Random access memory (usually known by its acronym, RAM) is a form of computer data storage. Today it takes the form of integrated circuits that allows the stored data to be accessed in any order (i.e., at random). The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.

This contrasts with storage mechanisms such as tapes, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.
The word RAM is mostly associated with volatile types of memory (such as DRAM memory modules), where the information is lost after the power is switched off. However, many other types of memory are RAM as well (i.e., Random Access Memory), including most types of ROM and a kind of flash memory called NOR-Flash.

Types of RAM
1 Megabit chip - one of the last models developed by VEB Carl Zeiss Jena in 1989
Modern types of writable RAM generally store a bit of data in either the state of a flip-flop, as in SRAM (static RAM), or as a charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM), EPROM, EEPROM and Flash. Some types have circuitry to detect and/or correct random faults called memory errors in the stored data, using parity bits or error correction codes. RAM of the read-only type, ROM, instead uses a metal mask to permanently enable/disable selected transistors, instead of storing a charge in them.
As both SRAM and DRAM are volatile, other forms of computer storage, such as disks and magnetic tapes, have been used as persistent storage in traditional computers. Many newer products instead rely on flash memory to maintain data when not in use, such as PDAs or small music players. Certain personal computers, such as many rugged computers and netbooks, have also replaced magnetic disks with flash drives. With flash memory, only the NOR type is capable of true random access, allowing direct code execution, and is therefore often used instead of ROM; the lower cost NAND type is commonly used for bulk storage in memory cards and solid-state drives.

Memory hierarchy
Many computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external caches, DRAM, paging systems, and virtual memory or swap space on a hard drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different access times, violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or interleave organization of the components make the access time variable, although not to the extent that rotating storage media or a tape is variable. The overall goal of using a memory hierarchy is to obtain the higher possible average access performance while minimizing the total cost of entire memory system. (Generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom.)
In many modern personal computers, the RAM comes in an easily upgraded form of modules called memory modules or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or too small for current purposes. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the CPU and other ICs on the motherboard, as well as in hard-drives, CD-ROMs, and several other parts of the computer system.

Swapping
If a computer becomes low on RAM during intensive application cycles, many CPU architectures and operating systems are able to perform an operation known as "swapping". This makes use of a paging file, an area on a hard drive temporarily used as additional working memory. Constantly relying on this mechanism is called thrashing and is generally undesirable because it lowers overall system performance, mainly because hard drives are slower than RAM.

mobile phones

A mobile phone (also called cellphone or handphone as well as wireless phone, cell phone, cellular phone, cellular telephone or cell telephone) is a long-range, electronic device used for mobile voice or data communication over a network of specialized base stations known as cell sites. In addition to the standard voice function of a mobile phone, telephone, current mobile phones may support many additional services, and accessories, such as SMS for text messaging, email, packet switching for access to the Internet, gaming, Bluetooth, infrared, camera with video recorder and MMS for sending and receiving photos and video, MP3 player, radio and GPS. Most current mobile phones connect to a cellular network of base stations (cell sites), which is in turn interconnected to the public switched telephone network (PSTN) (the exception is satellite phones).
In 2008 there were 4,100 million mobile cellular subscriptions in the world.A mobile phone proper typically has a telephone keypad, more advanced devices have a separate key for each letter. Some mobile phones have a touchscreen.

History

In 1908, U.S. Patent 887,357 for a wireless telephone was issued in to Nathan B. Stubblefield of Murray, Kentucky. He applied this patent to "cave radio" telephones and not directly to cellular telephony as the term is currently understood. Cells for mobile phone base stations were invented in 1947 by Bell Labs engineers at AT&T and further developed by Bell Labs during the 1960s. Radiophones have a long and varied history going back to Reginald Fessenden's invention and shore-to-ship demonstration of radio telephony, through the Second World War with military use of radio telephony links and civil services in the 1950s, while hand-held cellular radio devices have been available since 1973. A patent for the first wireless phone as we know today was issued in US Patent Number 3,449,750 to George Sweigert of Euclid, Ohio on June 10, 1969.
In 1945, the zero generation (0G) of mobile telephones was introduced. 0G mobile phones, such as Mobile Telephone Service, were not cellular, and so did not feature "handover" from one base station to the next and reuse of radio frequency channels.[citation needed] Like other technologies of the time, it involved a single, powerful base station covering a wide area, and each telephone would effectively monopolize a channel over that whole area while in use. The concepts of frequency reuse and handoff as well as a number of other concepts that formed the basis of modern cell phone technology are first described in U.S. Patent 4,152,647 , issued May 1, 1979 to Charles A. Gladden and Martin H. Parelman, both of Las Vegas, Nevada and assigned by them to the United States Government.
This is the first embodiment of all the concepts that formed the basis of the next major step in mobile telephony, the Analog cellular telephone. Concepts covered in this patent (cited in at least 34 other patents) also were later extended to several satellite communication systems. Later updating of the cellular system to a digital system credits this patent.
Martin Cooper, a Motorola researcher and executive is widely considered to be the inventor of the first practical mobile phone for handheld use in a non-vehicle setting. Cooper is the inventor named on "Radio telephone system" filed on October 17, 1973 with the US Patent Office and later issued as US Patent 3,906,166.Using a modern, if somewhat heavy portable handset, Cooper made the first call on a handheld mobile phone on April 3, 1973 to a rival, Dr. Joel S. Engel of Bell Labs.

The National Aeronautics and Space Administration (NASA, pronounced) is an agency of the United States government, responsible for the nation's public space program. NASA was established on July 29, 1958, by the National Aeronautics and Space Act.
In addition to the space program, it is also responsible for long-term civilian and military aerospace research. Since February 2006 NASA's self-described mission statement is to "pioneer the future in space exploration, scientific discovery, and aeronautics research."

History

After the Soviet space program's launch of the world's first human-made satellite (Sputnik 1) on October 4, 1957, the attention of the United States turned toward its own fledgling space efforts. The U.S. Congress, alarmed by the perceived threat to U.S. security and technological leadership (known as the "Sputnik crisis"), urged immediate and swift action; President Dwight D. Eisenhower and his advisors counseled more deliberate measures. Several months of debate produced an agreement that a new federal agency was needed to conduct all non-military activity in space. The Defense Advanced Research Projects Agency (DARPA) was also created at this time and many of DARPA's early space programs were soon transferred to NASA.

May 5, 1961 launch of Redstone rocket and NASA's Mercury Freedom 7 with Alan Shepard on the United States' first manned sub-orbital spaceflight.
Explorer 1, officially Satellite 1958 Alpha, was the first Earth artificial satellite of the United States, having been launched at 10:48 pm EST on January 31, 1958. On July 29, 1958, President Eisenhower signed the National Aeronautics and Space Act, establishing the National Aeronautics and Space Administration. When it began operations on October 1, 1958, NASA consisted mainly of the four laboratories and some 80 employees of the government's 46-year-old research agency, the National Advisory Committee for Aeronautics (NACA). A significant contributor to NASA's entry into the Space race was the technology from the German rocket program, led by Wernher von Braun, who became a naturalized citizen of the United States after World War II. He is today regarded as the father of the United States space program. Elements of the Army Ballistic Missile Agency (of which von Braun's team was a part) and the Naval Research Laboratory were incorporated into NASA.
NASA's earliest programs involved research into human spaceflight and were conducted under the pressure of the competition between the U.S. and the USSR (the Space Race) that existed during the Cold War. Project Mercury, initiated in 1958, started NASA down the path of human space exploration with missions designed to discover simply if man could survive in space. Representatives from the U.S. Army (M.L. Raines, LTC, USA), Navy (P.L. Havenstein, CDR, USN) and Air Force (K.G. Lindell, COL, USAF) were selected/requested to provide assistance to the NASA Space Task Group through coordination with the existing U.S. defense research and defense contracting infrastructure, and technical assistance resulting from experimental aircraft (and the associated military test pilot pool) development in the 1950s. On May 5, 1961, astronaut Alan Shepard—one of the seven Project Mercury astronauts selected as pilot for this mission—became the first American in space when he piloted Freedom 7 on a 15-minute suborbital flight. John Glenn became the first American to orbit the Earth on February 20, 1962 during the 5 and a quarter-hour flight of Friendship 7.
After the Mercury project, Project Gemini was launched to conduct experiments and work out issues relating to a moon mission. The first Gemini flight with astronauts on board, Gemini 3, was flown by Gus Grissom and John Young on March 23, 1965. Nine other missions followed, showing that long-duration human space flight was possible, proving that rendezvous and docking with another vehicle in space was possible, and gathering medical data on the effects of weightlessness on human beings.
During this time NASA also began to explore the solar system with unmanned probes. As with the manned program, the Soviets had the first successes, such as the first photographs of the lunar far side, but NASA's Mariner 2 was the first space probe to visit another planet, Venus, in 1962.