Random Access Memory

Random-access memory (RAM) is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in bursts, although the name DRAM / RAM has stuck. However, many types of SRAM, ROM, OTP, and NOR flash are still random access even in a strict sense. RAM is often associated with volatile types of memory (such as DRAM memory modules), 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-Flash. The first RAM modules to come into the market were created in 1951 and were sold until the late 1960s and early 1970s. However, other memory devices (magnetic tapes, disks) can access the storage data in a predetermined order, because mechanical designs only allow this.

Types of RAM
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. Of special consideration are SIMM and DIMM memory modules. Because of economic reasons, personal computers, workstations and game consoles do not control large bits of data stored in DRAM. The computer's cache memory uses static RAM disks of data buffers.
SRAM and DRAM are volatile. Other forms of computer storage, such as disks and magnetic tapes, have been used as persistent storage. 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. A memory chip is an integrated circuit (IC) made of millions of transistors and capacitors. In the most common form of computer memory, dynamic random access memory (DRAM), a transistor and a capacitor are paired to create a memory cell, which represents a single bit of data. The capacitor holds the bit of information — a 0 or a 1. The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.

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". Swapping uses a paging file, an area on a hard drive temporarily used as additional working memory. Excessive use of this mechanism is called thrashing and is generally undesirable because it lowers overall system performance, mainly because hard drives are far slower than RAM. However, if a program attempts to allocate memory and fails, it may crash.