547 Matching Annotations
  1. Jan 2020
    1. The white allotrope can be produced using several different methods. In the industrial process, phosphate rock is heated in an electric or fuel-fired furnace

      This shows me how Phosphorus is refined and specifically how white Phosphorus is made and heated in the process.

    1. These are mined in Brazil, Canada, Finland, Russia, South Africa and Zimbabwe. Phosphate rock is mined mostly by surface methods using draglines and bucket wheel excavators for large deposits and power shovels or earthmovers for smaller deposits.

      This clip is about where Phosphorus is mined and how it is mined. Phosphorus is mined in many different places around the world.

    1. Alloys[edit] Antimony forms a highly useful alloy with lead, increasing its hardness and mechanical strength. For most applications involving lead, varying amounts of antimony are used as alloying metal. In lead–acid batteries, this addition improves plate strength and charging characteristics.[49][59] For sailboats, lead keels are used as counterweights, ranging from 600 lbs to over 8000 lbs; to improve hardness and tensile strength of the lead keel, antimony is mixed with lead between 2% and 5% by volume. Antimony is used in antifriction alloys (such as Babbitt metal),[60] in bullets and lead shot, electrical cable sheathing, type metal (for example, for linotype printing machines[61]), solder (some "lead-free" solders contain 5% Sb),[62] in pewter,[63] and in hardening alloys with low tin content in the manufacturing of organ pipes. Other applications[edit] Three other applications consume nearly all the rest of the world's supply.[48] One application is as a stabilizer and catalyst for the production of polyethylene terephthalate.[48] Another is as a fining agent to remove microscopic bubbles in glass, mostly for TV screens;[64] antimony ions interact with oxygen, suppressing the tendency of the latter to form bubbles.[65] The third application is pigments.[48] Antimony is increasingly being used in semiconductors as a dopant in n-type silicon wafers[66] for diodes, infrared detectors, and Hall-effect devices. In the 1950s, the emitters and collectors of n-p-n alloy junction transistors were doped with tiny beads of a lead-antimony alloy.[67] Indium antimonide is used as a material for mid-infrared detectors.[68][69][70]

      Potential uses in phone batteries and silicon wafers As semi conductors.

    1. University of Pennsylvania has designed a new, cost-effective approach to recycle Yttrium.

      can be recycled.

    1. this will eventually lead to increasing concentrations in humans, animals and soil particles.

      blood clots could be made in a human or animal.

    1. Loss of landscape/aesthetic degradation, Soil contamination, Soil erosion, Deforestation and loss of vegetation cover, Surface water pollution / Decreasing water (physico-chemical, biological) quality, Mine tailing spills

      these are the problems that the community and environment are suffering because of the mines.

    1. Lead isotopes are the end products of each of the three series of naturally occurring radioactive elements.

      lead is radioactive, so when you touch it, it can effect your body.

    1. Keep work areas clean. Do not use compressed air to remove lead dust.

      when you use compressed air to remove lead dust, can blow the lead into the air, causing people to breath it in.

    1. Most of the antimony mined each year comes from China, which supplies over three-quarters of the world’s total. The remainder is from Russia, South Africa, Tajikistan, Bolivia, and a few other countries, including the United States. Some antimony is produced as a by-product of smelting ores of other metals, mainly gold, copper and silver, in countries such as the United States, Canada, and Australia.

      Top antimony producing countries

    1. Antimony is rarely found in its native (as an element) state. Instead, it usually occurs as a compound. The most common minerals of antimony are stibnite, tetrahedrite, bournonite, boulangerite, and jamesonite.

      How antimony is found in nature

    2. Antimony is a moderately active element. It does not combine with oxygen in the air at room temperature. It also does not react with cold water or with most cold acids. It does dissolve in some hot acids, however, and in aqua regia. Aqua regia is a mixture of hydrochloric and nitric acids. It often reacts with materials that do not react with either acid separately.

      Chemical properties of antimony

    3. Antimony is a silvery-white, shiny element that looks like a metal. It has a scaly surface and is hard and brittle like a non-metal. It can also be prepared as a black powder with a shiny brilliance to it. The melting point of antimony is 630°C (1,170°F) and its boiling point is 1,635°C (2,980°F). It is a relatively soft material that can be scratched by glass. Its density is 6.68 grams per cubic centimeter.

      Physical properties of antimony

    1. Occupational exposure may cause respiratory irritation, pneumoconiosis, antimony spots on the skin and gastrointestinal symptoms. In addition antimony trioxide is possibly carcinogenic to humans.

      Negative health effects of repeated exposure to antimony

    1. Antimony is used in the electronics industry to make some semiconductor devices, such as infrared detectors and diodes. It is alloyed with lead or other metals to improve their hardness and strength. A lead-antimony alloy is used in batteries. Other uses of antimony alloys include type metal (in printing presses), bullets and cable sheathing. Antimony compounds are used to make flame-retardant materials, paints, enamels, glass and pottery.

      Uses of antimony in technology

    1. large amount of waste rock, tailings, and smelting slags are produced during the Sb mining, processing, and smelting. These are partly used for filling the underground voids and piled up in the waste slack yard. The tailings produced in mineral processing are piped to the tailings dam located in the southwest of the mining area, which is located in the natural depression between two mountain bodies in the southwest of the mining area. Downstream is a large area of farmland, at risk from contamination from tailings release.

      What happens to waste produced during Sb refining.

    1. Europium has no known biological role. Europium salts could be mildly toxic by ingesiton, but its toxicity has not been fully investigated.

      Health and Community Problems

    2. Europium poses no environmental threath to plants or animals. The metal dust presents a fire and explosion hazard.

      Environmental Problems

    1. Europium has a strong tendency to absorb neutrons, making it useful in nuclear power production. A nuclear power plant produces electricity from the energy released by nuclear fission. Slow-moving neutrons collide with uranium or Plutonium atoms, breaking them apart and releasing energy as heat. The amount of energy produced in a nuclear power plant is controlled by the number of neutrons present. Europium is used to absorb neutrons in this kind of control system. Chemical properties Europium is the most active of the lanthanides. It reacts quickly with water to give off hydrogen. It also reacts strongly with oxygen in the air, catching fire spontaneously. Scientists must use great care in handling the metal.

      Properties

    2. Europium is prepared by heating its oxide with lanthanum metal: Europium metal is quite expensive to make and sells for about $3,000 to $4,500 a kilogram ($1,350 to $2,000 a pound).

      Refining

    1. europium are crucial to your iPhone’s function – they’re used in the phone’s battery, as well as to help give the display screen colour and make the phone vibrate when you get a text,

      Needed for normal function in Iphone.

    1. a relatively poor conductor of electricity

      lead is a poor electricity conductor but good and used in phone batteries.

    1. A 2009 study published in PLOS One concluded that the global warming potential of mining and processing nickel was the eighth highest of 63 metals over the previous year.

      The mining and processing of nickel has one of the biggest effects on global warming out of 63 metals.

    1. The pond, owned by the Inner Mongolia Baotou Steel Rare-Earth Hi-Tech Company, or Baotou Steel, lacks a proper lining and for the past 20 years its toxic contents have been seeping into groundwater, according to villagers and state media reports.

      Lanthanum Toxins have been leaking into the ground water which people use for tap water.

    1. Damage to water sources that residents attribute to mining, as well as increased demand due to population migration to mining sites, reduces communities’ access to water for drinking, washing and cooking. Women, who are primarily responsible for fetching water, are forced to walk longer distances or wait for long periods to obtain water from alternative sources. The dust produced by bauxite mining and transport smothers fields and enters homes, leaving families and health workers worried that reduced air quality threatens their health and environment.

      Mining is contaminating our water sources and water is one of the most important things for survival.

    1. How much money does this machinery cost i know it cant be cheap with how huge these things are

    2. Mining can include extraction of metals and minerals, like coal, diamond, gold, silver, platinum, copper, tin and iron. Mining can also include other things like oil and natural gas.

      How do you mine gas?

    1. Environmental impacts of mining can occur at local, regional, and global scales through direct and indirect mining practices.
    1. Tin mining on and offshore off Bangka island has a devastating effect on its environment. 

      Good information about environmental issues

  2. Jul 2019
  3. Jun 2019
    1. capital

      is a sum of money provided to a company to further its business objectives. The term also can refer to a company's acquisition of long-term assets such as real estate, manufacturing plants, and machinery.

    1. Computer Revolution

      Computer Revolution also called "Digital Revolution" refers to where the technology is very popular and its used by people around the world.

  4. May 2019
    1. Cities such as Lubumbashi and Likasi face serious air pollution because of the thousands of trucks that travel to and from the mines throughout the day. Chronic exposure to such dusts can lead to severe lung diseases. Water is unfit for human consumption and agriculture because soil and water in the immediate vicinity of the mines are polluted by discharges of wastewater.
    1. Tin is extracted by roasting the mineral casseterite with carbon in a furnace to approximately 2500 degrees Fahrenheit. The next step involves leaching with acid or water solutions to remove impurities. Electrostatic or magnetic separation helps to remove any heavy metal impurities. Compared to the processes used in the past, today’s advanced technology allows the industry to produce 10 times more tin. General Kinematics’ vibratory equipment innovation has made them a leader in the production of mining equipment for more than 45 years. A great many of the challenges faced by miners can be solved with the use of top quality vibratory equipment. With a proven track record in 35 countries, General Kinematics is making their mark on the future of tin mining.

      tin refining

    1. Lithium-ion batteries are key components for most consumer electronics, from cellphones and laptops to electric cars.

      the ions in the lithium is the competent that is found in phones.

    1. The governments of Ethiopia and Russia are set to expand business ties on multiple frontiers, ranging from nuclear energy development to Russians investment in mining in Ethiopia.

      That's interesting!

    1. the government reported the discovery of large gold deposits in Lege Dimbi, also in Sidamo.

      This shows that there was gold in Ethiopia!

    1. Due to its electrical conductivity, copper is used in electronics, cars, and wires. This makes copper critical for highly developed countries.

      true stuuff

    2. Developing economies’ copper demand has steadily grown over the last decades, fueling economic and social improvement. By 2011, China already represented 40% of the demand.

      Why does China need so much.

    3. Codelco is a state-owned Chilean mining company and the world’s largest copper producer. Based on their annual report and USGS statistics, they produced ~10% of the world’s copper in 2015 and own 8% of global reserves. They are also a large producer of greenhouse gas emissions. Last year, Codelco produced 3,2 t CO2e/millions tmf from both indirect and direct effects, and in 2011 it consumed 12% of the total national electricity supply.

      Goddamn they should start recylcling

    4. Copper is a key driver of growth and economic wealth for Chile.
    5. What can Codelco do, then? In addition to cost cutting, Codelco could consider entering the recycling copper industry, as energy prices are driving “virgin” copper costs up.

      thats good

    6. copper mining significantly contributes to climate change.
    1. Shaida’s mineralisation has been described as a porphyry copper deposit.
    2. The Shaida Copper Mine is situated in the western region of Afghanistan in the Adraskan District of the Herat province, 65 km South-West of the city of Herat and 50 km SSW of Gozareh

      Where it is located.

    1. Problems stemming from artisanal mining include disruption of families, mining-related illnesses, environmental damage, child labor, prostitution and rape.

      problems

    1. Neodymium is chiefly obtained from monazite and bastnasite, where it occurs as an impurity. It is mined in the USA, China, Russia, Australia, and India.

      some info about the ore and where it is mined

    1. The Neodymium metal element is initially separated from refined Rare Earth oxides in an electrolytic furnace. The "Rare Earth" elements are lanthanoids (also called lanthanides) and the term arises from the uncommon oxide minerals used to isolate the elements

      how neodymium is refined

    1. It is a preferred component in lithium-ion batteries that power laptops, cell phones, and electric vehicles.

      what it's used for

    2. The country’s economy is completely dependent on mining. Many poor families are completely dependent on their children working the mines. That $9/day is hard for a child to reject

      its weird how people are able to exploit these people legally, and if not legally than how have people not made an effort to stop them?

    3. Walt describes how the multibillion-dollar industry, that has made some people outside Africa really really rich, is not known to workers like Lukasa. He just sells his haul to Chinese traders who have seen their profits increase 400% over the last two years.

      I wonder if the people at the top of the company even know that this is going on with there "employees"

    4. Especially one child named Lukasa who gets up at 5 AM to work a 12-hour day for less than $9

      how is this allowed

    1. The Indonesian islands of Bangka and Belitung produce over 90 percent of the country's tin. President Joko Widodo has vowed to end illegal mining operations that put the lives of men and children in danger.

      That's a lot of tin! Glad the president is putting some safer laws regarding tin!

    1. 1China110,0002Indonesia95,2003Peru23,6684Bolivia19,3005Brazil12,0006Myanmar11,0007Australia6,4748Viet Nam5,4009Malaysia3,70010Congo, The Democratic Republic Of The3,00011Rwanda1,90012Lao People's Democratic Republic80013Nigeria57014Russian Federation42015Thailand20016Uganda4017Portugal4018Burundi20 Source: United States Geological Survey (USGS) Minerals Resources Program

      Tin production

    1. The metal is a solid material and metallic gray in color. Furthermore, the metal possess high melting and boiling points. At standard temperature and pressure, cobalt is not readily oxidized, which means it does not easily lose electrons from its surface.

      A description of cobalt.

    2. cobalt can be found among the d-block elements, transition metals. It has the chemical abbreviation Co and atomic number 27.

      Where to find cobalt on periodic table and cobalt's atomic number.

    1. Monseigneur de Hemptinne watched Yeke people working at Dikuluwe as late as 1924. They worked in the dry season and stopped when the first rains arrived. The mining camp was near a stream where millet could be planted. Women and children collected malachite from the surface, while men used iron picks to excavate pits and shafts, using fire to crack the rocks when needed. The mines were between 10 metres (33 ft) and 15 metres (49 ft) deep with galleries up to 20 metres (66 ft) long. The ore would be sorted and then taken to a nearby stream for concentration before being smelted

      info about how mines work

    2. The Katanga, or Shaba, copperbelt in the DRC is a belt about 70 kilometres (43 mi) wide and 250 kilometres (160 mi) long

      how large the coperbelt is

    1. his is the second round of mineral licensing conducted by the Afghan government, which in 2008 gave Metallurgical Corp of China rights to the Aynak copper deposit

      They seem to offer up mining contracts very easily.

    1. This cobalt is found in every lithium-ion rechargeable battery on the planet

      I wonder if there is a replacement for the cobalt

    2. Arthur. The 16-year-old boy joined a group of young men who spent two months digging a tunnel 26m straight down before they hit a heterogenite vein. Now they descend into darkness each day, spending up to 24 hours at a time in narrow tunnels unable to stand, hacking away for cobalt. Every minute is suffused with dread, because many tunnels have collapsed in Kasulo, burying alive everyone inside.
    3. Elodie is 15. Her two-month-old son is wrapped tightly in a frayed cloth around her back. He inhales potentially lethal mineral dust every time he takes a breath. Toxicity assaults at every turn; earth and water are contaminated with industrial runoff, and the air is brown with noxious haze. Elodie is on her own here, orphaned by cobalt mines that took both her parents. She spends the entire day bent over, digging with a small shovel to gather enough cobalt-containing heterogenite stone to rinse at nearby Lake Malo to fill one sack. It will take her an entire day to do so, after which Chinese traders will pay her about $0.65 (50p). Hopeless though it may be, it is her and her child’s only means of survival.
    1. Xiamen Tungsten and Xiamen Sanhong Molybdenum would invest more than 2 billion yuan (US$314.13 million) in the surveying and subsequent exploitation of the tungsten ore mine.
    2. The proved reserve of the mine has exceeded Jiangxi's total amount of available tungsten reserves, and has a potential economic value of more than 300 billion yuan (US$47.13 billion).
    3. The mine is located in Dahutang Area, Wuning County, Jiujiang City
  5. Feb 2019
    1. mas de reconhecer que, sem os votos desse contingente de remediados de ascensão recente (talvez não fosse exagero qualificá-los como semipobres), não haveria extrema direita no poder.

      Isso aqui é interessantíssimo, inclusive para estudos futuros: se uma classe "sobe" no estrato social, como surgiu a Classe C aqui, qual a tendência de voto dela? Teria o PT dado o próprio tiro no pé? Há algo que fazer pra reverter esse quadro?

  6. Nov 2018
    1. Metop-C Satellite :The Metop-C satellite, launched from the European Space Port in Kourou, French Guiana, is the third and last satellite of the Metop series, which forms the space segment of the EUMETSAT Polar system.
  7. Oct 2018
  8. Jul 2018
  9. Jun 2018
    1. The idea of the numeric_limits<T> trait class is due to John Barton and Lee Nackman

      numeric_limits<T>

  10. May 2018
    1. One group received citalopram (up to 60 mg/day) plus vitamin C (up to 1,000 mg/day). The other group received citalopram plus placebo.

      Considering the results of other trials with different depression severities and vitamin C dosage and forms (sustained release) (covered in this article), it is astonishing that the researchers used such a single, modest, non-sustained release dose.

      The plasma peak of vitamin C is obtained a few hours after the administration. For the rest of the day, the subject is back to baseline.

      This is common knowledge.

  11. Apr 2018
    1. Another way to look at it: The .NET Framework has essentially two forks. One fork is provided by Microsoft and is Windows only. The other fork is Mono which you can use on Linux and Mac. With .NET Core we’re able to develop an entire .NET stack as a full open source project. Thus, having to maintain separate forks will no longer be necessary: together with the Mono community we’ll make .NET Core great for Windows, Linux and Mac OSX. This also enables the Mono community to innovate on top of the leaner .NET Core stack as well as taking it to environments that Microsoft isn’t interested in.

      .NET core开源模式开发,适配三个平台 最终会取代.NET framework和mono

  12. Mar 2018
    1. Todd Veldhuizen - Techniques for Scientific C++ (RT542.pdf)

    2. Scienti candEngineeringC++

      Barton and Nackman, Scientific and Engineering C++

    1. Todd Veldhuizen, the co-discoverer of expres-sion templates (CIP 10:6, 1996 p. 552; CJP I 1 :3, 1997, p. 263), i now a graduate student at the Univer-sity of Waterloo.

      Todd Veldhuizen 创建Blitz++ 时是滑铁卢大学的一名研究生

    1. While that allows you to use the string class, the relevant operator<< is defined in the <string> header itself, so you must include that manually

      为什么有时候使用visual c++会提示 cout << 不能输出 string 类型: 因为 <iostream> 包含了头文件 <xstring>, 该头文件引入了 string 类型但是未引入 operator<<

  13. Feb 2018
    1. There are several authors of this article, like Stephen A. Flores who graduated from University of Texas medical school with honors

      "like"? Are all of these researchers from UT?

    1. There are, however, certain features of the American penal system that give the topic of disenfranchisement a pressing moral sa
  14. Jan 2018
  15. Nov 2017
    1. What we find is that think of it this way. Let’s say, the reference range of vitamin C in the bloodstream is one milligram per deciliter. After a 15,000, a 15-gram IBC infusion and we typically, do measure people after we give them an infusion, it’s about 100 milligrams per deciliter. It’s 100 times what you would normally have in your blood. Now, you can take as much oral vitamin C as you can and you might get it up from one to maybe three, maybe four, maybe five if you’ve already been taking vitamin C for a long time but to get beyond that is almost impossible.

      How much vitamin C

    2. Now, there are cases where people have taken, they’ve built themselves up to 50, 60,000 milligrams of oral vitamin C a day over months and years and they can achieve a higher blood level in the 20s, maybe the 30s. What we’re shooting for with cancer patients is for 400 milligrams per deciliter.

      How much vitamin C

    3. I think the concept of antioxidants has been misunderstood. What Dr. Tom Levy and I have introduced at our most recent conference is this concept of redox medicine that everything in nature is redox. Dave Asprey:                     Yes. Explain redox for our listeners. This is also in headstrong. Just like define that. It’s so important. You guys will have to hear this. Ron Hunninghake:           Yeah. Well, life is redox. There’s oxidation, there is reduction. Oxidation is when molecules lose electrons and it causes dysfunction of some sort. Reduction is when from some source, such as vitamin C or good quality food, you are able to get electrons back in order to stabilize the molecule and to stabilize the structures that it’s working within.

      Redox

    4. There are some other doctors that have looked at depriving cancer patients of all vitamin C for periods of time and then hitting them really hard with high doses of vitamin C as a possible way of inducing apoptosis in cancer cells. There’s a lot of research in this area and it’s not like it’s all worked out now. It’s more like we are in the early stages of flight and we need research is what we really need and unfortunately, it’s hard to come by at the funding for this particular area.

      Vitamin C Cancer

  16. Sep 2017
    1. Although children may be at the one-word stage in their productive language, research suggests that they are perceiving and processing language in five-to six-word segments.
  17. languagedev.wikispaces.com languagedev.wikispaces.com
    1. Children acquiring two languages prior lo age 3 is termed simultaneous bilin-gualism
    1. Plot a course through the genome Inspired by Google Maps, a suite of tools is allowing researchers to chart the complex conformations of chromosomes.

      mentioned tools are focused on (capture) Hi-C data

  18. Aug 2017
    1. Indy flies to Nepal (followed by a Nazi agent, Toht (Ronald Lacey)) to confront Marion Ravenwood, who runs a restaurant and bar (and who can outdrink anyone) because he needs the headpiece to the Staff of Ra, whose crystals will allow him to determine the exact location of the Ark. Marion, still bitter over their breakup, nonetheless accepts when Indy offers her $3,000 and the promise of more when they return stateside. She is cryptic about the headpiece, and after Indy leaves she ponders it as she wears it around her neck.

      C

    2. Toht and several Sherpa heavies enter the bar and hold Marion hostage, with Toht ready to torture her for the headpiece. Indy returns and a firefight erupts during which the fireplace is dislodged and the building begins burning down. Toht finds the headpiece but when he grabs it he's badly burned -- leaving an image of one side of the headpiece branded on his hand. He escapes while Indy and Marion do likewise and fly to Egypt to see Indy's pal, Sallah (John Rhys-Davies), who is working on the Nazi site and who reveals that the Nazis are aided by a French archaeologist (Belloq).

      C

  19. May 2017
  20. Mar 2017
    1. Good question. For C++ there are quite a number of established frameworks, including (but not limited to), CppUnit, Google Test, Boost.Test, Aeryn, Cute, Fructose and many, many more. Even for Objective-C there are a few, including OCUnit - which now comes bundled with XCode.
  21. Dec 2016
  22. Nov 2016
    1. TR: How do you account for the fact that C++ is both widely criticized and resented by many programmers but at the same time very broadly used? Why is it so successful?BS: The glib answer is, There are just two kinds of languages: the ones everybody complains about and the ones nobody uses.There are more useful systems developed in languages deemed awful than in languages praised for being beautiful–many more. The purpose of a programming language is to help build good systems, where “good” can be defined in many ways. My brief definition is, correct, maintainable, and adequately fast. Aesthetics matter, but first and foremost a language must be useful; it must allow real-world programmers to express real-world ideas succinctly and affordably.

      La idea, de Stroupstrup en C++, de un lenguaje para escribir sistemas (¿de computo?) constrasta con la de uno que sirva a la expresión creativa del espíritu humano, de Ingalls en Smalltalk. El programador profesional como destinatario del lenguaje en C++ también contrasta con los niños en Smalltalk.

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  23. Oct 2016
    1. Online

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  24. Sep 2016
  25. Jul 2016
    1. ...one of the most highly regarded and expertly designed C++ library projects in the world.

      code lại C++ cho hệ nhúng, OpenCV

  26. Jun 2016
    1. This basic process has implications for rhetorical heuristics: (1) students need to develop their own schemata to fit their particular topics/situations, and (2) if we give them schemata first, their goal should be to revise those schemata as a part of the invention process rather than follow them prescriptively

      Repurposing Taylor's Complexity Theory to Comp.

  27. Apr 2016
    1. JAMES AND FUNCTIONALISM

      Covered in C&W

    2. covered in C&W

    3. HUMANISM

      C&W uses Humanistic Perspective after Psychodynamic and Behavioral

    4. BIOPSYCHOLOGY AND EVOLUTIONARY PSYCHOLOGY

      covered in C&W after sociocultural

    5. MULTICULTURAL PSYCHOLOGY

      !! is this the same as Sociocultural Perspective - the term used in C&W?

    6. WUNDT AND STRUCTURALISM

      covered in C&W text

    1. Remember that the auto keyword does not automatically specify a pointer/reference for you, you need to add that yourself.
  28. Feb 2016
    1. son locos de quienes se fían aquellos sométicos (sodomitas) mujeriles, que no son más que mujeres bardajas de sus hombres barbudos, que se han rendido a ellos de miedo

      ¿Qué quiere decir esta frase?

  29. Jan 2016
    1. A thorough primer on C programming, specifically for the Texas Instruments TM4C123 or TM4C1294 microcontrollers. By Jonathan Valvano and Ramesh Yerraballi of UT Austin.

  30. Sep 2015
    1. information we give off about our selves, in photos and e-mails and MySpace pages and all the rest of it, has dramatically in­creased our social visibility

      No one really realizes how big their digital footprint is and what people are capable of finding through the digital footprint

  31. Jul 2015
  32. Jan 2014
    1. There are three kinds of storage locations: stack locations, heap locations, and registers.
    2. There are three kinds of values: (1) instances of value types, (2) instances of reference types, and (3) references. (Code in C# cannot manipulate instances of reference types directly; it always does so via a reference. In unsafe code, pointer types are treated like value types for the purposes of determining the storage requirements of their values.)
    3. Having made these points many times in the last few years, I've realized that the fundamental problem is in the mistaken belief that the type system has anything whatsoever to do with the storage allocation strategy. It is simply false that the choice of whether to use the stack or the heap has anything fundamentally to do with the type of the thing being stored. The truth is: the choice of allocation mechanism has to do only with the known required lifetime of the storage.

      The type system has nothing to do with the storage allocation strategy; the choice of allocation mechanism has to do only with the known required lifetime of the storage.

    1. A lot of people seem to think that heap allocation is expensive and stack allocation is cheap. They are actually about the same, typically. It’s the deallocation costs – the marking and sweeping and compacting and moving memory from generation to generation – that are massive for heap memory compared to stack memory.
    2. Now compare this to the stack. The stack is like the heap in that it is a big block of memory with a “high water mark”. But what makes it a “stack” is that the memory on the bottom of the stack always lives longer than the memory on the top of the stack; the stack is strictly ordered. The objects that are going to die first are on the top, the objects that are going to die last are on the bottom. And with that guarantee, we know that the stack will never have holes, and therefore will not need compacting. We know that the stack memory will always be “freed” from the top, and therefore do not need a free list. We know that anything low-down on the stack is guaranteed alive, and so we do not need to mark or sweep.
    3. This sketch is complicated by the fact that there are actually three such arenas; the CLR collector is generational. Objects start off in the “short lived” heap. If they survive they eventually move to the “medium lived” heap, and if they survive there long enough, they move to the “long lived” heap. The GC runs very often on the short lived heap and very seldom on the long lived heap; the idea is that we do not want to have the expense of constantly re-checking a long-lived object to see if it is still alive. But we also want short-lived objects to be reclaimed swiftly.
    4. When a garbage collection is performed there are three phases: mark, sweep and compact. In the “mark” phase, we assume that everything in the heap is “dead”. The CLR knows what objects were “guaranteed alive” when the collection started, so those guys are marked as alive. Everything they refer to is marked as alive, and so on, until the transitive closure of live objects are all marked. In the “sweep” phase, all the dead objects are turned into holes. In the “compact” phase, the block is reorganized so that it is one contiguous block of live memory, free of holes.
    5. If we’re in that situation when new memory is allocated then the “high water mark” is bumped up, eating up some of the previously “free” portion of the block. The newly-reserved memory is then usable for the reference type instance that has just been allocated. That is extremely cheap; just a single pointer move, plus zeroing out the newly reserved memory if necessary.
    6. The idea is that there is a large block of memory reserved for instances of reference types. This block of memory can have “holes” – some of the memory is associated with “live” objects, and some of the memory is free for use by newly created objects. Ideally though we want to have all the allocated memory bunched together and a large section of “free” memory at the top.
    1. I regret that the documentation does not focus on what is most relevant; by focusing on a largely irrelevant implementation detail, we enlarge the importance of that implementation detail and obscure the importance of what makes a value type semantically useful. I dearly wish that all those articles explaining what “the stack” is would instead spend time explaining what exactly “copied by value” means and how misunderstanding or misusing “copy by value” can cause bugs.

      Documentation should focus on semantically useful descriptions; another accompanying document (or annotation) can provide relevant implementation details upon request, but that deeper level of detail should be left out by default to avoid enlarging the importance of less relevant things.

    2. I find this characterization of a value type based on its implementation details rather than its observable characteristics to be both confusing and unfortunate
    3. Surely the most relevant fact about value types is not the implementation detail of how they are allocated, but rather the by-design semantic meaning of “value type”, namely that they are always copied “by value”.
    4. I blogged a while back about how “references” are often described as “addresses” when describing the semantics of the C# memory model. Though that’s arguably correct, it’s also arguably an implementation detail rather than an important eternal truth. Another memory-model implementation detail I often see presented as a fact is “value types are allocated on the stack”. I often see it because of course, that’s what our documentation says.