Random-access memory
Random-access memory 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, 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 modern technology, random-access memory takes the form of integrated circuit chips with MOS 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 and dynamic random-access memory.
Non-volatile RAM has also been developed 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 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 was reintroduced with the Samsung KM48SL2000 chip in 1992.
History
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 during the early 1970s.
Prior to the development of integrated read-only memory circuits, permanent random-access memory was often constructed using diode matrices driven by address decoders, or specially wound core rope memory planes.
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.
MOS RAM
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 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 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.Integrated bipolar static random-access memory 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 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.
File:Bundesarchiv Bild 183-1989-0406-022, VEB Carl Zeiss Jena, 1-Megabit-Chip.jpg|thumb|right|CMOS 1-megabit DRAM chip, one of the last models developed by VEB Carl Zeiss Jena, in 1989
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 8μm MOS process with a capacity of 1kbit, and was released in 1970.
The earliest DRAMs were often synchronized with the CPU clock 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 16Mbit. The first commercial double data rate SDRAM was Samsung's 64Mbit DDR SDRAM, released in June 1998. GDDR is a form of SGRAM, which was first released by Samsung as a 16Mbit memory chip in 1998.
Types
In general, the term RAM refers solely to solid-state memory devices, and more specifically the main memory in most computers. The two widely used forms of modern RAM are static RAM and dynamic RAM. In SRAM, a bit of data is stored using the state of a memory cell, typically using six MOSFETs. This form of RAM is more expensive to produce, but is generally faster and requires less static 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, which together comprise a DRAM cell. The capacitor holds a high or low charge, 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 when power is removed from the system. By contrast, read-only memory stores data by permanently enabling or disabling selected transistors, such that the memory cannot be altered. Writable variants of ROM share properties of both ROM and RAM, enabling data to persist without power and to be updated without requiring special equipment.
ECC memory includes special circuitry to detect and/or correct random faults in the stored data, using parity bits or error correction codes.
Memory cell
The memory cell is the fundamental building block of computer memory. The memory cell is an electronic circuit that stores one bit of binary information. The cell can be set to store a logic 1 and reset to store a logic 0. Its value is maintained until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it.In SRAM, the memory cell is a type of flip-flop circuit, usually implemented using FETs. This means that SRAM requires very low power when not being accessed, but it is complex, expensive and has low storage density.
A second type, DRAM, is based around a capacitor. Charging and discharging this capacitor can store a 1 or a 0 in the cell. However, the charge in this capacitor slowly leaks away and must be refreshed periodically. Because of this refresh process, DRAM uses more power, but it can achieve greater storage densities and lower unit costs compared to SRAM.