RAM

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• Random-access memory (RAM; /ræm/) is a form of electronic computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory, in contrast with other direct-access data storage media (such as hard disks and magnetic tape), where the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement.


• In today's technology, random-access memory takes the form of integrated circuit (IC) chips with MOS (metal–oxide–semiconductor) memory cells. RAM is normally associated with volatile types of memory where stored information is lost if power is removed. The two main types of volatile random-access semiconductor memory are static random-access memory (SRAM) and dynamic random-access memory (DRAM).


• Non-volatile RAM has also been developed[3] and other types of non-volatile memories allow random access for read operations, but either do not allow write operations or have other kinds of limitations. These include most types of ROM and NOR flash memory.


• The use of semiconductor RAM dates back to 1965 when IBM introduced the monolithic (single-chip) 16-bit SP95 SRAM chip for their System/360 Model 95 computer, and Toshiba used bipolar DRAM memory cells for its 180-bit Toscal BC-1411 electronic calculator, both based on bipolar transistors. While it offered higher speeds than magnetic-core memory, bipolar DRAM could not compete with the lower price of the then-dominant magnetic-core memory. In 1966, Dr. Robert Dennard invented modern DRAM architecture in which there's a single MOS transistor per capacitor. The first commercial DRAM IC chip, the 1K Intel 1103, was introduced in October 1970. Synchronous dynamic random-access memory (SDRAM) was reintroduced with the Samsung KM48SL2000 chip in 1992.

• Early computers used relays, mechanical counters or delay lines for main memory functions. Ultrasonic delay lines were serial devices which could only reproduce data in the order it was written. Drum memory could be expanded at relatively low cost but efficient retrieval of memory items requires knowledge of the physical layout of the drum to optimize speed. Latches built out of triode vacuum tubes, and later, out of discrete transistors, were used for smaller and faster memories such as registers. Such registers were relatively large and too costly to use for large amounts of data; generally only a few dozen or few hundred bits of such memory could be provided.


• The first practical form of random-access memory was the Williams tube. It stored data as electrically charged spots on the face of a cathode-ray tube. Since the electron beam of the CRT could read and write the spots on the tube in any order, memory was random access. The capacity of the Williams tube was a few hundred to around a thousand bits, but it was much smaller, faster, and more power-efficient than using individual vacuum tube latches. Developed at the University of Manchester in England, the Williams tube provided the medium on which the first electronically stored program was implemented in the Manchester Baby computer, which first successfully ran a program on 21 June, 1948. In fact, rather than the Williams tube memory being designed for the Baby, the Baby was a testbed to demonstrate the reliability of the memory.


• Magnetic-core memory was invented in 1947 and developed up until the mid-1970s. It became a widespread form of random-access memory, relying on an array of magnetized rings. By changing the sense of each ring's magnetization, data could be stored with one bit stored per ring. Since every ring had a combination of address wires to select and read or write it, access to any memory location in any sequence was possible. Magnetic core memory was the standard form of computer memory until displaced by semiconductor memory in integrated circuits (ICs) during the early 1970s.


• Prior to the development of integrated read-only memory (ROM) circuits, permanent (or read-only) random-access memory was often constructed using diode matrices driven by address decoders, or specially wound core rope memory planes.[citation needed] Semiconductor memory appeared in the 1960s with bipolar memory, which used bipolar transistors. Although it was faster, it could not compete with the lower price of magnetic core memory.

• In 1957, Frosch and Derick manufactured the first silicon dioxide field-effect transistors at Bell Labs, the first transistors in which drain and source were adjacent at the surface. Subsequently, in 1960, a team demonstrated a working MOSFET at Bell Labs. This led to the development of metal–oxide–semiconductor (MOS) memory by John Schmidt at Fairchild Semiconductor in 1964. In addition to higher speeds, MOS semiconductor memory was cheaper and consumed less power than magnetic core memory. The development of silicon-gate MOS integrated circuit (MOS IC) technology by Federico Faggin at Fairchild in 1968 enabled the production of MOS memory chips. MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s.


• ntegrated bipolar static random-access memory (SRAM) was invented by Robert H. Norman at Fairchild Semiconductor in 1963. It was followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964. SRAM became an alternative to magnetic-core memory, but required six MOS transistors for each bit of data. Commercial use of SRAM began in 1965, when IBM introduced the SP95 memory chip for the System/360 Model 95. Dynamic random-access memory (DRAM) allowed replacement of a 4 or 6-transistor latch circuit by a single transistor for each memory bit, greatly increasing memory density at the cost of volatility. Data was stored in the tiny capacitance of each transistor and had to be periodically refreshed every few milliseconds before the charge could leak away.


• Toshiba's Toscal BC-1411 electronic calculator, which was introduced in 1965, used a form of capacitor bipolar DRAM, storing 180-bit data on discrete memory cells, consisting of germanium bipolar transistors and capacitors. Capacitors had also been used for earlier memory schemes, such as the drum of the Atanasoff–Berry Computer, the Williams tube and the Selectron tube. While it offered higher speeds than magnetic-core memory, bipolar DRAM could not compete with the lower price of the then-dominant magnetic-core memory.


• In 1966, Robert Dennard, while examining the characteristics of MOS technology, found it was capable of building capacitors, and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, and the MOS transistor could control writing the charge to the capacitor. This led to his development of modern DRAM architecture for which there is a single MOS transistor per capacitor. In 1967, Dennard filed a patent under IBM for a single-transistor DRAM memory cell, based on MOS technology. The first commercial DRAM IC chip was the Intel 1103, which was manufactured on an 8 μm MOS process with a capacity of 1 kbit, and was released in 1970.


• The earliest DRAMs were often synchronized with the CPU clock (clocked) and were used with early microprocessors. In the mid-1970s, DRAMs moved to the asynchronous design, but in the 1990s returned to synchronous operation. In 1992 Samsung released KM48SL2000, which had a capacity of 16 Mbit. and mass-produced in 1993. The first commercial DDR SDRAM (double data rate SDRAM) memory chip was Samsung's 64 Mbit DDR SDRAM chip, released in June 1998. GDDR (graphics DDR) is a form of DDR SGRAM (synchronous graphics RAM), which was first released by Samsung as a 16 Mbit memory chip in 1998.

1. SRAM
2. DRAM

• The two widely used forms of modern RAM are static RAM (SRAM) and dynamic RAM (DRAM). In SRAM, a bit of data is stored using the state of a six-transistor memory cell, typically using six MOSFETs. This form of RAM is more expensive to produce, but is generally faster and requires less dynamic power than DRAM. In modern computers, SRAM is often used as cache memory for the CPU. DRAM stores a bit of data using a transistor and capacitor pair (typically a MOSFET and MOS capacitor, respectively), which together comprise a DRAM cell. The capacitor holds a high or low charge (1 or 0, respectively), and the transistor acts as a switch that lets the control circuitry on the chip read the capacitor's state of charge or change it. As this form of memory is less expensive to produce than static RAM, it is the predominant form of computer memory used in modern computers.


• Both static and dynamic RAM are considered volatile, as their state is lost or reset when power is removed from the system. By contrast, read-only memory (ROM) stores data by permanently enabling or disabling selected transistors, such that the memory cannot be altered. Writable variants of ROM (such as EEPROM and NOR flash) share properties of both ROM and RAM, enabling data to persist without power and to be updated without requiring special equipment. ECC memory (which can be either SRAM or DRAM) includes special circuitry to detect and/or correct random faults (memory errors) in the stored data, using parity bits or error correction codes.


• In general, the term RAM refers solely to solid-state memory devices (either DRAM or SRAM), and more specifically the main memory in most computers. In optical storage, the term DVD-RAM is somewhat of a misnomer since, it is not random access; it behaves much like a hard disc drive if somewhat slower. Aside, unlike CD-RW or DVD-RW, DVD-RAM does not need to be erased before reuse.