is added, poured from a height from one cup to another several times in alternating fashion and in quick succession(SONY VGP-BPL10 battery), to create a tea with entrapped air bubbles creating a frothy "head" in the cup. This beverage, teh tarik, literally, "pulled tea," has a creamier taste than flat milk tea and is extremely popular in the region. Tea pouring in Malaysia has been further developed into an art form in which a dance is done by people pouring tea from one container to another(SONY VGP-BPS10 battery), which in any case takes skill and precision. The participants, each holding two containers, one full of tea, pour it from one to another. They stand in lines and squares and pour the tea into each others' pots. The dance must be choreographed to allow anyone who has both pots full to empty them and refill those of whoever has no tea at any one point(Dell Latitude E6500 battery).

Economics

Tea is the most popular drink in the world in terms of consumption. Its consumption equals all other manufactured drinks in the world — including coffee, chocolate, soft drinks, and alcohol — put together. Most tea consumed outside East Asia is produced on large plantations in India or Sri Lanka(Dell Latitude E6400 battery), and is destined to be sold to large businesses. Opposite this large-scale industrial production there are many small "gardens," sometimes minuscule plantations, that produce highly sought-after teas prized by gourmets. These teas are both rare and expensive, and can be compared to some of the most expensive wines in this respect(Dell Latitude E5400 battery).

India is the world's largest tea-drinking nation although the per capita consumption of tea remains a modest 750 grams per person every year. Turkey, with 2.5 kg of tea consumed per person per year, is the world's greatest per capita consumer(SONY VAIO VGN-NR11Z battery).

Production

In 2003, world tea production was 3.21 million tonnes annually. In 2008, world tea production reached over 4.73 million tonnes. The largest producers of tea are thePeople's Republic of China, India, Kenya, Sri Lanka, and Turkey(SONY VAIO VGN-NR11S battery).

The following table shows the amount of tea production (in tonnes) by leading countries in recent years. Data is generated by the Food and Agriculture Organization (FAO) of the United Nations as of January 2010(SONY VAIO VGN-NR110E battery).

Certification

Workers who pick and pack tea in developing countries can face harsh working conditions and can earn below the living wage.

There are a number of bodies that independently certify the production of tea. Tea from certified estates can be sold with a certification label on pack. The most important certification schemes are Rainforest Alliance(SONY Vaio VGN-FW31J Battery), Fairtrade, UTZ Certified, and Organic. All these schemes certify other crops (like coffee, cocoa and fruit) as well. Rainforest Alliance certified tea is sold by Unilever brands Lipton and PG Tips in Western Europe, Australia and the US. Fairtrade certified tea is sold by a large number of suppliers around the world. UTZ Certified announce a partnership in 2008 with Sara Lee brand Pickwick tea(SONY Vaio VGN-FW32J Battery).

Production of organic tea is rising; 3,500 tonnes of organic tea were grown in 2003. The majority of this tea (about 75%) is sold in France, Germany, Japan, the United Kingdom and the United States.

Trade

According to the FAO, in 2007 the largest importer of tea, by weight, was the Russian Federation, followed by the United Kingdom(SONY Vaio VGN-FW17W Battery), Pakistan, and the United States. Kenya, China, India and Sri-Lanka were the largest exporters of tea in 2007 (with exports of: 374229, 292199, 193459 and 190203 tonnes respectively). The largest exporter of black tea in the world is Kenya, while the largest producer (and consumer) of black tea in the world is India(SONY Vaio VGN-FW31E Battery).

Packaging

Tea bags

In 1907, American tea merchant Thomas Sullivan began distributing samples of his tea in small bags of Chinese silk with a drawstring. Consumers noticed that they could simply leave the tea in the bag and re-use it with fresh tea. However, the potential of this distribution/packaging method would not be fully realized until later on(SONY Vaio VGN-FW139E Battery). DuringWorld War II, tea was rationed. In 1953 (after rationing in the UK ended), Tetley launched the tea bag to the UK and it was an immediate success.

Tea leaves are packed into a small envelope (usually composed of paper) known as a tea bag. The use of tea bags is easy and convenient, making tea bags popular for many people today(SONY Vaio VGN-FW139E/H Battery). However, the tea used in tea bags has an industry name—it is called fannings or "dust" and is the waste product produced from the sorting of higher quality loose leaf tea, although this certainly is not true for all brands of tea, especially in the case of many specialty, high quality teas now available in bag form(SONY Vaio VGN-FW31M Battery). It is commonly held among tea aficionados that this method provides an inferior taste and experience. The paper used for the bag can also be tasted by many, which can detract from the tea's flavor. Because fannings and dust are a lower quality of the tea to begin with, the tea found in tea bags is less finicky when it comes to brewing time and temperature(SONY VAIO VGN-FZ21E Battery).

Additional reasons why bag tea is considered less well-flavored include:

Dried tea loses its flavor quickly on exposure to air. Most bag teas (although not all) contain leaves broken into small pieces; the great surface area to volume ratio of the leaves in tea bags exposes them to more air, and therefore causes them to go stale faster. Loose tea leaves are likely to be in larger pieces, or to be entirely intact(SONY VAIO VGN-FZ21Z Battery).

Breaking up the leaves for bags extracts flavored oils.

The small size of the bag does not allow leaves to diffuse and steep properly.

Some tea bags are made using a wet paper strength-reinforcing coating using epichlorohydrin, a known carcinogen

Pyramid tea bags(SONY VAIO VGN-FZ21J Battery)

The "pyramid tea bag," introduced by Lipton and PG Tips/Scottish Blend in 1996, has a unique design that addresses one of connoisseurs' arguments against paper tea bags, because its three-dimensional tetrahedron shape allows more room for tea leaves to expand while steeping(SONY Vaio VGN-FW11 Battery). However, some types of pyramid tea bags have been criticized as being environmentally unfriendly, since their synthetic material does not break down in landfills as loose tea leaves and paper tea bags do. This type of tea bag is also called a sachet by other brands(SONY Vaio VGN-FW11M Battery).

Loose tea

The tea leaves are packaged loosely in a canister or other container. Rolled gunpowder tea leaves, which resist crumbling, are commonly vacuum packed for freshness inaluminized packaging for storage and retail. The portions must be individually measured by the consumer for use in a cup, mug, or teapot(SONY Vaio VGN-FW11S Battery). This allows greater flexibility, letting the consumer brew weaker or stronger tea as desired, but convenience is sacrificed. Strainers, "tea presses," filtered teapots, and infusion bags are available commercially to avoid having to drink the floating loose leaves and to prevent over-brewing(SONY Vaio VGN-FW21E Battery). A more traditional, yet perhaps more efficient way around this problem is to use a three-piece lidded teacup, called a gaiwan. The lid of the gaiwan can be tilted to decant the leaves while pouring the tea into a different cup for consumption.

Compressed tea

Some teas (particularly Pu-erh tea) are still compressed for transport, storage, and aging convenience(SONY Vaio VGN-FW21J Battery). The tea brick remains in use in the Himalayan countries. The tea is prepared and steeped by first loosening leaves off the compressed cake using a small knife. Compressed teas can usually be stored for longer periods of time without spoilage when compared with loose leaf tea(SONY Vaio VGN-FW21L Battery).

Instant tea

In recent times, "instant teas" are becoming popular, similar to freeze dried instant coffee. Similar products also exist for instant iced tea, due to the convenience of not requiring boiling water. Instant tea was developed in the 1930s, but not commercialized until later. Nestea introduced the first instant tea in 1946, while Redi-Tea introduced the first instant iced tea in 1953(SONY Vaio VGN-FW21M Battery).

These products often come with added flavors, such as vanilla, honey or fruit, and may also contain powdered milk. Tea connoisseurs tend to criticize these products for sacrificing the delicacies of tea flavor in exchange for convenience.

Canned tea

Canned tea was first launched in 1981 in Japan. As such, it is a fairly recent innovation, and it has mostly benefits in marketing(SONY VGP-BPS13Q Battery).

Storage

Tea has a shelf life that varies with storage conditions and type of tea. Black tea has a longer shelf life than green tea. An exception, Pu-erh tea improves with age. Tea stays freshest when stored in a dry, cool, dark place in an air-tight container. Black tea stored in a bag inside a sealed opaque canister may keep for two years(SONY VGP-BPS13B/Q Battery). Green tea loses its freshness more quickly, usually in less than a year. Gunpowder tea, its leaves being tightly rolled, keeps longer than the more open-leafed Chun Mee tea. Storage life for all teas can be extended by using desiccant packets or oxygen absorbing packets, and by vacuum sealing(SONY VGN NR11S/S battery).

When storing green tea, discreet use of refrigeration or freezing is recommended. In particular, drinkers need to take precautions against temperature variation.

Improperly stored tea may lose flavor, acquire disagreeable flavors or odors from other foods, or become moldy(SONY VGN NR11M/S battery).

Thursday, April 28, 2011

Central processing unit

The central processing unit (CPU) is the portion of a computer system that carries out the instructions of a computer program, and is the primary element carrying out the computer's functions(Sony Vaio VGN-FZ battery). The central processing unit carries out each instruction of the program in sequence, to perform the basic arithmetical, logical, and input/output operations of the system. This term has been in use in the computer industry at least since the early 1960s. The form, design and implementation of CPUs have changed dramatically since the earliest examples(Sony VGP-BPS8 battery), but their fundamental operation remains much the same.

Early CPUs were custom-designed as a part of a larger, sometimes one-of-a-kind, computer. However, this costly method of designing custom CPUs for a particular application has largely given way to the development of mass-produced processors that are made for one or many purposes(Sony VGP-BPL9 battery). This standardization trend generally began in the era of discrete transistor mainframes and minicomputers and has rapidly accelerated with the popularization of the integrated circuit (IC). The IC has allowed increasingly complex CPUs to be designed and manufactured to tolerances on the order of nanometers(Sony VGP-BPS9 battery). Both the miniaturization and standardization of CPUs have increased the presence of these digital devices in modern life far beyond the limited application of dedicated computing machines. Modern microprocessors appear in everything from automobiles to cell phones and children's toys(Sony VGP-BPL11 battery).

History

Computers such as the ENIAC had to be physically rewired in order to perform different tasks, which caused these machines to be called "fixed-program computers." Since the term "CPU" is generally defined as a software (computer program) execution device, the earliest devices that could rightly be called CPUs came with the advent of the stored-program computer(Sony VGP-BPL15 battery).

The idea of a stored-program computer was already present in the design of J. Presper Eckert and John William Mauchly's ENIAC, but was initially omitted so the machine could be finished sooner. On June 30, 1945, before ENIAC was completed, mathematician John von Neumanndistributed the paper entitled First Draft of a Report on the EDVAC(Sony VGN-FZ460E battery). It outlined the design of a stored-program computer that would eventually be completed in August 1949. EDVAC was designed to perform a certain number of instructions (or operations) of various types. These instructions could be combined to create useful programs for the EDVAC to run. Significantly(Sony VGP-BPS11 battery), the programs written for EDVAC were stored in high-speed computer memory rather than specified by the physical wiring of the computer. This overcame a severe limitation of ENIAC, which was the considerable time and effort required to reconfigure the computer to perform a new task. With von Neumann's design, the program(SONY VAIO VGN-FZ4000 Battery), or software, that EDVAC ran could be changed simply by changing the contents of the computer's memory.

While von Neumann is most often credited with the design of the stored-program computer because of his design of EDVAC, others before him, such as Konrad Zuse, had suggested and implemented similar ideas(Sony VGP-BPS10 battery). The so-called Harvard architecture of the Harvard Mark I, which was completed before EDVAC, also utilized a stored-program design using punched paper tape rather than electronic memory(Sony VGP-BPS13 battery). The key difference between the von Neumann and Harvard architectures is that the latter separates the storage and treatment of CPU instructions and data, while the former uses the same memory space for both. Most modern CPUs are primarily von Neumann in design, but elements of the Harvard architecture are commonly seen as well( Sony VGP-BPS3 battery).

As a digital device, a CPU is limited to a set of discrete states, and requires some kind of switching elements to differentiate between and change states. Prior to commercial development of the transistor, electrical relays and vacuum tubes (thermionic valves) were commonly used as switching elements(Sony VGP-BPS2 battery). Although these had distinct speed advantages over earlier, purely mechanical designs, they were unreliable for various reasons. For example, building direct current sequential logic circuits out of relays requires additional hardware to cope with the problem of contact bounce(Sony Vaio VGN-FZ21M battery ). While vacuum tubes do not suffer from contact bounce, they must heat up before becoming fully operational, and they eventually cease to function due to slow contamination of their cathodes that occurs in the course of normal operation. If a tube's vacuum seal leaks, as sometimes happens, cathode contamination is accelerated(Sony VGN-FZ150E battery). Usually, when a tube failed, the CPU would have to be diagnosed to locate the failed component so it could be replaced. Therefore, early electronic (vacuum tube based) computers were generally faster but less reliable than electromechanical (relay based) computers(Sony VGN-FZ15 battery).

Tube computers like EDVAC tended to average eight hours between failures, whereas relay computers like the (slower, but earlier) Harvard Mark I failed very rarely.[1] In the end, tube based CPUs became dominant because the significant speed advantages afforded generally outweighed the reliability problems(Sony VGN-FZ15L battery). Most of these early synchronous CPUs ran at low clock rates compared to modern microelectronic designs (see below for a discussion of clock rate). Clock signal frequencies ranging from 100 kHz to 4 MHz were very common at this time, limited largely by the speed of the switching devices they were built with(Sony Vaio VGN-FZ18M battery).

The Control Unit

The control unit of the CPU contains circuitry that uses electrical signals to direct the entire computer system to carry out, stored program instructions.The control unit does not execute program instructions; rather, it directs other parts of the system to do so(Sony VGN-FZ15T battery). The control unit must communicate with both the arithmetic/logic unit and memory.

Discrete transistor and integrated circuit CPUs

The design complexity of CPUs increased as various technologies facilitated building smaller and more reliable electronic devices. The first such improvement came with the advent of the transistor(Sony VGN-FZ480E battery). Transistorized CPUs during the 1950s and 1960s no longer had to be built out of bulky, unreliable, and fragile switching elements like vacuum tubes and electrical relays. With this improvement more complex and reliable CPUs were built onto one or several printed circuit boards containing discrete (individual) components(Sony VGN-FZ61B battery).

During this period, a method of manufacturing many transistors in a compact space gained popularity. The integrated circuit (IC) allowed a large number of transistors to be manufactured on a single semiconductor-based die, or "chip." At first only very basic non-specialized digital circuits such as NOR gates were miniaturized into ICs(Sony VGN-FZ31E battery). CPUs based upon these "building block" ICs are generally referred to as "small-scale integration" (SSI) devices. SSI ICs, such as the ones used in the Apollo guidance computer, usually contained transistor counts numbering in multiples of ten. To build an entire CPU out of SSI ICs required thousands of individual chips, but still consumed much less space and power than earlier discrete transistor designs(Sony VGN-FZ340E battery). As microelectronic technology advanced, an increasing number of transistors were placed on ICs, thus decreasing the quantity of individual ICs needed for a complete CPU. MSI and LSI (medium- and large-scale integration) ICs increased transistor counts to hundreds, and then thousands(Sony VGN-FZ180E battery).

In 1964 IBM introduced its System/360 computer architecture which was used in a series of computers that could run the same programs with different speed and performance. This was significant at a time when most electronic computers were incompatible with one another, even those made by the same manufacturer(Sony VGN-FZ18E battery). To facilitate this improvement, IBM utilized the concept of a microprogram (often called "microcode"), which still sees widespread usage in modern CPUs. The System/360 architecture was so popular that it dominated themainframe computer market for decades and left a legacy that is still continued by similar modern computers like the IBM zSeries(Sony VGP-BPL4 battery). In the same year (1964), Digital Equipment Corporation (DEC) introduced another influential computer aimed at the scientific and research markets, the PDP-8. DEC would later introduce the extremely popular PDP-11 line that originally was built with SSI ICs but was eventually implemented with LSI components once these became practical(Sony VGN-FZ160E battery). In stark contrast with its SSI and MSI predecessors, the first LSI implementation of the PDP-11 contained a CPU composed of only four LSI integrated circuits.

Transistor-based computers had several distinct advantages over their predecessors. Aside from facilitating increased reliability and lower power consumption(Sony VGN-FZ15M battery), transistors also allowed CPUs to operate at much higher speeds because of the short switching time of a transistor in comparison to a tube or relay. Thanks to both the increased reliability as well as the dramatically increased speed of the switching elements (which were almost exclusively transistors by this time) (Sony Vaio VGN-FZ18G battery), CPU clock rates in the tens of megahertz were obtained during this period. Additionally while discrete transistor and IC CPUs were in heavy usage, new high-performance designs like SIMD (Single Instruction Multiple Data) vector processors began to appear(Sony VGN-FZ345E battery). These early experimental designs later gave rise to the era of specialized supercomputers like those made by Cray Inc.

Microprocessors

The introduction of the microprocessor in the 1970s significantly affected the design and implementation of CPUs. Since the introduction of the first commercially available microprocessor (the Intel 4004) in 1970 and the first widely used microprocessor (the Intel 8080) in 1974(Sony VGP-BPS14 battery), this class of CPUs has almost completely overtaken all other central processing unit implementation methods. Mainframe and minicomputer manufacturers of the time launched proprietary IC development programs to upgrade their older computer architectures(Sony PCGA-BP2E battery), and eventually produced instruction set compatible microprocessors that were backward-compatible with their older hardware and software. Combined with the advent and eventual vast success of the now ubiquitous personal computer, the term CPU is now applied almost exclusively to microprocessors. Several CPUs can be combined in a single processing chip(Sony VGN-FZ11S battery).

Previous generations of CPUs were implemented as discrete components and numerous small integrated circuits (ICs) on one or more circuit boards. Microprocessors, on the other hand, are CPUs manufactured on a very small number of ICs; usually just one(Sony VGN-FZ17L battery). The overall smaller CPU size as a result of being implemented on a single die means faster switching time because of physical factors like decreased gate parasitic capacitance. This has allowed synchronous microprocessors to have clock rates ranging from tens of megahertz to several gigahertz(Sony VGN-FZ17G battery). Additionally, as the ability to construct exceedingly small transistors on an IC has increased, the complexity and number of transistors in a single CPU has increased dramatically. This widely observed trend is described by Moore's law, which has proven to be a fairly accurate predictor of the growth of CPU (and other IC) complexity to date(Sony VGN-FZ17 battery).

While the complexity, size, construction, and general form of CPUs have changed drastically over the past sixty years, it is notable that the basic design and function has not changed much at all. Almost all common CPUs today can be very accurately described as von Neumann stored-program machines(Sony VGP-BPS18 battery). As the aforementioned Moore's law continues to hold true, concerns have arisen about the limits of integrated circuit transistor technology. Extreme miniaturization of electronic gates is causing the effects of phenomena like electromigration and subthreshold leakage to become much more significant(Sony VGP-BPS15 battery). These newer concerns are among the many factors causing researchers to investigate new methods of computing such as the quantum computer, as well as to expand the usage of parallelism and other methods that extend the usefulness of the classical von Neumann model(Sony VGP-BPS12 battery).

Operation

The fundamental operation of most CPUs, regardless of the physical form they take, is to execute a sequence of stored instructions called a program. The program is represented by a series of numbers that are kept in some kind of computer memory. There are four steps that nearly all CPUs use in their operation(Sony VGP-BPL12 battery): fetch, decode,execute, and writeback.

The first step, fetch, involves retrieving an instruction (which is represented by a number or sequence of numbers) from program memory. The location in program memory is determined by a program counter (PC), which stores a number that identifies the current position in the program(Sony VGP-BPL7 battery). After an instruction is fetched, the PC is incremented by the length of the instruction word in terms of memory units. Often, the instruction to be fetched must be retrieved from relatively slow memory, causing the CPU to stall while waiting for the instruction to be returned(Sony VGN-FZ290 battery). This issue is largely addressed in modern processors by caches and pipeline architectures (see below).

The instruction that the CPU fetches from memory is used to determine what the CPU is to do. In the decode step, the instruction is broken up into parts that have significance to other portions of the CPU. The way in which the numerical instruction value is interpreted is defined by the CPU's instruction set architecture (ISA) (Sony VGN-FZ29VN battery). Often, one group of numbers in the instruction, called the opcode, indicates which operation to perform. The remaining parts of the number usually provide information required for that instruction, such as operands for an addition operation. Such operands may be given as a constant value (called an immediate value) (Sony VGN-SZ56 battery), or as a place to locate a value: a register or a memory address, as determined by some addressing mode. In older designs the portions of the CPU responsible for instruction decoding were unchangeable hardware devices. However, in more abstract and complicated CPUs and ISAs(Sony VGN-SZ55 battery), a microprogram is often used to assist in translating instructions into various configuration signals for the CPU. This microprogram is sometimes rewritable so that it can be modified to change the way the CPU decodes instructions even after it has been manufactured(Sony VGN-FZ220E battery).

After the fetch and decode steps, the execute step is performed. During this step, various portions of the CPU are connected so they can perform the desired operation. If, for instance, an addition operation was requested, the arithmetic logic unit (ALU) will be connected to a set of inputs and a set of outputs(Sony VGN-FZ11Z battery). The inputs provide the numbers to be added, and the outputs will contain the final sum. The ALU contains the circuitry to perform simple arithmetic and logical operations on the inputs (like addition and bitwise operations). If the addition operation produces a result too large for the CPU to handle, an arithmetic overflow flag in a flags register may also be set(Sony VGN-FZ11M battery).

The final step, writeback, simply "writes back" the results of the execute step to some form of memory. Very often the results are written to some internal CPU register for quick access by subsequent instructions. In other cases results may be written to slower, but cheaper and larger, main memory(Sony Vaio VGN-FZ31M battery). Some types of instructions manipulate the program counter rather than directly produce result data. These are generally called "jumps" and facilitate behavior like loops, conditional program execution (through the use of a conditional jump), and functions in programs. Many instructions will also change the state of digits in a "flags" register(Sony Vaio VGN-FZ31Z battery). These flags can be used to influence how a program behaves, since they often indicate the outcome of various operations. For example, one type of "compare" instruction considers two values and sets a number in the flags register according to which one is greater. This flag could then be used by a later jump instruction to determine program flow(Sony VGN-FZ19VN battery).

After the execution of the instruction and writeback of the resulting data, the entire process repeats, with the next instruction cycle normally fetching the next-in-sequence instruction because of the incremented value in the program counter(Sony Vaio VGN-FZ38M battery). If the completed instruction was a jump, the program counter will be modified to contain the address of the instruction that was jumped to, and program execution continues normally. In more complex CPUs than the one described here, multiple instructions can be fetched, decoded(Sony Vaio VGN-FZ31S battery), and executed simultaneously. This section describes what is generally referred to as the "Classic RISC pipeline", which in fact is quite common among the simple CPUs used in many electronic devices (often called microcontroller). It largely ignores the important role of CPU cache, and therefore the access stage of the pipeline(Sony VGN-FZ11L battery).

Design and implementation

Integer range

The way a CPU represents numbers is a design choice that affects the most basic ways in which the device functions. Some early digital computers used an electrical model of the common decimal (base ten)numeral system to represent numbers internally. A few other computers have used more exotic numeral systems like ternary (base three) (Sony VGN-FZ15G battery). Nearly all modern CPUs represent numbers in binary form, with each digit being represented by some two-valued physical quantity such as a "high" or "low" voltage.

Related to number representation is the size and precision of numbers that a CPU can represent. In the case of a binary CPU, a bit refers to one significant place in the numbers a CPU deals with. The number of bits (or numeral places) a CPU uses to represent numbers is often called "word size"(Sony VGP-BPS9/B battery), "bit width", "data path width", or "integer precision" when dealing with strictly integer numbers (as opposed to Floating point). This number differs between architectures, and often within different parts of the very same CPU. For example, an 8-bit CPU deals with a range of numbers that can be represented by eight binary digits (each digit having two possible values) (Sony VGN-FZ190 battery), that is, 28 or 256 discrete numbers. In effect, integer size sets a hardware limit on the range of integers the software run by the CPU can utilize.

Integer range can also affect the number of locations in memory the CPU can address (locate). For example, if a binary CPU uses 32 bits to represent a memory address, and each memory address represents one octet (8 bits) (Sony VGN-FZ190E battery), the maximum quantity of memory that CPU can address is 232 octets, or 4 GiB. This is a very simple view of CPU address space, and many designs use more complex addressing methods like paging in order to locate more memory than their integer range would allow with a flat address space(Sony VGN-FZ18 battery).

Higher levels of integer range require more structures to deal with the additional digits, and therefore more complexity, size, power usage, and general expense. It is not at all uncommon, therefore, to see 4- or 8-bitmicrocontrollers used in modern applications, even though CPUs with much higher range (such as 16, 32, 64, even 128-bit) are available(Sony Vaio VGN-FZ21E battery ). The simpler microcontrollers are usually cheaper, use less power, and therefore dissipate less heat, all of which can be major design considerations for electronic devices. However, in higher-end applications, the benefits afforded by the extra range (most often the additional address space) are more significant and often affect design choices(Sony Vaio VGN-FZ21S battery). To gain some of the advantages afforded by both lower and higher bit lengths, many CPUs are designed with different bit widths for different portions of the device. For example, the IBM System/370 used a CPU that was primarily 32 bit(Sony VGN-FZ230E battery), but it used 128-bit precision inside its floating point units to facilitate greater accuracy and range in floating point numbers. Many later CPU designs use similar mixed bit width, especially when the processor is meant for general-purpose usage where a reasonable balance of integer and floating point capability is required(Sony Vaio VGN-FZ210CE battery).

Clock rate

The clock rate is the speed at which a microprocessor executes instructions. Every computer contains an internal clock that regulates the rate at which instructions are executed and synchronizes all the various computer components. The CPU requires a fixed number of clock ticks (or clock cycles) to execute each instruction(Sony Vaio VGN-FZ18S battery). The faster the clock, the more instructions the CPU can execute per second.

Most CPUs, and indeed most sequential logic devices, are synchronous in nature. That is, they are designed and operate on assumptions about a synchronization signal. This signal, known as a clock signal, usually takes the form of a periodic square wave(Sony VGN-FW11 battery). By calculating the maximum time that electrical signals can move in various branches of a CPU's many circuits, the designers can select an appropriate period for the clock signal.

This period must be longer than the amount of time it takes for a signal to move, or propagate, in the worst-case scenario. In setting the clock period to a value well above the worst-case propagation delay(Sony VGN-FW11M battery), it is possible to design the entire CPU and the way it moves data around the "edges" of the rising and falling clock signal. This has the advantage of simplifying the CPU significantly, both from a design perspective and a component-count perspective. However, it also carries the disadvantage that the entire CPU must wait on its slowest elements, even though some portions of it are much faster(Sony VGN-FZ18L battery). This limitation has largely been compensated for by various methods of increasing CPU parallelism. (see below)

However, architectural improvements alone do not solve all of the drawbacks of globally synchronous CPUs. For example, a clock signal is subject to the delays of any other electrical signal(Sony Vaio VGN-FZ31B battery). Higher clock rates in increasingly complex CPUs make it more difficult to keep the clock signal in phase (synchronized) throughout the entire unit. This has led many modern CPUs to require multiple identical clock signals to be provided in order to avoid delaying a single signal significantly enough to cause the CPU to malfunction. Another major issue as clock rates increase dramatically is the amount of heat that is dissipated by the CPU(Sony VGN-FZ145E battery). The constantly changing clock causes many components to switch regardless of whether they are being used at that time. In general, a component that is switching uses more energy than an element in a static state. Therefore, as clock rate increases, so does heat dissipation, causing the CPU to require more effective cooling solutions(Sony Vaio VGN-FZ31J battery ).

One method of dealing with the switching of unneeded components is called clock gating, which involves turning off the clock signal to unneeded components (effectively disabling them). However, this is often regarded as difficult to implement and therefore does not see common usage outside of very low-power designs(Sony VGN-FZ72B battery). One notable late CPU design that uses clock gating is that of the IBM PowerPC-based Xbox 360. It utilizes extensive clock gating in order to reduce the power requirements of the aforementioned videogame console in which it is used(Sony VGN-FZ71B battery). Another method of addressing some of the problems with a global clock signal is the removal of the clock signal altogether. While removing the global clock signal makes the design process considerably more complex in many ways, asynchronous (or clockless) designs carry marked advantages in power consumption and heat dissipation in comparison with similar synchronous designs. While somewhat uncommon(Sony VGN-FZ70B battery), entire asynchronous CPUs have been built without utilizing a global clock signal. Two notable examples of this are the ARM compliant AMULET and the MIPS R3000 compatible MiniMIPS. Rather than totally removing the clock signal, some CPU designs allow certain portions of the device to be asynchronous, such as using asynchronous ALUs in conjunction with superscalar pipelining to achieve some arithmetic performance gains(Sony VGN-FZ62B battery). While it is not altogether clear whether totally asynchronous designs can perform at a comparable or better level than their synchronous counterparts, it is evident that they do at least excel in simpler math operations. This, combined with their excellent power consumption and heat dissipation properties, makes them very suitable for embedded computers(Sony VGN-FZ140E battery).

Parallelism

The description of the basic operation of a CPU offered in the previous section describes the simplest form that a CPU can take. This type of CPU, usually referred to assubscalar, operates on and executes one instruction on one or two pieces of data at a time(Sony VGN-FZ90HS battery).

This process gives rise to an inherent inefficiency in subscalar CPUs. Since only one instruction is executed at a time, the entire CPU must wait for that instruction to complete before proceeding to the next instruction. As a result the subscalar CPU gets "hung up" on instructions which take more than one clock cycle to complete execution(Sony VGN-FZ90NS battery). Even adding a second execution unit (see below) does not improve performance much; rather than one pathway being hung up, now two pathways are hung up and the number of unused transistors is increased. This design, wherein the CPU's execution resources can operate on only one instruction at a time(Sony Vaio VGN-FZ21J battery), can only possibly reach scalar performance (one instruction per clock). However, the performance is nearly always subscalar (less than one instruction per cycle).

Attempts to achieve scalar and better performance have resulted in a variety of design methodologies that cause the CPU to behave less linearly and more in parallel. When referring to parallelism in CPUs(Sony VGN-FZ32B battery), two terms are generally used to classify these design techniques. Instruction level parallelism (ILP) seeks to increase the rate at which instructions are executed within a CPU (that is, to increase the utilization of on-die execution resources), and thread level parallelism (TLP) purposes to increase the number of threads (effectively individual programs) that a CPU can execute simultaneously(Sony VGN-FZ35 battery). Each methodology differs both in the ways in which they are implemented, as well as the relative effectiveness they afford in increasing the CPU's performance for an application.

Instruction level parallelism

One of the simplest methods used to accomplish increased parallelism is to begin the first steps of instruction fetching and decoding before the prior instruction finishes executing(Sony VGN-FZ37 battery). This is the simplest form of a technique known as instruction pipelining, and is utilized in almost all modern general-purpose CPUs. Pipelining allows more than one instruction to be executed at any given time by breaking down the execution pathway into discrete stages. This separation can be compared to an assembly line, in which an instruction is made more complete at each stage until it exits the execution pipeline and is retired(Sony VGN-FZ38 battery).

Pipelining does, however, introduce the possibility for a situation where the result of the previous operation is needed to complete the next operation; a condition often termed data dependency conflict. To cope with this, additional care must be taken to check for these sorts of conditions and delay a portion of the instruction pipeline if this occurs(Sony VGN-FZ440N battery). Naturally, accomplishing this requires additional circuitry, so pipelined processors are more complex than subscalar ones (though not very significantly so). A pipelined processor can become very nearly scalar, inhibited only by pipeline stalls (an instruction spending more than one clock cycle in a stage) (Sony VGN-FZ32 battery).

Further improvement upon the idea of instruction pipelining led to the development of a method that decreases the idle time of CPU components even further. Designs that are said to be superscalar include a long instruction pipeline and multiple identical execution units(Sony VGN-FZ430E battery). In a superscalar pipeline, multiple instructions are read and passed to a dispatcher, which decides whether or not the instructions can be executed in parallel (simultaneously). If so they are dispatched to available execution units, resulting in the ability for several instructions to be executed simultaneously. In general(Sony VGN-FZ11E battery), the more instructions a superscalar CPU is able to dispatch simultaneously to waiting execution units, the more instructions will be completed in a given cycle.

Most of the difficulty in the design of a superscalar CPU architecture lies in creating an effective dispatcher. The dispatcher needs to be able to quickly and correctly determine whether instructions can be executed in parallel(Sony VGP-BPS13B/B battery), as well as dispatch them in such a way as to keep as many execution units busy as possible. This requires that the instruction pipeline is filled as often as possible and gives rise to the need in superscalar architectures for significant amounts of CPU cache. It also makeshazard-avoiding techniques like branch prediction(Sony VGP-BPS13A/B battery), speculative execution, and out-of-order execution crucial to maintaining high levels of performance. By attempting to predict which branch (or path) a conditional instruction will take, the CPU can minimize the number of times that the entire pipeline must wait until a conditional instruction is completed(Sony VGP-BPS13/S battery). Speculative execution often provides modest performance increases by executing portions of code that may not be needed after a conditional operation completes. Out-of-order execution somewhat rearranges the order in which instructions are executed to reduce delays due to data dependencies(Sony VGP-BPS13/B battery). Also in case of Single Instructions Multiple Data — a case when a lot of data from the same type has to be processed, modern processors can disable parts of the pipeline so that when a single instruction is executed many times(Sony PCGA-BP2EA battery), the CPU skips the fetch and decode phases and thus greatly increasing performance on certain occasions, especially in highly monotonous program engines such as video creation software and photo processing.

In the case where a portion of the CPU is superscalar and part is not, the part which is not suffers a performance penalty due to scheduling stalls(Sony VGP-BPS13AS battery). The Intel P5 Pentium had two superscalar ALUs which could accept one instruction per clock each, but its FPU could not accept one instruction per clock. Thus the P5 was integer superscalar but not floating point superscalar. Intel's successor to the P5 architecture, P6, added superscalar capabilities to its floating point features(Sony VGP-BPS13S battery), and therefore afforded a significant increase in floating point instruction performance.

Both simple pipelining and superscalar design increase a CPU's ILP by allowing a single processor to complete execution of instructions at rates surpassing one instruction per cycle (IPC). Most modern CPU designs are at least somewhat superscalar(Sony VGP-BPS13B/S battery), and nearly all general purpose CPUs designed in the last decade are superscalar. In later years some of the emphasis in designing high-ILP computers has been moved out of the CPU's hardware and into its software interface, or ISA. The strategy of the very long instruction word (VLIW) causes some ILP to become implied directly by the software(Sony VGP-BPS13A/S battery), reducing the amount of work the CPU must perform to boost ILP and thereby reducing the design's complexity.

Thread-level parallelism

Another strategy of achieving performance is to execute multiple programs or threads in parallel. This area of research is known as parallel computing. In Flynn's taxonomy, this strategy is known as Multiple Instructions-Multiple Data or MIMD(Sony VGN-FW31J battery).

One technology used for this purpose was multiprocessing (MP). The initial flavor of this technology is known as symmetric multiprocessing(SMP), where a small number of CPUs share a coherent view of their memory system. In this scheme, each CPU has additional hardware to maintain a constantly up-to-date view of memory(Sony VGP-BPS21 battery). By avoiding stale views of memory, the CPUs can cooperate on the same program and programs can migrate from one CPU to another. To increase the number of cooperating CPUs beyond a handful, schemes such as non-uniform memory access (NUMA) and directory-based coherence protocolswere introduced in the 1990s(Sony VGP-BPS21B battery). SMP systems are limited to a small number of CPUs while NUMA systems have been built with thousands of processors. Initially, multiprocessing was built using multiple discrete CPUs and boards to implement the interconnect between the processors. When the processors and their interconnect are all implemented on a single silicon chip, the technology is known as a multi-coremicroprocessor(Sony VGP-BPS21/S battery).

It was later recognized that finer-grain parallelism existed with a single program. A single program might have several threads (or functions) that could be executed separately or in parallel. Some of earliest examples of this technology implemented input/output processing such as direct memory access as a separate thread from the computation thread(Sony VGP-BPS21A/B battery). A more general approach to this technology was introduced in the 1970s when systems were designed to run multiple computation threads in parallel. This technology is known as multi-threading (MT). This approach is considered more cost-effective than multiprocessing(Sony VAIO PCG-5G2L battery), as only a small number of components within a CPU is replicated in order to support MT as opposed to the entire CPU in the case of MP. In MT, the execution units and the memory system including the caches are shared among multiple threads. The downside of MT is that the hardware support for multithreading is more visible to software than that of MP and thus supervisor software like operating systems have to undergo larger changes to support MT(Sony VAIO PCG-5G3L battery). One type of MT that was implemented is known as block multithreading, where one thread is executed until it is stalled waiting for data to return from external memory. In this scheme, the CPU would then quickly switch to another thread which is ready to run, the switch often done in one CPU clock cycle, such as the UltraSPARC Technology(Sony VAIO PCG-5J1L battery). Another type of MT is known assimultaneous multithreading, where instructions of multiple threads are executed in parallel within one CPU clock cycle.

For several decades from the 1970s to early 2000s, the focus in designing high performance general purpose CPUs was largely on achieving high ILP through technologies such as pipelining, caches(Sony VAIO PCG-5J2L battery), superscalar execution, out-of-order execution, etc. This trend culminated in large, power-hungry CPUs such as the Intel Pentium 4. By the early 2000s, CPU designers were thwarted from achieving higher performance from ILP techniques due to the growing disparity between CPU operating frequencies and main memory operating frequencies as well as escalating CPU power dissipation owing to more esoteric ILP techniques(Sony VAIO PCG-5K2L battery).

CPU designers then borrowed ideas from commercial computing markets such as transaction processing, where the aggregate performance of multiple programs, also known as throughput computing, was more important than the performance of a single thread or program(Sony VAIO PCG-5L1L battery).

This reversal of emphasis is evidenced by the proliferation of dual and multiple core CMP (chip-level multiprocessing) designs and notably, Intel's newer designs resembling its less superscalar P6 architecture. Late designs in several processor families exhibit CMP, including the x86-64Opteron and Athlon 64 X2(Sony VAIO PCG-6S2L battery), the SPARC UltraSPARC T1, IBM POWER4 and POWER5, as well as several video game console CPUs like theXbox 360's triple-core PowerPC design, and the PS3's 7-core Cell microprocessor.

Data parallelism

A less common but increasingly important paradigm of CPUs (and indeed, computing in general) deals with data parallelism. The processors discussed earlier are all referred to as some type of scalar device(Sony VAIO PCG-6S3L battery). As the name implies, vector processors deal with multiple pieces of data in the context of one instruction. This contrasts with scalar processors, which deal with one piece of data for every instruction. Using Flynn's taxonomy, these two schemes of dealing with data are generally referred to as SIMD (single instruction, multiple data) (Sony VAIO PCG-6V1L battery) and SISD (single instruction, single data), respectively. The great utility in creating CPUs that deal with vectors of data lies in optimizing tasks that tend to require the same operation (for example, a sum or a dot product) to be performed on a large set of data. Some classic examples of these types of tasks are multimedia applications (images, video, and sound) (Sony VAIO PCG-6W1L battery), as well as many types of scientific and engineering tasks. Whereas a scalar CPU must complete the entire process of fetching, decoding, and executing each instruction and value in a set of data, a vector CPU can perform a single operation on a comparatively large set of data with one instruction. Of course, this is only possible when the application tends to require many steps which apply one operation to a large set of data(Sony VAIO PCG-7111L battery).

Most early vector CPUs, such as the Cray-1, were associated almost exclusively with scientific research and cryptography applications. However, as multimedia has largely shifted to digital media, the need for some form of SIMD in general-purpose CPUs has become significant(Sony VAIO PCG-6W3L battery). Shortly after floating point execution units started to become commonplace to include in general-purpose processors, specifications for and implementations of SIMD execution units also began to appear for general-purpose CPUs. Some of these early SIMD specifications like HP's Multimedia Acceleration eXtensions (MAX) and Intel's MMX were integer-only(Sony VAIO PCG-7113L battery). This proved to be a significant impediment for some software developers, since many of the applications that benefit from SIMD primarily deal with floating point numbers. Progressively, these early designs were refined and remade into some of the common, modern SIMD specifications, which are usually associated with one ISA. Some notable modern examples are Intel's SSE and the PowerPC-relatedAltiVec (also known as VMX) (Sony VAIO PCG-7133L battery).

Performance

The performance or speed of a processor depends on the clock rate and the instructions per clock (IPC), which together are the factors for the instructions per second (IPS) that the CPU can perform. Many reported IPS values have represented "peak" execution rates on artificial instruction sequences with few branches(Sony VAIO PCG-7Z1L battery), whereas realistic workloads consist of a mix of instructions and applications, some of which take longer to execute than others. The performance of the memory hierarchy also greatly affects processor performance, an issue barely considered in MIPS calculations. Because of these problems(Sony VAIO PCG-7Z2L battery), various standardized tests such as SPECint have been developed to attempt to measure the real effective performance in commonly used applications.

Processing performance of computers is increased by using multi-core processors, which essentially is plugging two or more individual processors (called cores in this sense) into one integrated circuit. Ideally, a dual core processor would be nearly twice as powerful as a single core processor(Sony VAIO PCG-8Y1L battery). In practice, however, the performance gain is far less, only about fifty percent, due to imperfect software algorithms and implementation.