Anti-imperialism

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Anti-imperialism

Islamic commander Omar Mukhtar, popularly known as the "Lion of the Desert", led the Libyan Mujahidin against the imperialist forces of Fascist Italy during the Interwar Period

Anti-imperialism in political science and international relations is a term used in a variety of contexts, usually by nationalist movements who want to secede from a larger polity (usually in the form of an empire, but also in a multi-ethnic sovereign state) or as a specific theory opposed to capitalism in Leninist discourse, derived from Vladimir Lenin's work Imperialism, the Highest Stage of Capitalism. Less common usage refers to opponents of an interventionist foreign policy.

People who categorize themselves as anti-imperialists often state that they are opposed to colonialism, colonial empires, hegemony, imperialism and the territorial expansion of a country beyond its established borders. An influential movement independent of the Western Left that advocated religious anti-imperialism was Pan-Islamism; which challenged the Western civilisational model and rose to prominence across various parts of the Islamic World during the 19th and 20th centuries. Its most influential ideologue was the Sunni theologian Muhammad Rashid Rida, a fierce opponent of Western ideas, who called upon Muslims to rise up in armed resistance by waging Jihad against imperialism and re-establish an Islamic Caliphate. Through his resolution in the Second World Congress of Comintern (1920), Lenin accused the anti-imperialism of Pan-Islamists of favouring the interests of the bourgeoisie, feudal landlords and religious clerics; and enjoined fellow communists to compulsorily fight Pan-Islamism. Since then, Soviet authorities regularly employed the charge of Pan-Islamism to target Islamic dissidents for anti-Soviet activities and fomenting anti-communist rebellions.

The phrase gained a wide currency after the Second World War and at the onset of the Cold War as political movements in colonies of European powers promoted national sovereignty. Some anti-imperialist groups who opposed the United States supported the power of the Soviet Union, while in some Marxist schools, such as Maoism, this was criticized as social imperialism. Islamist movements traditionally view Russia and China as imperial and neo-colonial forces engaged in persecution and oppression of Muslim communities domestically and abroad, in addition to the U.S. and its allies like Israel.

Theory

Anti-imperialist painting in Caracas, specifically targeting American imperialism

In the late 1870s, the term "imperialism" was introduced to the English language by opponents of the aggressively imperial policies of British Prime Minister Benjamin Disraeli (1874–1880). It was shortly appropriated by supporters of "imperialism" such as Joseph Chamberlain. For some, imperialism designated a policy of idealism and philanthropy; others alleged that it was characterized by political self-interest; and a growing number associated it with capitalist greed. John A. Hobson and Vladimir Lenin added a more theoretical macroeconomic connotation to the term. Many theoreticians on the left have followed either or both in emphasizing the structural or systemic character of "imperialism". Such writers have expanded the time period associated with the term so that it now designates neither a policy, nor a short space of decades in the late 19th century, but a global system extending over a period of centuries, often going back to Christopher Columbus. As the application of the term has expanded, its meaning has shifted along five distinct but often parallel axes: the moral, the economic, the systemic, the cultural and the temporal. Those changes reflect—among other shifts in sensibility—a growing unease with the fact of power, specifically Western power.

The relationships among capitalism, aristocracy and imperialism have been discussed and analysed by theoreticians, historians, political scientists such as John A. Hobson and Thorstein Veblen, Joseph Schumpeter and Norman Angell. Those intellectuals produced much of their works about imperialism before the World War I (1914–1918), yet their combined work informed the study of the impact of imperialism upon Europe and contributed to the political and ideologic reflections on the rise of the military–industrial complex in the United States from the 1950s onwards.

Hobson

Main article: Imperialism (Hobson)

John A. Hobson strongly influenced the anti-imperialism of both Marxists and liberals, worldwide through his 1902 book on Imperialism. He argued that the "taproot of imperialism" is not in nationalist pride, but in Capitalism. As a form of economic organization, imperialism is unnecessary and immoral, the result of the mis-distribution of wealth in a capitalist society. That created an irresistible desire to extend the national markets into foreign lands, in search of profits greater than those available in the Mother Country. In the capitalist economy, rich capitalists received a disproportionately higher income than did the working class. If the owners invested their incomes to their factories, the greatly increased productive capacity would exceed the growth in demand for the products and services of said factories. Lenin adopted Hobson's ideas to argue that capitalism was doomed and would eventually be replaced by socialism, the sooner the better.

Hobson was also influential in liberal circles, especially the British Liberal Party. Historians Peter Duignan and Lewis H. Gann argue that Hobson had an enormous influence in the early 20th century that caused widespread distrust of imperialism:

Hobson's ideas were not entirely original; however his hatred of moneyed men and monopolies, his loathing of secret compacts and public bluster, fused all existing indictments of imperialism into one coherent system....His ideas influenced German nationalist opponents of the British Empire as well as French Anglophobes and Marxists; they colored the thoughts of American liberals and isolationist critics of colonialism. In days to come they were to contribute to American distrust of Western Europe and of the British Empire. Hobson helped make the British averse to the exercise of colonial rule; he provided indigenous nationalists in Asia and Africa with the ammunition to resist rule from Europe.

On the positive side, Hobson argued that domestic social reforms could cure the international disease of imperialism by removing its economic foundation. Hobson theorized that state intervention through taxation could boost broader consumption, create wealth and encourage a peaceful multilateral world order. Conversely, should the state not intervene, rentiers (people who earn income from property or securities) would generate socially negative wealth that fostered imperialism and protectionism.

Political movement

As a self-conscious political movement, anti-imperialism originated in Europe in the late 19th and early 20th centuries in opposition to the growing European colonial empires and the United States control of the Philippines after 1898. However, it reached its highest level of popular support in the colonies themselves, where it formed the basis for a wide variety of national liberation movements during the mid-20th century and later. These movements, and their anti-imperialist ideas, were instrumental in the decolonization process of the 1950s and 1960s, which saw most European colonies in Asia and Africa achieving their independence.

International context

United States

Main article: American Anti-Imperialist League

An early use of the term "anti-imperialist" occurred after the United States entered the Spanish–American War in 1898. Most activists supported the war itself, but opposed the annexation of new territory, especially the Philippines. The Anti-Imperialist League was founded on June 15, 1898, in Boston in opposition of the acquisition of the Philippines, which would happen anyway. The anti-imperialists opposed the expansion because they believed imperialism violated the credo of republicanism, especially the need for "consent of the governed". Appalled by American imperialism, the Anti-Imperialist League, which included famous citizens such as Andrew Carnegie, Henry James, William James and Mark Twain, formed a platform which stated:

We hold that the policy known as imperialism is hostile to liberty and tends toward militarism, an evil from which it has been our glory to be free. We regret that it has become necessary in the land of Washington and Lincoln to reaffirm that all men, of whatever race or color, are entitled to life, liberty and the pursuit of happiness. We maintain that governments derive their just powers from the consent of the governed. We insist that the subjugation of any people is "criminal aggression" and open disloyalty to the distinctive principles of our Government... We cordially invite the cooperation of all men and women who remain loyal to the Declaration of Independence and the Constitution of the United States.

Fred Harrington states that "the anti-imperialist's did not oppose expansion because of commercial, religious, constitutional, or humanitarian reasons but instead because they thought that an imperialist policy ran counter to the political doctrines of the Declaration of Independence, Washington's Farewell Address, and Lincoln's Gettysburg Address".

An important influence on American intellectuals was the work of British writer John A. Hobson. especially Imperialism: A Study (1902). Historians Peter Duignan and Lewis H. Gann argue that Hobson had an enormous influence in the early 20th century that caused widespread distrust of imperialism:

Hobson's ...hatred of moneyed men and monopolies, his loathing of secret compacts and public bluster, fused all existing indictments of imperialism into one coherent system....His ideas influenced German nationalist opponents of the British Empire as well as French Anglophobes and Marxists; they colored the thoughts of American liberals and isolationist critics of colonialism. In days to come they were to contribute to American distrust of Western Europe and of the British Empire. Hobson helped make the British averse to the exercise of colonial rule; he provided indigenous nationalists in Asia and Africa with the ammunition to resist rule from Europe.

The American rejection of the League of Nations in 1919 was accompanied with a sharp American reaction against European imperialism. American textbooks denounced imperialism as a major cause of the World War. The uglier aspects of British colonial rule were emphasized, recalling the long-standing anti-British sentiments in the United States.

In Britain and Canada

Anti-imperialism within Britain emerged in the 1890s, especially from within the Liberal Party. For over a century, back to the days of Adam Smith in 1776, economists had been hostile to imperialism on the grounds that it is a violation of the principles of free trade; they never formed a popular movement. Indeed, imperialism seems to have been generally popular before the 1890s. The key impetus around 1900 came from strong public disapproval with the British actions during with the Second Boer War (1899–1902). The war was fought against the Afrikaners, who were Dutch colonists who had built new homelands in South Africa. Opposition to the Second Boer War was modest when the war began and was generally less widespread than support for it. However, influential groups formed immediately against the war, including the South African Conciliation Committee and W. T. Stead's Stop the War Committee. Much of the opposition in Britain came from the Liberal Party. Intellectuals and activists Britain based in the socialist, labour and Fabian movements generally oppose imperialism and John A. Hobson, a Liberal, took many of his ideas from their writings. After the Boer war, opponents of imperialism turned their attention to the British crown colonies in Africa and Asia. By the 1920s, the government was sponsoring large-scale exhibits promoting imperialism, notably the 1924 British Empire Exhibition in London and the 1938 Glasgow Empire Exhibition. Some intellectuals used the opportunity to criticise imperialism as a policy.^[29]

Moderately active anti-imperial movements emerged in Canada and Australia. The French Canadians were hostile to British expansion whilst in Australia, it was the Irish Catholics who were opposed.^[30] French Canadians argue that Canadian nationalism was the proper and true goal and it sometimes conflicted with loyalty to the British Empire. Many French Canadians claimed that they would fight for Canada but would not fight for the Empire.

Protestant Canadians, typically of British descent, generally supported British imperialism enthusiastically. They sent thousands of volunteers to fight alongside British and imperial forces against the Boers and in the process identified themselves even more strongly with the British Empire. A little opposition also came from some English immigrants such as the intellectual leader Goldwin Smith. In Canada, the Irish Catholics were fighting the French Canadians for control of the Catholic Church, so the Irish generally supported the pro-British position. Anti-imperialism also grew rapidly in India and formed a core element of the demand by Congress for independence.

Marxism–Leninism

To the Russian revolutionary Vladimir Lenin, imperialism was the highest, but degenerate, stage of capitalism

To the Latin American revolutionary Che Guevara, imperialism was a capitalistic geopolitical system of control and repression which must be understood as such in order to be defeated

In the mid-19th century, Karl Marx mentioned imperialism to be part of the prehistory of the capitalist mode of production in Das Kapital (1867–1894). Much more important was Vladimir Lenin, who defined imperialism as "the highest stage of capitalism", the economic stage in which monopoly finance capital becomes the dominant application of capital. As such, said financial and economic circumstances impelled national governments and private business corporations to worldwide competition for control of natural resources and human labour by means of colonialism.

The Leninist views of imperialism and related theories, such as dependency theory, address the economic dominance and exploitation of a country, rather than the military and the political dominance of a people, their country and its natural resources. Hence, the primary purpose of imperialism is economic exploitation, rather than mere control of either a country or of a region. The Marxist and the Leninist denotation thus differs from the usual political science denotation of imperialism as the direct control (intervention, occupation and rule) characteristic of colonial and neo-colonial empires as used in the realm of international relations.

In Imperialism, the Highest Stage of Capitalism (1917), Lenin outlined the five features of capitalist development that lead to imperialism:

1. Concentration of production and capital leading to the dominance of national and multinational monopolies and cartels.
2. Industrial capital as the dominant form of capital has been replaced by finance capital, with the industrial capitalists increasingly reliant on capital provided by monopolistic financial institutions. "Again and again, the final word in the development of banking is monopoly".
3. The export of the aforementioned finance capital is emphasized over the export of goods.
4. The economic division of the world by multinational cartels.
5. The political division of the world into colonies by the great powers, in which the great powers monopolise investment.

Generally, the relationship among Marxist-Leninists and radical, left-wing organisations who are anti-war, often involves persuading such political activists to progress from pacifism to anti-imperialism—that is, to progress from the opposition of war, in general, to the condemnation of the capitalist economic system, in particular.

In the 20th century, the Soviet Union represented themselves as the foremost enemy of imperialism and thus politically and financially supported Third World revolutionary organisations who fought for national independence. This was accomplished through the export of both financial capital and Soviet military apparatuses, with the Soviet Union sending military advisors to Ethiopia, Angola, Egypt and Afghanistan.

However, anarchists as well as many other Marxist organizations, have characterized Soviet foreign policy as imperialism and cited it as evidence that the philosophy of Marxism would not resolve and eliminate imperialism. Mao Zedong developed the theory that the Soviet Union was a social imperialist nation, a socialist people with tendencies to imperialism, an important aspect of Maoist analysis of the history of the Soviet Union. Contemporarily, the term "anti-imperialism" is most commonly applied by Marxist-Leninists, and political organisations of like ideological persuasion who oppose capitalism, present a class analysis of society and the like.

About the nature of imperialism and how to oppose and defeat it, Che Guevara said:

imperialism is a world system, the last stage of capitalism—and it must be defeated in a world confrontation. The strategic end of this struggle should be the destruction of imperialism. Our share, the responsibility of the exploited and underdeveloped of the world, is to eliminate the foundations of imperialism: our oppressed nations, from where they extract capitals, raw materials, technicians, and cheap labor, and to which they export new capitals—instruments of domination—arms and all kinds of articles; thus submerging us in an absolute dependence.

— Che Guevara, Message to the Tricontinental, 1967

Opposition to Soviet imperialism

See also: Soviet empire, Anti-Sovietism, and Social imperialism

Depiction of the first shoot-down of Soviet helicopter gunships by the Afghan mujahideen using Western-supplied Stinger Missiles, widely regarded as the turning point in the Soviet-Afghan War

Polish anti-Soviet poster depicting Lev Trotsky.

Soviet foreign policy have been characterized as imperialist and colonialist. The nations which were part of the Soviet sphere of influence were nominally independent countries with separate governments that set their own policies, but those policies had to stay within certain limits decided by the Soviet Union. These limits were enforced by the threat of intervention by Soviet forces, and later the Warsaw Pact. Major military invasions took place in East Germany in 1953, Hungary in 1956, Czechoslovakia in 1968, Poland in 1980–81 and Afghanistan from 1979 to 1989. Countries in the Soviet Bloc were considered satellite states.

The Soviet Union exhibited tendencies common to historic empires. The notion of "Soviet empire" often refers to a form of "classic" or "colonial" empire with communism only replacing conventional imperial ideologies such as Christianity or monarchy, rather than creating a revolutionary state. Academically the idea is seen as emerging with Richard Pipes' 1957 book The Formation of the Soviet Union: Communism and Nationalism, 1917–1923, but it has been reinforced, along with several other views, in continuing scholarship. Several scholars hold that the Soviet Union was a hybrid entity containing elements common to both multinational empires and nation states. The Soviet Union practiced colonialism similar to conventional imperial powers.

Islamic anti-imperialism

See also: Pan-Islamism and Islamism

Resistance fighters of Caucasian Mujahidin defend the village of Salta from the invading Russian Imperial Army during the Caucasian War

Imam Muhammad Rashid Rida was an ardent advocate of Pan-Islamist insurgency against imperialism. His teachings inspired figures like Hasan al-Banna, Sayyid Qutb and Abdullah Azzam

The 18th and 19th centuries witnessed the rise of numerous anti-colonial and anti-imperial Islamic resistance movements across various parts of the Muslim World. These included the Jihad movement led by the Imamate of Caucasus and the Circassian Confederacy against Russian imperialism during the Caucasus Wars (1763-1864 C.E). Prominent leaders in this resistance campaign included Ghazi Mullah, Gamzat-bek, Shamil, Hajji Qerandiqo Berzeg, Jembulat Boletoqo, etc. Other major anti-imperial movements included the Padri War, Java War, and the Aceh War against the Dutch colonisation of Indonesia, Moro Rebellion against the United States, the South Asian Jihad movement of Sayyid Ahmad Shahid, Mahdist State in Sudan and the Arabian Muwahhidun that fought British colonialism, Emir Abd al-Qadir's military insurgency against French in Algeria, North-West Frontier Uprisings of the Pashtun tribes against the British Raj, Omar Mukhtar's Jihad against Italian Fascists in Libya, etc. The establishment and defense of Islamic statehood that enforces Sharia (Islamic law) based on Qur'an and Sunnah, elimination of superstitions and heterodox local practices and folk rituals, etc. were key objectives of these reform movements.

These anti-colonial movements inspired the rise of Pan-Islamism during the late 19th century; which gave birth to numerous Islamist organisations advocating anti-imperialism across the Muslim World; such as the Muslim Brotherhood (Ikhwan al-Muslimeen) and Jamaat-e-Islami. Syro-Egyptian Islamist theoretician Muhammad Rashid Rida (1865 C.E/1282 A.H- 1935 C.E/1354 A.H), a Salafi theologian greatly influenced by preceding militant Islamic revivalist movements, was an ardent opponent of European imperial powers; and he called for armed Jihad to defend the Islamic World from encroaching colonialism, complemented by a political programme to establish Islamic states which would implement Sharia (Islamic laws). He extended this anti-imperialist campaign to the theological level through the Arab Salafiyya movement; which professed the key theme of returning to the values of Salaf al-Salih. This encompassed a theological assault on Western ideological currents emanating from the principles of secularism and nationalism as well as denunciation of Western cultural imperialism.

After Rashid Rida, the mantle of Islamist anti-imperialism was spearheaded by the Egyptian Muslim Brotherhood founder Hasan al-Banna, South Asian revolutionary Islamist leader Sayyid Abul A'la Maududi and Egyptian Jihadist theoretician Sayyid Qutb. Mawdudi held the belief that West was in decline and that restoration of Islamic prowess was inevitable. Openly equating Western colonialism with atheism, Mawdudi called upon Muslims to rally in Jihad against the imperialist forces to regain their spiritual, cultural, economic and military sovereignty and self-sufficiency. Sayyid Qutb, an Egyptian scholar influenced by both Mawdudi and Rashid Rida, took their ideas to its logical culmination; proclaiming the necessity of a permanent, un-ending Islamist revolution not only against the imperialists but also its allied regimes in the Muslim World. This revolution against the apostate regimes has to be waged as an armed Jihad by an ideological vanguard committed to establish the Islamic state and uphold Tawhid (Islamic monotheism). These ideas gained prominence and arose in influence across the Islamic World during the post World War 2 era. During the Cold War period, the Islamist intellectuals from the Muslim Brotherhood and Jamaat-e Islami also launched fervent anti-communist campaigns, ideologically critiquing socialism and Marxism and chiding leftists as agents of Soviet Imperialism.

In his book "Al Jihad Fil Islam", South Asian revolutionary Islamist scholar Abul A'la Mawdudi made a comprehensive Islamic refutation of imperialism. He argued that oppressive rulers justify imperialism in the name of progress and socio-political reforms. Describing the main features of imperialism, Mawdudi wrote:

"the basic quality of imperialism is the dominance of one particular nation or country... Thus, the doors of imperialism remain closed to people of other nationalities and for this reason, they can play no major role in running its affairs. This gives rise to the development of other faults in the system and characters of the subject nation. They develop a weakness of character, lose self-esteem and the sense of righteousness. Even if the ruling nation does not treat the subjects with outright cruelty and arrogance, their (the subject nation’s) character sinks to such a low ebb of ignobility that they become quite incapable of striving for attaining and maintaining self-rule for a very long time."

— Abul A'la Maududi, Al Jihad fil Islam

According to the Egyptian Jihadist theoretician Sayyid Qutb, the imperialism of secular Western powers was a by-product of their historical Crusading spirit and driven by ideological differences

The Indian Jamaat-e-Islami Hind launched a ten-day nationwide campaign titled Anti-Imperialism Campaign in December 2009. Contemporary Jihadist movements such as Al-Qaeda, influenced by Sayyid Qutb's thought, declares itself as a "global revolutionary vanguard" waging Jihad to defend Muslims from atrocities committed by the forces of Western imperialism and its allies.

In the worldview of Egyptian Jihadist theoretician Sayyid Qutb, imperialist policies of the secular Western regimes were a continuation of their historical "Crusading Spirit". In his commentary of the Qur'anic verse 2:120 "{Never will the Jews be pleased with you, (O Prophet), nor the Christians until you follow their way..}", Sayyid Qutb writes:

"The conflict between the Judeo-Christian world on the one side, and the Muslim community on the other, remains in essence one of ideology, although over the years it has appeared in various guises and has grown more sophisticated and, at times, more insidious. We have seen the original ideological conflict succeeded by economic, political and military confrontation, on the basis that ‘religious’ or ‘ideological’ conflicts are outdated and are usually prosecuted by ‘fanatics’ and backward people. Unfortunately, some naïve and confused Muslims have fallen for this stratagem and persuaded themselves that the religious and ideological aspects of the conflict are no longer relevant. But in reality world Zionism and Christian Imperialism, as well as world Communism, are conducting the fight against Islam and the Muslim community, first and foremost, on ideological grounds... The confrontation is not over control of territory or economic resources, or for military domination. If we believe that, we would play into our enemies’ hands and would have no one but ourselves to blame for the consequences."

— Sayyid Qutb, Fi Zilal al-Quran

Liberal anti-imperialism

Sometimes liberals also oppose imperialism. However, liberal anti-imperialists are distinct from socialist anti-imperialists because they do not support anti-capitalism.

South Korean liberals have opposed Chinese and Japanese imperialism. "No Japan Movement" is related to anti-imperialist sentiment in South Korea. On August 14, 2019, seven politicians of the DPK's descendants of independence activists said at a press conference, "In the spirit of Great Korean Independence 100 years ago, let's overcome the economic invasion of Shinzo Abe's government." (100년 전 대한독립의 정신으로 아베 정부 경제침략을 이겨내자.) South Korean liberals, unlike protectionist anti-imperialists, believing that the Japanese government's actions that undermined the "free trade principle" (자유무역 원칙 or 자유무역 철칙) during the Japan–South Korea trade dispute were far-right imperialist 'economic invasion'. (South Korean liberals argue that the Japanese government caused unfair damage to the South Korean economy to avoid compensation for Korean victims of Japanese war crimes during the past imperialist Japan.) South Korean liberals also oppose the appropriation of Korean culture of the Chinese people.

Some modern liberals in the United States, including Dennis Kucinich, support non-interventionism.

Criticism

Antonio Negri and Michael Hardt assert that traditional anti-imperialism is no longer relevant. In the book Empire, Negri and Hardt argue that imperialism is no longer the practice or domain of any one nation or state. Rather, they claim, the "Empire" is a conglomeration of all states, nations, corporations, media, popular and intellectual culture and so forth; and thus, traditional anti-imperialist methods and strategies can no longer be applied against them.

Plant reproduction

From Wikipedia, the free encyclopedia

Plant reproduction is the production of new offspring in plants, which can be accomplished by sexual or asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from either parent. Asexual reproduction produces new individuals without the fusion of gametes, resulting in clonal plants that are genetically identical to the parent plant and each other, unless mutations occur.

Asexual reproduction

See also: Asexual reproduction

Asexual reproduction does not involve the production and fusion of male and female gametes. Asexual reproduction may occur through budding, fragmentation, spore formation, regeneration and vegetative propagation.

Asexual reproduction is a type of reproduction where the offspring comes from one parent only, thus inheriting the characteristics of the parent. Asexual reproduction in plants occurs in two fundamental forms, vegetative reproduction and agamospermy. Vegetative reproduction involves a vegetative piece of the original plant producing new individuals by budding, tillering, etc. and is distinguished from apomixis, which is a replacement of sexual reproduction, and in some cases involves seeds. Apomixis occurs in many plant species such as dandelions (Taraxacum species) and also in some non-plant organisms. For apomixis and similar processes in non-plant organisms, see parthenogenesis.

Natural vegetative reproduction is a process mostly found in perennial plants, and typically involves structural modifications of the stem or roots and in a few species leaves. Most plant species that employ vegetative reproduction do so as a means to perennialize the plants, allowing them to survive from one season to the next and often facilitating their expansion in size. A plant that persists in a location through vegetative reproduction of individuals gives rise to a clonal colony. A single ramet, or apparent individual, of a clonal colony is genetically identical to all others in the same colony. The distance that a plant can move during vegetative reproduction is limited, though some plants can produce ramets from branching rhizomes or stolons that cover a wide area, often in only a few growing seasons. In a sense, this process is not one of reproduction but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called vegetative growth. However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. A major disadvantage of vegetative reproduction is the transmission of pathogens from parent to offspring. It is uncommon for pathogens to be transmitted from the plant to its seeds (in sexual reproduction or in apomixis), though there are occasions when it occurs.

Seeds generated by apomixis are a means of asexual reproduction, involving the formation and dispersal of seeds that do not originate from the fertilization of the embryos. Hawkweeds (Hieracium), dandelions (Taraxacum), some species of Citrus and Kentucky blue grass (Poa pratensis) all use this form of asexual reproduction. Pseudogamy occurs in some plants that have apomictic seeds, where pollination is often needed to initiate embryo growth, though the pollen contributes no genetic material to the developing offspring. Other forms of apomixis occur in plants also, including the generation of a plantlet in replacement of a seed or the generation of bulbils instead of flowers, where new cloned individuals are produced.

Structures

A rhizome is a modified underground stem serving as an organ of vegetative reproduction; the growing tips of the rhizome can separate as new plants, e.g., polypody, iris, couch grass and nettles.

Prostrate aerial stems, called runners or stolons, are important vegetative reproduction organs in some species, such as the strawberry, numerous grasses, and some ferns.

Adventitious buds form on roots near the ground surface, on damaged stems (as on the stumps of cut trees), or on old roots. These develop into above-ground stems and leaves. A form of budding called suckering is the reproduction or regeneration of a plant by shoots that arise from an existing root system. Species that characteristically produce suckers include elm (Ulmus) and many members of the rose family such as Rosa, Kerria and Rubus.

Bulbous plants such as onion (Allium cepa), hyacinths, narcissi and tulips reproduce vegetatively by dividing their underground bulbs into more bulbs. Other plants like potatoes (Solanum tuberosum) and dahlias reproduce vegetatively from underground tubers. Gladioli and crocuses reproduce vegetatively in a similar way with corms.

Gemmae are single cells or masses of cells that detach from plants to form new clonal individuals. These are common in Liverworts and mosses and in the gametophyte generation of some filmy fern. They are also present in some Club mosses such as Huperzia lucidula . They are also found in some higher plants such as species of Drosera.

Usage

The most common form of plant reproduction used by people is seeds, but a number of asexual methods are used which are usually enhancements of natural processes, including: cutting, grafting, budding, layering, division, sectioning of rhizomes, roots, tubers, bulbs, stolons, tillers, etc., and artificial propagation by laboratory tissue cloning. Asexual methods are most often used to propagate cultivars with individual desirable characteristics that do not come true from seed. Fruit tree propagation is frequently performed by budding or grafting desirable cultivars (clones), onto rootstocks that are also clones, propagated by stooling.

In horticulture, a cutting is a branch that has been cut off from a mother plant below an internode and then rooted, often with the help of a rooting liquid or powder containing hormones. When a full root has formed and leaves begin to sprout anew, the clone is a self-sufficient plant, genetically identical.

Examples include cuttings from the stems of blackberries (Rubus occidentalis), African violets (Saintpaulia), verbenas (Verbena) to produce new plants. A related use of cuttings is grafting, where a stem or bud is joined onto a different stem. Nurseries offer for sale trees with grafted stems that can produce four or more varieties of related fruits, including apples. The most common usage of grafting is the propagation of cultivars onto already rooted plants, sometimes the rootstock is used to dwarf the plants or protect them from root damaging pathogens.

Since vegetatively propagated plants are clones, they are important tools in plant research. When a clone is grown in various conditions, differences in growth can be ascribed to environmental effects instead of genetic differences.

Sexual reproduction

Main article: Sexual reproduction of plants

Sexual reproduction involves two fundamental processes: meiosis, which rearranges the genes and reduces the number of chromosomes, and fertilization, which restores the chromosome to a complete diploid number. In between these two processes, different types of plants and algae vary, but many of them, including all land plants, undergo alternation of generations, with two different multicellular structures (phases), a gametophyte and a sporophyte. The evolutionary origin and adaptive significance of sexual reproduction are discussed in the pages Evolution of sexual reproduction and Origin and function of meiosis.

The gametophyte is the multicellular structure (plant) that is haploid, containing a single set of chromosomes in each cell. The gametophyte produces male or female gametes (or both), by a process of cell division, called mitosis. In vascular plants with separate gametophytes, female gametophytes are known as mega gametophytes (mega=large, they produce the large egg cells) and the male gametophytes are called micro gametophytes (micro=small, they produce the small sperm cells).

The fusion of male and female gametes (fertilization) produces a diploid zygote, which develops by mitotic cell divisions into a multicellular sporophyte.

The mature sporophyte produces spores by meiosis, sometimes referred to as reduction division because the chromosome pairs are separated once again to form single sets.

In mosses and liverworts, the gametophyte is relatively large, and the sporophyte is a much smaller structure that is never separated from the gametophyte. In ferns, gymnosperms, and flowering plants (angiosperms), the gametophytes are relatively small and the sporophyte is much larger. In gymnosperms and flowering plants the megagametophyte is contained within the ovule (that may develop into a seed) and the microgametophyte is contained within a pollen grain.

History of sexual reproduction of plants

Main article: Evolution of sexual reproduction

Unlike animals, plants are immobile, and cannot seek out sexual partners for reproduction. In the evolution of early plants, abiotic means, including water and much later, wind, transported sperm for reproduction. The first plants were aquatic, as described in the page Evolutionary history of plants, and released sperm freely into the water to be carried with the currents. Primitive land plants such as liverworts and mosses had motile sperm that swam in a thin film of water or were splashed in water droplets from the male reproduction organs onto the female organs. As taller and more complex plants evolved, modifications in the alternation of generations evolved. In the Paleozoic era progymnosperms reproduced by using spores dispersed on the wind. The seed plants including seed ferns, conifers and cordaites, which were all gymnosperms, evolved 350 million years ago. They had pollen grains that contained the male gametes for protection of the sperm during the process of transfer from the male to female parts.

It is believed that insects fed on the pollen, and plants thus evolved to use insects to actively carry pollen from one plant to the next. Seed producing plants, which include the angiosperms and the gymnosperms, have a heteromorphic alternation of generations with large sporophytes containing much-reduced gametophytes. Angiosperms have distinctive reproductive organs called flowers, with carpels, and the female gametophyte is greatly reduced to a female embryo sac, with as few as eight cells. Each pollen grains contains a greatly reduced male gametophyte consisting of three or four cells. The sperm of seed plants are non-motile, except for two older groups of plants, the Cycadophyta and the Ginkgophyta, which have flagella.

Flowering plants

Further information: Plant reproductive morphology

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Flowering plants, the dominant plant group, reproduce both by sexual and asexual means. Their distinguishing feature is that their reproductive organs are contained in flowers. Sexual reproduction in flowering plants involves the production of separate male and female gametophytes that produce gametes.

The anther produces pollen grains which contain male gametophytes. The pollen grains attach to the stigma on top of a carpel, in which the female gametophytes (inside ovules) are located. Plants may either self-pollinate or cross-pollinate. The transfer of pollen (the male gametophytes) to the female stigmas occurs is called pollination. After pollination occurs, the pollen grain germinates to form a pollen tube that grows through the carpel's style and transports male nuclei to the ovule to fertilize the egg cell and central cell within the female gametophyte in a process termed double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus a binucleate female cell) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The fertilized ovules develop into seeds within a fruit formed from the ovary. When the seeds are ripe they may be dispersed together with the fruit or freed from it by various means to germinate and grow into the next generation.

Pollination

An orchid flower

Plants that use insects or other animals to move pollen from one flower to the next have developed greatly modified flower parts to attract pollinators and to facilitate the movement of pollen from one flower to the insect and from the insect to the next flower. Flowers of wind-pollinated plants tend to lack petals and or sepals; typically large amounts of pollen are produced and pollination often occurs early in the growing season before leaves can interfere with the dispersal of the pollen. Many trees and all grasses and sedges are wind-pollinated.

Plants have a number of different means to attract pollinators including color, scent, heat, nectar glands, edible pollen and flower shape. Along with modifications involving the above structures two other conditions play a very important role in the sexual reproduction of flowering plants, the first is the timing of flowering and the other is the size or number of flowers produced. Often plant species have a few large, very showy flowers while others produce many small flowers, often flowers are collected together into large inflorescences to maximize their visual effect, becoming more noticeable to passing pollinators. Flowers are attraction strategies and sexual expressions are functional strategies used to produce the next generation of plants, with pollinators and plants having co-evolved, often to some extraordinary degrees, very often rendering mutual benefit.

Flower heads showing disk and ray florets.

The largest family of flowering plants is the orchids (Orchidaceae), estimated by some specialists to include up to 35,000 species, which often have highly specialized flowers that attract particular insects for pollination. The stamens are modified to produce pollen in clusters called pollinia, which become attached to insects that crawl into the flower. The flower shapes may force insects to pass by the pollen, which is "glued" to the insect. Some orchids are even more highly specialized, with flower shapes that mimic the shape of insects to attract them to attempt to 'mate' with the flowers, a few even have scents that mimic insect pheromones.

Another large group of flowering plants is the Asteraceae or sunflower family with close to 22,000 species, which also have highly modified inflorescences composed of many individual flowers called florets. Heads with florets of one sex, when the flowers are pistillate or functionally staminate or made up of all bisexual florets, are called homogamous and can include discoid and liguliflorous type heads. Some radiate heads may be homogamous too. Plants with heads that have florets of two or more sexual forms are called heterogamous and include radiate and disciform head forms, though some radiate heads may be heterogamous too.

Ferns

Ferns typically produce large diploids with stem, roots, and leaves. On fertile leaves sporangia are produced, grouped together in sori and often protected by an indusium. If the spores are deposited onto a suitable moist substrate they germinate to produce short, thin, free-living gametophytes called prothalli that are typically heart-shaped, small and green in color. The gametophytes produce both motile sperm in the antheridia and egg cells in separate archegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the antheridia where they fertilize the egg. To promote out crossing or cross-fertilization the sperm is released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of the different thallus. A zygote is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is called alternation of generations. Other plants with similar reproductive means include the Psilotum, Lycopodium, Selaginella and Equisetum.

Bryophytes

Main articles: Alternation of generations and Bryophyte life cycle

The bryophytes, which include liverworts, hornworts and mosses, reproduce both sexually and vegetatively. The gametophyte is the most commonly known phase of the plant. Bryophytes are typically small plants that grow in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominant form, which is a multicellular haploid body with leaf-like structures that photosynthesize. Haploid gametes are produced in antheridia and archegonia by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells, thus producing zygotes that are diploid. The zygote divides by mitotic division and grows into a sporophyte that is diploid. The multicellular diploid sporophyte produces structures called spore capsules. The spore capsules produce spores by meiosis, and when ripe, the capsules burst open and the spores are released. Bryophytes show considerable variation in their breeding structures and the above is a basic outline. In some species each gametophyte is one sex while other species may be monoicous, producing both antheridia and archegonia on the same gametophyte which is thus hermaphrodite.

Dispersal and offspring care

Main article: Seed dispersal

One of the outcomes of plant reproduction is the generation of seeds, spores, gemmae and other vegetative organs that allow plants to move to new locations or new habitats.

Plants do not have nervous systems or any will for their actions. Even so, scientists are able to observe mechanisms that help their "children" thrive as they grow. All organisms have mechanisms to increase survival in offspring.

Offspring care is observed in the Mammillaria hernandezii, a small cactus found in Mexico. A cactus is a type of succulent, meaning it retains water when it is available for future droughts. M. hernandezii also stores a portion of its seeds in its stem, and releases the rest to grow. This can be advantageous for many reasons. By delaying the release of some of its seeds, the cactus can protect these from potential threats from insects, herbivores, or mold caused by micro-organisms. A study found that the presence of adequate water in the environment causes M. Hernandezii to release more seeds to allow for germination. The plant was able to perceive a water potential gradient in the surroundings, and act by giving its seeds a better chance in this preferable environment. This evolutionary strategy gives a better potential outcome for seed germination

There are similar reproductive strategies found in both mammals and plants. A divergence between the two is that in harsh environmental conditions, mammals produce fewer and larger offspring, whereas plants produce more seeds.

Transgender pregnancy

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Transgender_pregnancy

Transgender pregnancy is the gestation of one or more embryos or fetuses by transgender people. As of 2023, the possibility is restricted to those born with female reproductive systems. However, transition-related treatments may impact fertility. Transgender men and nonbinary people who are or wish to become pregnant face social, medical, legal, and psychological concerns. As uterus transplantations are currently experimental, and none have successfully been performed on trans women, they cannot become pregnant.

Trans men

Pregnant trans man Fernando Machado and his transgender wife, Diane Rodríguez.

Pregnancy is possible for transgender men who retain functioning ovaries and a uterus, such as in the case of Thomas Beatie. Regardless of prior hormone replacement therapy (HRT) treatments, the progression of pregnancy and birthing procedures are typically the same as those of cisgender women. Delivery options include conventional methods such as vaginal delivery and cesarean section, and patient preference should be taken into consideration in order to reduce gender dysphoric feelings associated with certain physical changes and sensations. It has been shown that historical HRT use may not negatively impact ovarian stimulation outcomes, with no significant differences in the markers of follicular function or oocyte maturity between transgender men with and without a history of testosterone use.

Among the wide array of transgender-related therapies available, including surgical and medical interventions, some offer the option of preserving fertility while others may compromise one's ability to become pregnant (including bilateral salpingo-oophorectomy and/or total hysterectomy).

Effect of masculinizing hormone therapy

Testosterone therapy affects fertility, but many trans men who have become pregnant were able to do so within six months of stopping testosterone. Testosterone has been shown to be an ineffective form of contraception. Exposing a fetus to high levels of exogenous testosterone may damage an unborn child. This is particularly impactful in the first trimester of development when many pregnancies haven't been discovered yet. Additionally, patients experiencing amenorrhea (a common side effect of exogenous testosterone exposure) may experience additional challenges in identifying early pregnancies due to the lack of regular menstrual cycling that could indicate a pregnancy if missed, for example. For this reason, it is important for patients and healthcare practitioners to comprehensively discuss fertility goals, family planning and contraceptive options during gender-affirming care. Previous studies of pregnancies in women suggest that high levels of endogenous androgens are associated with reduced birth weight, although it is unclear how prior testosterone in a childbearing trans person may affect birth weight. Future pregnancies can be achieved by oophyte banking, but the process may increase gender dysphoria or may not be accessible due to lack of insurance coverage. Testosterone therapy is not a sufficient method of contraception, and trans men may experience unintended pregnancy, especially if they miss doses.

Another important postpartum consideration for trans men is whether to resume testosterone therapy. There is currently no evidence that testosterone enters breast milk in a significant quantity. However, elevated testosterone levels may suppress lactation and healthcare guidelines have previously recommended that trans men do not undergo testosterone therapy while chestfeeding. Trans men who undergo chest reconstruction surgery may maintain the ability to chestfeed.

Mental health

Special consideration of the mental health of transgender people during pregnancy is important. It has previously been shown that transgender individuals often experience higher rates of suicidality than cisgender people and lesser degrees of social support from their environment and familial relationships. Relatedly, many transgender individuals experiencing pregnancy reported that choices of healthcare providers were substantially impacted by the views of the healthcare worker, and many transgender people prefer midwifery services rather than experience labor and delivery in a hospital.

Some individuals reported having gender dysphoria and feelings of isolation due to the public reception of their gender identity and drastic changes in appearance which occur during pregnancy, such as enlarged breasts.

According to the National Transgender Discrimination Survey, postpartum rates of suicide and depression in trans individuals has been found to be higher than the adult average. This may be attributed to factors such as lack of social support, discrimination, and lack of adequate healthcare practitioner training.

Medical discrimination

Some trans men who carry pregnancies are subjected to discrimination, which can include a variety of negative social, emotional, and medical experiences, as pregnancy is regarded as an exclusively feminine or female activity. Several studies indicate a lack of awareness, services, and medical assistance available to pregnant trans men. Inaccessibility to these services may lead to difficulty in finding comfortable and supportive services concerning prenatal care, as well as an increased risk for unsafe or unhealthy practices.

Unintended pregnancies can result in transgender men or nonbinary people considering or attempting self-induced abortion. Many transgender men report attempting a self-induced abortion because of lack of safe, effective abortion methods.

Statistics

According to figures compiled by Medicare for Australia, one of the few national surveys as of 2020, 75 male-identified people gave birth naturally or via C-section in the country in 2016, and 40 in 2017.

Non-binary people

Non-binary people with a functioning female reproductive system can give birth.

Not all non-binary people (or trans people of any gender identity) medically transition through hormone replacement therapy (HRT) or any kind of surgeries due to various factors ranging from medical conditions, accessibility and/or expenses, but those that do have to interrupt their HRT in order to carry the pregnancy. Unintended pregnancies by non-binary people on testosterone therapy may be more common if they are on a low dose of testosterone. Non-binary parents choose whether to be called "mom" or "dad", or to utilize newly coined gender-neutral or non-binary titles.

Non-binary people who have written or been profiled about their experiences of pregnancy include Rory Mickelson, Braiden Schirtzinger, and Mariah MacCarthy.

Trans women

Lili Elbe in October 1930, a year before her death from a rejected uterus transplant

Further information: Uterus transplantation

Pregnancy is not possible for transgender women as they lack a female reproductive system. As of 2019, uterus transplantation has not been successfully performed in transgender women. The Danish transgender painter Lili Elbe died in 1931 from surgical complications following an attempt at such an operation.

Uterine transplantation, or UTx, is in its infancy and is not yet publicly available. As of 2019, in cisgender women, more than 42 UTx procedures had been performed, with 12 live births resulting from the transplanted uteruses as of publication. The International Society of Uterine Transplantation (ISUTx) was established internationally in 2016, with 70 clinical doctors and scientists, and currently has 140 intercontinental delegates.

In 2012, McGill University published the "Montreal Criteria for the Ethical Feasibility of Uterine Transplantation", a proposed set of criteria for carrying out uterine transplants, in Transplant International. Under these criteria, and because no research has been conducted in non-females, only a genetic female could ethically be considered a transplant recipient. The exclusion of trans women from candidacy is justified by the lack of research to determine how to conduct the surgery, rather than an inherent bar. In 2021, authors of the Montreal Criteria published a revised set of criteria in Bioethics with an ethical framework for consideration of genetic males' (and other genetic XY individuals') eligibility for uterine transplants.

Society and culture

In 1583, an intersex person that had masculine gender expression reportedly became pregnant in Beaumaris, Wales.

Unicode introduced "pregnant man" and "pregnant person" emojis in version 14.0, approved September 2021. However, this came with some controversy, as some viewed it to be absurd.

Virtual memory

From Wikipedia, the free encyclopedia

 

This article is about the computer memory management technique. For the technique of pooling multiple storage devices, see Storage virtualization. For the TBN game show, see Virtual Memory (game show).

Virtual memory combines active RAM and inactive memory on DASD to form a large range of contiguous addresses.

In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very large (main) memory".

The computer's operating system, using a combination of hardware and software, maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory. Main storage, as seen by a process or task, appears as a contiguous address space or collection of contiguous segments. The operating system manages virtual address spaces and the assignment of real memory to virtual memory. Address translation hardware in the CPU, often referred to as a memory management unit (MMU), automatically translates virtual addresses to physical addresses. Software within the operating system may extend these capabilities, utilizing, e.g., disk storage, to provide a virtual address space that can exceed the capacity of real memory and thus reference more memory than is physically present in the computer.

The primary benefits of virtual memory include freeing applications from having to manage a shared memory space, ability to share memory used by libraries between processes, increased security due to memory isolation, and being able to conceptually use more memory than might be physically available, using the technique of paging or segmentation.

Properties

Virtual memory makes application programming easier by hiding fragmentation of physical memory; by delegating to the kernel the burden of managing the memory hierarchy (eliminating the need for the program to handle overlays explicitly); and, when each process is run in its own dedicated address space, by obviating the need to relocate program code or to access memory with relative addressing.

Memory virtualization can be considered a generalization of the concept of virtual memory.

Usage

Virtual memory is an integral part of a modern computer architecture; implementations usually require hardware support, typically in the form of a memory management unit built into the CPU. While not necessary, emulators and virtual machines can employ hardware support to increase performance of their virtual memory implementations. Older operating systems, such as those for the mainframes of the 1960s, and those for personal computers of the early to mid-1980s (e.g., DOS), generally have no virtual memory functionality, though notable exceptions for mainframes of the 1960s include:

• the Atlas Supervisor for the Atlas
• THE multiprogramming system for the Electrologica X8 (software based virtual memory without hardware support)
• MCP for the Burroughs B5000
• MTS, TSS/360 and CP/CMS for the IBM System/360 Model 67
• Multics for the GE 645
• The Time Sharing Operating System for the RCA Spectra 70/46

During the 1960s and early '70s, computer memory was very expensive. The introduction of virtual memory provided an ability for software systems with large memory demands to run on computers with less real memory. The savings from this provided a strong incentive to switch to virtual memory for all systems. The additional capability of providing virtual address spaces added another level of security and reliability, thus making virtual memory even more attractive to the marketplace.

Most modern operating systems that support virtual memory also run each process in its own dedicated address space. Each program thus appears to have sole access to the virtual memory. However, some older operating systems (such as OS/VS1 and OS/VS2 SVS) and even modern ones (such as IBM i) are single address space operating systems that run all processes in a single address space composed of virtualized memory.

Embedded systems and other special-purpose computer systems that require very fast and/or very consistent response times may opt not to use virtual memory due to decreased determinism; virtual memory systems trigger unpredictable traps that may produce unwanted and unpredictable delays in response to input, especially if the trap requires that data be read into main memory from secondary memory. The hardware to translate virtual addresses to physical addresses typically requires a significant chip area to implement, and not all chips used in embedded systems include that hardware, which is another reason some of those systems don't use virtual memory.

History

In the 1950s, all larger programs had to contain logic for managing primary and secondary storage, such as overlaying. Virtual memory was therefore introduced not only to extend primary memory, but to make such an extension as easy as possible for programmers to use. To allow for multiprogramming and multitasking, many early systems divided memory between multiple programs without virtual memory, such as early models of the PDP-10 via registers.

A claim that the concept of virtual memory was first developed by German physicist Fritz-Rudolf Güntsch at the Technische Universität Berlin in 1956 in his doctoral thesis, Logical Design of a Digital Computer with Multiple Asynchronous Rotating Drums and Automatic High Speed Memory Operation does not stand up to careful scrutiny. The computer proposed by Güntsch (but never built) had an address space of 10⁵ words which mapped exactly onto the 10⁵ words of the drums, i.e. the addresses were real addresses and there was no form of indirect mapping, a key feature of virtual memory. What Güntsch did invent was a form of cache memory, since his high-speed memory was intended to contain a copy of some blocks of code or data taken from the drums. Indeed, he wrote (as quoted in translation): "The programmer need not respect the existence of the primary memory (he need not even know that it exists), for there is only one sort of addresses (sic) by which one can program as if there were only one storage." This is exactly the situation in computers with cache memory, one of the earliest commercial examples of which was the IBM System/360 Model 85. In the Model 85 all addresses were real addresses referring to the main core store. A semiconductor cache store, invisible to the user, held the contents of parts of the main store in use by the currently executing program. This is exactly analogous to Güntsch's system, designed as a means to improve performance, rather than to solve the problems involved in multi-programming.

The University of Manchester Atlas Computer was the first computer to feature true virtual memory.

The first true virtual memory system was that implemented at the University of Manchester to create a one-level storage system as part of the Atlas Computer. It used a paging mechanism to map the virtual addresses available to the programmer onto the real memory that consisted of 16,384 words of primary core memory with an additional 98,304 words of secondary drum memory. The addition of virtual memory into the Atlas also eliminated a looming programming problem: planning and scheduling data transfers between main and secondary memory and recompiling programs for each change of size of main memory. The first Atlas was commissioned in 1962 but working prototypes of paging had been developed by 1959. In 1961, the Burroughs Corporation independently released the first commercial computer with virtual memory, the B5000, with segmentation rather than paging.

IBM developed the concept of hypervisors in their CP-40 and CP-67, and in 1972 provided it for the S/370 as Virtual Machine Facility/370. IBM introduced the Start Interpretive Execution (SIE) instruction as part of 370-XA on the 3081, and VM/XA versions of VM to exploit it.

Before virtual memory could be implemented in mainstream operating systems, many problems had to be addressed. Dynamic address translation required expensive and difficult-to-build specialized hardware; initial implementations slowed down access to memory slightly. There were worries that new system-wide algorithms utilizing secondary storage would be less effective than previously used application-specific algorithms. By 1969, the debate over virtual memory for commercial computers was over; an IBM research team led by David Sayre showed that their virtual memory overlay system consistently worked better than the best manually controlled systems. Throughout the 1970s, the IBM 370 series running their virtual-storage based operating systems provided a means for business users to migrate multiple older systems into fewer, more powerful, mainframes that had improved price/performance. The first minicomputer to introduce virtual memory was the Norwegian NORD-1; during the 1970s, other minicomputers implemented virtual memory, notably VAX models running VMS.

Virtual memory was introduced to the x86 architecture with the protected mode of the Intel 80286 processor, but its segment swapping technique scaled poorly to larger segment sizes. The Intel 80386 introduced paging support underneath the existing segmentation layer, enabling the page fault exception to chain with other exceptions without double fault. However, loading segment descriptors was an expensive operation, causing operating system designers to rely strictly on paging rather than a combination of paging and segmentation.

Paged virtual memory

See also: Memory paging

Nearly all current implementations of virtual memory divide a virtual address space into pages, blocks of contiguous virtual memory addresses. Pages on contemporary systems are usually at least 4 kilobytes in size; systems with large virtual address ranges or amounts of real memory generally use larger page sizes.

Page tables

Page tables are used to translate the virtual addresses seen by the application into physical addresses used by the hardware to process instructions; such hardware that handles this specific translation is often known as the memory management unit. Each entry in the page table holds a flag indicating whether the corresponding page is in real memory or not. If it is in real memory, the page table entry will contain the real memory address at which the page is stored. When a reference is made to a page by the hardware, if the page table entry for the page indicates that it is not currently in real memory, the hardware raises a page fault exception, invoking the paging supervisor component of the operating system.

Systems can have, e.g., one page table for the whole system, separate page tables for each address space or process, separate page tables for each segment; similarly, systems can have, e.g., no segment table, one segment table for the whole system, separate segment tables for each address space or process, separate segment tables for each region in a tree of region tables for each address space or process. If there is only one page table, different applications running at the same time use different parts of a single range of virtual addresses. If there are multiple page or segment tables, there are multiple virtual address spaces and concurrent applications with separate page tables redirect to different real addresses.

Some earlier systems with smaller real memory sizes, such as the SDS 940, used page registers instead of page tables in memory for address translation.

Paging supervisor

This part of the operating system creates and manages page tables and lists of free page frames. In order to ensure that there will be enough free page frames to quickly resolve page faults, the system may periodically steal allocated page frames, using a page replacement algorithm, e.g., a Least recently used (LRU) algorithm. Stolen page frames that have been modified are written back to auxiliary storage before they are added to the free queue. On some systems the paging supervisor is also responsible for managing translation registers that are not automatically loaded from page tables.

Typically, a page fault that cannot be resolved results in an abnormal termination of the application. However, some systems allow the application to have exception handlers for such errors. The paging supervisor may handle a page fault exception in several different ways, depending on the details:

• If the virtual address is invalid, the paging supervisor treats it as an error.
• If the page is valid and the page information is not loaded into the MMU, the page information will be stored into one of the page registers.
• If the page is uninitialized, a new page frame may be assigned and cleared.
• If there is a stolen page frame containing the desired page, that page frame will be reused.
• For a fault due to a write attempt into a read-protected page, if it is a copy-on-write page then a free page frame will be assigned and the contents of the old page copied; otherwise it is treated as an error.
• If the virtual address is a valid page in a memory-mapped file or a paging file, a free page frame will be assigned and the page read in.

In most cases, there will be an update to the page table, possibly followed by purging the Translation Lookaside Buffer (TLB), and the system restarts the instruction that causes the exception.

If the free page frame queue is empty then the paging supervisor must free a page frame using the same page replacement algorithm for page stealing.

Pinned pages

Operating systems have memory areas that are pinned (never swapped to secondary storage). Other terms used are locked, fixed, or wired pages. For example, interrupt mechanisms rely on an array of pointers to their handlers, such as I/O completion and page fault. If the pages containing these pointers or the code that they invoke were pageable, interrupt-handling would become far more complex and time-consuming, particularly in the case of page fault interruptions. Hence, some part of the page table structures is not pageable.

Some pages may be pinned for short periods of time, others may be pinned for long periods of time, and still others may need to be permanently pinned. For example:

• The paging supervisor code and drivers for secondary storage devices on which pages reside must be permanently pinned, as otherwise paging wouldn't even work because the necessary code wouldn't be available.
• Timing-dependent components may be pinned to avoid variable paging delays.
• Data buffers that are accessed directly by peripheral devices that use direct memory access or I/O channels must reside in pinned pages while the I/O operation is in progress because such devices and the buses to which they are attached expect to find data buffers located at physical memory addresses; regardless of whether the bus has a memory management unit for I/O, transfers cannot be stopped if a page fault occurs and then restarted when the page fault has been processed. For example, the data could come from a measurement sensor unit and lost real time data that got lost because of a page fault can't be recovered.

In IBM's operating systems for System/370 and successor systems, the term is "fixed", and such pages may be long-term fixed, or may be short-term fixed, or may be unfixed (i.e., pageable). System control structures are often long-term fixed (measured in wall-clock time, i.e., time measured in seconds, rather than time measured in fractions of one second) whereas I/O buffers are usually short-term fixed (usually measured in significantly less than wall-clock time, possibly for tens of milliseconds). Indeed, the OS has a special facility for "fast fixing" these short-term fixed data buffers (fixing which is performed without resorting to a time-consuming Supervisor Call instruction).

Multics used the term "wired". OpenVMS and Windows refer to pages temporarily made nonpageable (as for I/O buffers) as "locked", and simply "nonpageable" for those that are never pageable. The Single UNIX Specification also uses the term "locked" in the specification for mlock(), as do the mlock() man pages on many Unix-like systems.

Virtual-real operation

In OS/VS1 and similar OSes, some parts of systems memory are managed in "virtual-real" mode, called "V=R". In this mode every virtual address corresponds to the same real address. This mode is used for interrupt mechanisms, for the paging supervisor and page tables in older systems, and for application programs using non-standard I/O management. For example, IBM's z/OS has 3 modes (virtual-virtual, virtual-real and virtual-fixed).

 Thrashing

When paging and page stealing are used, a problem called "thrashing" can occur, in which the computer spends an unsuitably large amount of time transferring pages to and from a backing store, hence slowing down useful work. A task's working set is the minimum set of pages that should be in memory in order for it to make useful progress. Thrashing occurs when there is insufficient memory available to store the working sets of all active programs. Adding real memory is the simplest response, but improving application design, scheduling, and memory usage can help. Another solution is to reduce the number of active tasks on the system. This reduces demand on real memory by swapping out the entire working set of one or more processes.

A system thrashing is often a result of a sudden spike in page demand from a small number of running programs. Swap-token is a lightweight and dynamic thrashing protection mechanism. The basic idea is to set a token in the system, which is randomly given to a process that has page faults when thrashing happens. The process that has the token is given a privilege to allocate more physical memory pages to build its working set, which is expected to quickly finish its execution and to release the memory pages to other processes. A time stamp is used to handover the token one by one. The first version of swap-token was implemented in Linux 2.6. The second version is called preempt swap-token and is also in Linux 2.6. In this updated swap-token implementation, a priority counter is set for each process to track the number of swap-out pages. The token is always given to the process with a high priority, which has a high number of swap-out pages. The length of the time stamp is not a constant but is determined by the priority: the higher the number of swap-out pages of a process, the longer the time stamp for it will be.

Segmented virtual memory

Some systems, such as the Burroughs B5500, use segmentation instead of paging, dividing virtual address spaces into variable-length segments. A virtual address here consists of a segment number and an offset within the segment. The Intel 80286 supports a similar segmentation scheme as an option, but it is rarely used. Segmentation and paging can be used together by dividing each segment into pages; systems with this memory structure, such as Multics and IBM System/38, are usually paging-predominant, segmentation providing memory protection.

In the Intel 80386 and later IA-32 processors, the segments reside in a 32-bit linear, paged address space. Segments can be moved in and out of that space; pages there can "page" in and out of main memory, providing two levels of virtual memory; few if any operating systems do so, instead using only paging. Early non-hardware-assisted x86 virtualization solutions combined paging and segmentation because x86 paging offers only two protection domains whereas a VMM, guest OS or guest application stack needs three. The difference between paging and segmentation systems is not only about memory division; segmentation is visible to user processes, as part of memory model semantics. Hence, instead of memory that looks like a single large space, it is structured into multiple spaces.

This difference has important consequences; a segment is not a page with variable length or a simple way to lengthen the address space. Segmentation that can provide a single-level memory model in which there is no differentiation between process memory and file system consists of only a list of segments (files) mapped into the process's potential address space.

This is not the same as the mechanisms provided by calls such as mmap and Win32's MapViewOfFile, because inter-file pointers do not work when mapping files into semi-arbitrary places. In Multics, a file (or a segment from a multi-segment file) is mapped into a segment in the address space, so files are always mapped at a segment boundary. A file's linkage section can contain pointers for which an attempt to load the pointer into a register or make an indirect reference through it causes a trap. The unresolved pointer contains an indication of the name of the segment to which the pointer refers and an offset within the segment; the handler for the trap maps the segment into the address space, puts the segment number into the pointer, changes the tag field in the pointer so that it no longer causes a trap, and returns to the code where the trap occurred, re-executing the instruction that caused the trap. This eliminates the need for a linker completely and works when different processes map the same file into different places in their private address spaces.

Address space swapping

Some operating systems provide for swapping entire address spaces, in addition to whatever facilities they have for paging and segmentation. When this occurs, the OS writes those pages and segments currently in real memory to swap files. In a swap-in, the OS reads back the data from the swap files but does not automatically read back pages that had been paged out at the time of the swap out operation.

IBM's MVS, from OS/VS2 Release 2 through z/OS, provides for marking an address space as unswappable; doing so does not pin any pages in the address space. This can be done for the duration of a job by entering the name of an eligible main program in the Program Properties Table with an unswappable flag. In addition, privileged code can temporarily make an address space unswappable using a SYSEVENT Supervisor Call instruction (SVC); certain changes in the address space properties require that the OS swap it out and then swap it back in, using SYSEVENT TRANSWAP.

Swapping does not necessarily require memory management hardware, if, for example, multiple jobs are swapped in and out of the same area of storage.