RAM

Random-access memory (RAM) is a form of computer data storage.A random-access device allows stored data to be accessed quickly in any random order. In contrast, other data storage media such as hard disk,CDs, DVDs and magnetic tape, as well as early primary memory types such as drum memory, read and write data only in a predetermined order, consecutively, because of mechanical design limitations. Therefore the time to access a given data location varies significantly depending on its physical location.
Today, random-access memory takes the form of integrated circuit. Strictly speaking, modern types of DRAM are not random access, as data is read in bursts, although the nameDRAM / RAM has stuck. However, many types of SRAM,  ROM, OTP, and NOR Flash are still even in a strict sense. RAM is often associated with voliate types of memory, where its stored information is lost if the power is removed. Many other types of non-volatile memory are RAM as well, including most types of ROM and a type of Flash memory called NOR-MEMORY. The first RAM modules to come into the market were created in 1951 and were sold until the late 1960s and early 1970s.

Early computers used relay, or delay line for "main" memory functions. Ultrasonic delay lines could only reproduce data in the order it was written. Drum memory could be expanded at low cost but retrieval of non-sequential memory items required knowledge of the physical layout of the drum to optimize speed. Latches built out of vacum of memory, and later, out of discrete transistor, were used for smaller and faster memories such as random-access register banks and registers. Such registers were relatively large, power-hungry and too costly to use for large amounts of data; generally only a few hundred or few thousand bits of such memory could be provided.
The first practical form of random-access memory was the William Tube starting in 1947. 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.
Magnetic core memory, invented in 1947 and developed up until the mid 1970s, became a widespread form of random-access memory. It relied on an array of magnetized rings; by changing the sense of magnetization, data could be stored, with each bit represented physically by one 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 memory system until displaced by solid-state memory in integrated circuits, starting in the early 1970s.Robert H Denart invented (DRAM) in 1968; this 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 in a few milliseconds before the charge could leak away.
Prior to the development of integrated (ROM) circuits, permanent (or read-only) random-access memory was often constructed using

Sumber : http://en.wikipedia.org/wiki/Random-access_memory
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