Saturday, September 18, 2010
iT IS AN EXAMPLE OF OLD
TYPE STORAGE MEDIA TO STORE LARGE AMOUNT OF DATA PERMANENTLY. mAGNETIC
tAPE CONSISTS OF A THIN RIBBion of plastic. The tap is coated with
magnetic material. The process of reading or writing of data on the tape
is very slow. In magnetic tape, data can only be accessed sequentially. It
is mostly used for taking backup of data.
Friday, September 17, 2010
Rom stands for read only memory. It cansists of those instructions that prepare the computer for use. Rom instructions are automatically loaded into the main memory. These instructions cab only be read but cannot be changed or deleted. It is not possible to write new instructions into the Rom. It stores data and instructions permanently. When we swithch off the computer, the instructions stored in the Rom are not lost. Therefor it called a non_volatile memory.
Thursday, September 16, 2010
Willett's 1907 proposal argued that DST increases opportunities for outdoor leisure activities during afternoon sunlight hours. The longer days nearer the summer solstice in high latitudes offer more room to shift daylight from morning to evening so that early morning daylight is not wasted. DST is commonly not observed during most of winter, because its mornings are darker: workers may have no sunlit leisure time, and children may need to leave for school in the dark.
General agreement about the day's layout confers so many advantages that a standard DST schedule usually outranks ad hoc efforts to get up earlier, even for people who personally dislike the DST schedule. The advantages of coordination are so great that many people ignore whether DST is in effect by altering their nominal work schedules to coordinate with television broadcasts or daylight.
In a typical case where a one-hour shift occurs at 02:00 local time, in spring the clock jumps forward from 02:00 standard time to 03:00 DST and that day has 23 hours, whereas in autumn the clock jumps backward from 02:00 DST to 01:00 standard time, repeating that hour, and that day has 25 hours. A digital display of local time does not read 02:00 exactly at the shift, but instead jumps from 01:59:59.9 either forward to 03:00:00.0 or backward to 01:00:00.0. In this example, a location observing UTC+10 during standard time is atUTC+11 during DST; conversely, a location at UTC−10 during standard time is at UTC−9 during DST.
Clock shifts are usually scheduled near a weekend midnight to lessen disruption to weekday schedules. A one-hour shift is customary, but Australia's Lord Howe Island uses a half-hour shift. Twenty-minute and two-hour shifts have been used in the past.
Coordination strategies differ when adjacent time zones shift clocks. The European Union shifts all at once, at 01:00 UTC; for example, Eastern European Time is always one hour ahead of Central European Time. Most of North America shifts at 02:00 local time, so its zones do not shift at the same time; for example, Mountain Time can be temporarily either zero or two hours ahead of Pacific Time. In the past, Australian districts went even further and did not always agree on start and end dates; for example, in 2008 most DST-observing areas shifted clocks forward on October 5 but Western Australia shifted on October 26. In some cases only part of a country shifts; for example, in the U.S., Hawaii and most of Arizona do not observe DST.
Start and end dates vary with location and year. Since 1996 European Summer Time has been observed from the last Sunday in March to the last Sunday in October; previously the rules were not uniform across the European Union. Starting in 2007, most of the United States and Canada observe DST from the second Sunday in March to the first Sunday in November, almost two-thirds of the year. The 2007 U.S. change was part of the Energy Policy Act of 2005; previously, from 1987 through 2006, the start and end dates were the first Sunday in April and the last Sunday in October, and Congress retains the right to go back to the previous dates now that an energy-consumption study has been done.
Beginning and ending dates are the reverse in the southern hemisphere. For example, mainland Chile observes DST from the second Saturday in October to the second Saturday in March, with transitions at 24:00 local time. The time difference between the United Kingdom and mainland Chile may therefore be five hours during the Northern summer, three hours during the Northern winter and four hours a few weeks per year because of mismatch of changing dates.
Western China, Iceland, and other areas skew time zones westward, in effect observing DST year-round without complications from clock shifts. For example, Saskatoon,Saskatchewan, is at 106° 39′ W longitude, slightly west of center of the idealized Mountain Time Zone (105° W), but the time in Saskatchewanis Central Standard Time (90° W) year-round, so Saskatoon is always about 67 minutes ahead of mean solar time. Conversely, northeastIndia and a few other areas skew time zones eastward, in effect observing negative DST. The United Kingdom and Irelandexperimented with year-round DST from 1968 to 1971 but abandoned it because of its unpopularity, particularly in northern regions.
Western France, Spain, and other areas skew time zones and shift clocks, in effect observing DST in winter with an extra hour in summer. For example, Nome, Alaska, is at 165° 24′ W longitude, which is just west of center of the idealized Samoa Time Zone (165° W), but Nome observes Alaska Time (135° W) with DST, so it is slightly more than two hours ahead of the sun in winter and three in summer. Double daylight saving time has been used on occasion; for example, Britain used it during World War II.
DST is generally not observed near the equator, where sunrise times do not vary enough to justify it. Some countries observe it only in some regions; for example, southern Brazil observes it while equatorial Brazil does not. Only a minority of the world's population uses DST because Asia and Africa generally do not observe it.
Although not punctual in the modern sense, ancient civilizations adjusted daily schedules to the sun more flexibly than modern DST does, often dividing daylight into twelve equal hours regardless of day length, so that each daylight hour was longer during summer. For example, Roman water clocks had different scales for different months of the year: at Rome's latitude the third hour from sunrise, hora tertia, started by modern standards at 09:02 solar time and lasted 44 minutes at the winter solstice, but at the summer solstice it started at 06:58 and lasted 75 minutes. After ancient times, equal-length civil hours eventually supplanted unequal, so civil time no longer varies by season. Unequal hours are still used in a few traditional settings, such as some Mount Athos monasteries and some Jewish ceremonies.
During his time as an American envoy to France, Benjamin Franklin, author of the proverb, "Early to bed, and early to rise, makes a man healthy, wealthy and wise", anonymously published a letter suggesting thatParisians economize on candles by rising earlier to use morning sunlight.This 1784 satire proposed taxing shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise. Franklin did not propose DST; like ancient Rome, 18th-century Europe did not keep precise schedules. However, this soon changed as rail and communication networks came to require a standardization of time unknown in Franklin's day.
Modern DST was first proposed by the New Zealand entomologist George Vernon Hudson, whose shift-work job gave him leisure time to collect insects, and made him aware of the value of after-hours daylight. In 1895 he presented a paper to the Wellington Philosophical Society proposing a two-hour daylight-saving shift, and after considerable interest was expressed in Christchurch, New Zealand he followed up in an 1898 paper. Many publications incorrectly credit DST's invention to the prominent English builder and outdoorsman William Willett, who independently conceived DST in 1905 during a pre-breakfast ride, when he observed with dismay how many Londoners slept through a large part of a summer day. An avid golfer, he also disliked cutting short his round at dusk. His solution was to advance the clock during the summer months, a proposal he published two years later. The proposal was taken up by the Liberal Member of Parliament (MP) Robert Pearce, who introduced the first Daylight Saving Bill to the House of Commons on 12 February 1908. A select committee was set up to examine the issue, but Pearce's bill did not become law, and several other bills failed in the following years. Willett lobbied unsuccessfully for the proposal in the UK until his death in 1915.
Starting on 30 April 1916, Germany and its World War I allies were the first to use DST (ger.: Sommerzeit) as a way to conserve coal during wartime. Britain, most of its allies, and many European neutrals soon followed suit. Russia and a few other countries waited until the next year and the United States adopted it in 1918. Since then, the world has seen many enactments, adjustments, and repeals