Saturday, September 18, 2010

System Software

System software consists of the programs that control or maintains the operations of the computer and its devices. System software serves as the There are further two types of thesystem software.

Computer Software



A "port" seves as an interface between the computer and other computers or peripheral devices. physically, a port is a specialized outlet on a piece of equipment to which a plug or cable connects.


The mother board sometime called a system board is the main circuit board of the system unit. It is used to connect all kind of hardware devices.


DVD, also known as "Digital Versatile Disc: or :Digital Video Disc:, is an optical disc storage media format. Its main uses are video and data storage. DVDs are of the same dimensions as compact discs (CDs), but store more than six times as much data.

Compact Disk (CD)

CD is the most
popular optical medium. It can hold 650 MB to 800 MB od data. The data on
the CD
cannot be erased like magnetic diskettes.

Optical Disks

Optical disk issued laser technology to read or write information. Laser Stands for Light Amplification through stimulated emission of Radiation Laser beam is used to make tiny holes on the surface of the disk.

Magnetic Tape (Sequential Access)

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.

Magnetic Disk

It is most widely used storage media for all type of computer. A magnetic disk is a thin circular metal plate or platter coated with magnetic material. Information can be record on or read from the magnetic surface through manetism. Magnetic disk is a random access storage media.

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.


Ram can not store data and instruction permanently. When we switch off the computer, all data and instructions from ram are washed or vanished. Therefore it is called volatile memory


Ram Stands for Random Access Memory. Ram is also called main memory or primary storagel it consists of memory chips that can be read from and written to by the processor and other devices. Ram varies in size like 128 mb, 156 mb, 512mb to 2gb


Data to be processd by the appplication programs and resulting information.


Application programs tha carry out a specific task such as word processing etc.


Memory stores three basic categories of data.
1. Operating system and other system software that control or maintain the computer and its devices

Primary Storage or Min Memory (RAM)

Primary Storage, persently known as memory or simly RAM, is the only one directly accessible to the CPU. the CPU continouusly reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner.


Storage refers gto the media either data are stored temporarily or permanently. There are two main types of storage.

1. Primary Storage or Main Memory
2. Secondary Storage


Machine cycle

For every instruction, a porcessor ropeats a set of four basic operations.
1. Fetching
3.Excuting and if necessary,

Thursday, September 16, 2010

Benefits and drawbacks

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.[24] 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.[40]

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.[41] 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.

How it works

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.[27] 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.[28] 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.[29] In some cases only part of a country shifts; for example, in the U.S., Hawaii and most of Arizona do not observe DST.[30]

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.[28] 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.[31] 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.[32]

Time graph. The horizontal axis shows dates in 2008. The vertical axis shows the UTC offsets of eastern Brazil and eastern U.S. The difference between the two starts at 3 hours, then goes to 2 hours on February 17 at 24:00 Brazil eastern time, then goes to 1 hour on March 9 at 02:00 U.S. eastern time.
In early 2008 central Brazil was one, two, or three hours ahead of eastern U.S., depending on the date.

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.[33] 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.

Map of the time zone boundaries of the world. Generally the borders run north-south and there are about 24 zones, but there are many exceptions where the borders follow national boundaries and a few half-hour or quarter-hour zones exist.
Time zones often lie west of their idealized boundaries, resulting in year-round DST.

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.[34] Conversely, northeastIndia and a few other areas skew time zones eastward, in effect observing negative DST.[35] The United Kingdom and Irelandexperimented with year-round DST from 1968 to 1971 but abandoned it because of its unpopularity, particularly in northern regions.[36]

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.[37] Double daylight saving time has been used on occasion; for example, Britain used it during World War II.[28]

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.[38] 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.[12] 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.[13] 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[14] and some Jewish ceremonies.[15]

A seated older Benjamin Franklin from the waist up, with body facing to viewer's right but head turned toward the artist. Franklin's waistcoat is bulging a bit, his expression is inscrutable, and his hair hangs down to his shoulders.
Benjamin Franklin satirically suggested firing cannons at sunrise to wake Parisians

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.[16]This 1784 satire proposed taxing shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise.[17] 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.[18]

Fuzzy head-and-shoulders photo of a 40-year-old man in a cloth cap and mustache.
G.V. Hudson invented modern DST, proposing it first in 1895.

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.[2] In 1895 he presented a paper to the Wellington Philosophical Society proposing a two-hour daylight-saving shift,[19] and after considerable interest was expressed in Christchurch, New Zealand he followed up in an 1898 paper.[20] Many publications incorrectly credit DST's invention to the prominent English builder and outdoorsman William Willett,[21] 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.[22] An avid golfer, he also disliked cutting short his round at dusk.[23] His solution was to advance the clock during the summer months, a proposal he published two years later.[24] 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.[25] 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