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The rad is a unit of absorbed radiation dose, equal to 1 centigray. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the [malignant mammalian] cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by one gram of matter. It was restated in SI units in 1970 as the dose causing 0.01 joule of energy to be absorbed per kilogram of matter.
The United States Nuclear Regulatory Commission requires the use of the units curie, rad and rem as part of the Code of Federal Regulations 10CFR20.
However, Systeme Internationale has introduced as a rival unit the gray; 1 rad is equal to 10 milligray, and 100 rads are equal to 1 Gy. The continued use of the rad is "strongly discouraged" by the author style guide of the U.S.National Institute of Standards and Technology. Nevertheless, use of the rad remains widespread and is still an industry standard.
To gauge biological effects the dose in rads is multiplied by a 'quality factor' which is dependent on the type of ionizing radiation. The modified dose is now measured in rems (roentgen equivalent mammal, or man). 100 rem = 1 sievert (Sv). A dose of under 100 rems is subclinical and will produce nothing other than blood changes. 100 to 200 rems will cause illness but will rarely be fatal. Doses of 200 to 1000 rems will likely cause serious illness with poor outlook at the upper end of the range. Doses of more than 1000 rems are almost invariably fatal.
A control unit in general is a central (or sometimes distributed but clearly distinguishable) part of the machinery that controls its operation, provided that a piece of machinery is complex and organized enough to contain any such unit. One domain in which the term is specifically used is the area of computer design. In the automotive industry, the control unit helps maintain various functions of the motor vehicle.
The rest of this article describes control unit in terms of computer design. There is no further article on other uses under this lemma as yet. (Disambiguation and/or integration of this article inComputer with respective linkageâ€”and retention/creation of a more broad-sense articleâ€”may be appropriate.)
Control Unit Co-ordinates the input and output devices of a computer system. It fetches the code of all of the instructions in the microprograms.
Application in Computer Design
In computers, the control unit was historically defined as one distinct part of the 1946 reference model of Von Neumann architecture. In modern computer designs, the control unit is typically an internal part of the CPU with its overall role and operation unchanged.
The outputs of the control unit control the activity of the rest of the device. A control unit can be thought of as a finite state machine.
The control unit is the circuitry that controls the flow of data through the processor, and coordinates the activities of the other units within it. In a way, it is the "brain within the brain", as it controls what happens inside the processor, which in turn controls the rest of the PC.
A few examples of devices that require a control unit are CPUs and GPUs. The modern information age would not be possible without complex control unit designs.
At one time, control units for CPUs were ad-hoc logic, and they were difficult to design. These can be identified as the main part of the computer and the main device that helps the computer to function in an appropriate manner. It is constructed of logic gates, flip-flops, encoder circuits, decoder circuits, digital counters and other digital circuits. Their control is based on fixed architecture i.e. it requires changes in the wiring if the instruction set is modified or changed. This architecture is preferred in RISC computers as it consists of a lesser instruction set.
Hardwired control units are implemented through use of sequential logic units, featuring a finite number of gates that can act as a generator of specific results, based on the instructions that were used to invoke those responses. These instructions are apparent in the design of the architecture, but can also be represented in other ways.
Microprogram Control Unit
The idea of microprogramming was introduced by M. V. Wilkes in 1951 as an intermediate level to execute computer program instructions (see also: microcode). Microprograms were organized as a sequence of microinstructions and stored in special control memory. The algorithm for the microprogram control unit is usually specified by flow-chart description. The main advantage of the microprogram control unit is the simplicity of its structure. Outputs of the controller are organized in microinstructions and they can be easily replaced.
Functions of the Control Unit
The functions performed by the control unit vary greatly by the internal architecture of the CPU, since the control unit really implements this architecture. On a regular processor that executes x86 instructions natively the control unit performs the tasks of fetching, decoding, managing execution and then storing results. On a x86 processor with a RISC core, the control unit has significantly more work to do. It manages the translation of x86 instructions to RISC micro-instructions, manages scheduling the micro-instructions between the various execution units, and juggles the output from these units to make sure they end up where they are supposed to go. On one of these processors the control unit may be broken into other units (such as a scheduling unit to handle scheduling and a retirement unit to deal with results coming from the pipeline) due to the complexity of the job it must perform.
Units of measurement for time have historically been based on the movement of the Sun (as seen from Earth; giving the solar day and the year) and the Moon (giving the month). Shorter intervals were measured by physiological periods such as drawing breath, winking or the heartbeat.
Units of time consisting of a number of years include the lustrum (five years) and the olympiad (four years). The month could be divided into half-months or fortnights, and quarters or weeks. Longer periods were given in lifetimes or generations (saecula, aion), subdivisions of the solar day in hours. The Sothic cycle was a period of 1,461 years of 365 days in the Ancient Egyptian calendar. Medieval (Pauranic) Hindu cosmologyÂ is notorious for introducing names for fabulously long time periods, such as kalpaÂ (4.32 billion years).
In classical antiquity, the hour divided the daylight period into 12 equal parts. The duration of an hour thus varied over the course of the year. In classical China, the kÃ¨(åˆ») was a unit ofdecimal time, dividing a day into 100 equal intervals of 14.4 minutes. Alongside the ke, there were double hours (shÃchen) also known as watches. Because one cannot divide 12 double hours into 100 ke evenly, each ke was subdivided into 60 fÄ“n (åˆ†).
The introduction of the minute (minuta; â€²) as the 60th part of an hour, the second (seccunda; â€²â€²) as the 60th part of a minute, and the third (tertia; â€²â€²â€²) as the 60th part of the second dates to the medieval period, used by Al-Biruni around AD 1000, and by Roger Bacon in the 13th century. Bacon further subdivided the tertia into a quarta or fourth (â€²â€²â€²â€²). Hindu chronologyÂ divides the civil day (daylight hours) into vipalas, palas and ghatikas. A tithiis the 30th part of thesynodic month.
The introduction of the division of the solar day into 24 hours of equal length, as it were the length of a classical hour at equinox used regardless of daylight hours, dates to the 14th century, due to the development of the first mechanical clocks.
Today, the fundamental unit of time suggested by the International System of Units is the second, since 1967 defined as the second of International Atomic Time, based on the radiation emitted by a Caesium-133 atom in the ground state. Its definition is still so calibrated that 86,400 seconds correspond to a solar day. 31,557,600 (86,400 Ã— 365.25) seconds are a Julian year, exceeding the true length of a solar year by about 21 ppm.
Based on the second as the base unit, the following time units are in use:
- minute (1 min = 60 s)
- hour (1 h = 60 min = 3.6 ks)
- Julian day (1 d = 24 h = 86.4 ks)
- week (7 d = 604.8 ks)
- Julian year (1 a = 365.25 d = 31.5576 Ms)
- century (100 a = 3.15576 Gs)
- millennium (1 ka = 31.5576 Gs)
There are a number of proposals for decimal time, or decimal calendars, notably in the French Republican Calendar of 1793. Such systems have either ten days per week, a multiple of ten days in a month, or ten months per year.
A suggestion for hexadecimal time divides the Julian day into 16 hexadecimal hours of 1h 30 min each, or 65,536 hexadecimal seconds (1 hexsec â‰ˆ 1.32 s).
Our modern kilogram has its origins in the pre-French Revolution days of France. Louis XVI created a Consultative Commission for Units to create a new decimal-based system of measurement. This royal commission, which included such aristocrats as Antoine Lavoisier, founded the very beginnings of the â€œmetric systemâ€�, which later evolved into the contemporary International System of Units (SI).
On 7 April 1795, the â€œgrammeâ€�, upon which the kilogram is based, was decreed to be equal to â€œthe absolute weight of a volume of pure water equal to a cube of one hundredth of a metre, and at the temperature of the melting iceâ€�. Although this was the definition of the gram, the regulation of trade and commerce required a â€œpractical realisationâ€�: a single-piece, metallic reference standard that was one thousand times more massive that would be known as â€œgraveâ€� (symbol G). This mass unit, whose name is derived from the word â€œgravityâ€�, was used since 1793. Notwithstanding that the definition of the base unit of mass was the gramme (alternatively â€œgravetâ€�), this new, practical realisation would ultimately become the base unit of mass. A provisional kilogram standard was made and work was commissioned to determine precisely how massive a cubic decimetre (later to be defined as equal to one litre) of water was.
Although the decreed definition of the kilogram specified water at 0 Â°C â€” a highly stable temperature point â€” the scientists tasked with producing the new practical realisation chose to redefine the standard and perform their measurements at the most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4 Â°C. They concluded that one cubic decimetre of water at its maximum density was equal to 99.92072% of the mass of the provisional kilogram made earlier that year. Four years later in 1799, an all-platinum standard, the â€œKilogramme des Archivesâ€�, was fabricated with the objective that it would equal, as close as was scientifically feasible for the day, to the mass of cubic decimetre of water at 4 Â°C. The kilogram was defined to be equal to the mass of the Kilogramme des Archives and this standard stood for the next ninety years.
Note that the new metric system did not come into effect until after the French Revolution, when the new revolutionary government captured the idea of the metric system. The decision of the Republican government to name this new unit the â€œkilogrammeâ€� had been mainly politically motivated, because the name â€œgraveâ€� was at that time considered politically incorrect as it resembled the aristocratic German title of the Graf, an alternative name for the title of Count that, like other nobility titles, was inconsistent with the new French Republic notion of equality (Ã©galitÃ©). Accordingly, the name of the original, defined unit of mass, â€œgrammeâ€�, which was too small to serve as a practical realisation, was adopted and the new prefix â€œkiloâ€� was appended to it to form the name â€œkilogrammeâ€�. Consequently, the kilogram is the only SI base unit that has an SI prefix as part of its unit name.
English units of measurement principal system of weights and measures used in a few nations, the only major industrial one being the United States. It actually consists of two related systemsâ€”the U.S. Customary System of units, used in the United States and dependencies, and the British Imperial System. The names of the units and the relationships between them are generally the same in both systems, but the sizes of the units differ, sometimes considerably. Customary Units of Weights and Measures Units of Weight The pound (lb) is the basic unit of weight (which is proportional to mass). Within the English units of measurement there are three different systems of weights. In the avoirdupois system, the most widely used of the three, the pound is divided into 16 ounces (oz) and the ounce into 16 drams. The ton, used to measure large masses, is equal to 2,000 lb (short ton) or 2,240 lb (long ton). In Great Britain the stone, equal to 14 lb, is also used. The troy system (named for Troyes, France, where it is said to have originated) is used only for precious metals. The troy pound is divided into 12 ounces and the troy ounce into 20 pennyweights or 480 grains; the troy pound is thus 5,760 grains. The grain is also a unit in the avoirdupois system, 1 avoirdupois pound being 7,000 grains, so that the troy pound is 5,760/7,000 of an avoirdupois pound. Apothecaries' weights are based on troy weights; in addition to the pound, ounce, and grain, which are equal to the troy units of the same name, other units are the dram (1/8 oz) and the scruple (1/24 oz or 1/3 dram). Units of Length and Area The basic unit of length is the yard (yd); fractions of the yard are the inch (1/36 yd) and the foot (1/3 yd), and commonly used multiples are the rod (5 1/2 yd), the furlong (220 yd), and the mile (1,760 yd). The acre, equal to 4,840 square yards or 160 square rods, is used for measuring land area. Units of Liquid Measure For liquid measure, or liquid capacity, the basic unit is the gallon, which is divided into 4 quarts, 8 pints, or 32 gills. The U.S. gallon, or wine gallon, is 231 cubic inches (cu in.); the British imperial gallon is the volume of 10 lb of pure water at 62Â°F and is equal to 277.42 cu in. The British units of liquid capacity are thus about 20% larger than the corresponding American units. The U.S. fluid ounce is 1/16 of a U.S. pint; the British unit of the same name is 1/20 of an imperial pint and is thus slightly smaller than the U.S. fluid ounce. Units of Dry Measure For dry measure, or dry capacity, the basic unit is the bushel, which is divided into 4 pecks, 32 dry quarts, or 64 dry pints. The U.S. bushel, or Winchester bushel, is 2,150.42 cu in. and is about 3% smaller than the British imperial bushel of 2,219.36 cu in., with a similar difference existing between U.S. and British subdivisions. The barrel is a unit for measuring the capacity of larger quantities and has various legal definitions depending on the quantity being measured, the most common value being 105 dry quarts. Differences between American and British Systems Many American units of weights and measures are based on units in use in Great Britain before 1824, when the British Imperial System was established. Since the Mendenhall Order of 1893, the U.S. yard and pound and all other units derived from them have been defined in terms of the metric units of length and mass, the meter and the kilogram ; thus, there is no longer any direct relationship between American units and British units of the same name. In 1959 an international agreement was reached among English-speaking nations to use the same metric equivalents for the yard and pound for purposes of science and technology; these values are 1 yd=0.9144 meter (m) and 1 lb=0.45359237 kilogram (kg). In the United States, the older definition of the yard as 3,600/3,937 m is still used for surveying, the corresponding foot (1,200/3,937 m) being known as the survey foot. The English units of measurement have many drawbacks: the complexity of converting from one unit to another, the differences between American and British units, the use of the same name for different units (e.g., ounce for both weight and liquid capacity, quart and pint for both liquid and dry capacity), and the existence of three different systems of weights (avoirdupois, troy, and apothecaries'). Because of these disadvantages and because of the wide use of the much simpler metric system in most other parts of the world, there have been proposals to do away with the U.S. Customary System and replace it with the metric system. Bibliography See L. J. Chisholm, Units of Weights and Measure: International and U.S. Customary (U.S. National Bureau of Standards, 1967).