Memory is a computer system's primary workspace. It works in tandem with the CPU, or microprocessor, to store data, programs, and processed information that can be made immediately accessible to the CPU or to other system devices. Memory is central to a computer's operation because it forms the critical link between software and the CPU. Computer memory also determines the size and number of programs that can be run simultaneously, and helps to optimize the capabilities of increasingly powerful microprocessors.

There are many different kinds of memory, each with its own features and benefits. Unfortunately, with so many different types of memory, it can be easy to get them confused. Use the following information as a quick reference to refresh your own memory from time to time.

RAM (Random Access Memory)

Internally, computer memory is arranged as a matrix of "memory cells" laid out in rows and columns, like squares on a checkerboard. Each memory cell is used to store a bit of data, which can be instantaneously retrieved by indicating the row and column location (or address) of the data. Because these bits of data can be individually accessed, retrieved, and modified at random, the type of main memory used in computers is called Random Access Memory (RAM).

RAM is a volatile form of memory, which means that it must have power in order to retain data. When the power is turned off, data in RAM is lost. Compare this to other storage media such as disks, tapes, and CDs that retain data even without power.

TYPES OF RAM

DRAM (Dynamic RAM) is the most common type of computer memory. A bank of memory modules using DRAM chips usually forms the core of a computer's main memory. The system uses this memory to temporarily store programs, data, and processed information that moves to and from the processor, video card, and other peripherals.

It is called "dynamic" RAM because it must be refreshed, or re-energized, hundreds of times each second in order to retain data in its memory cells. It has to be refreshed because its memory cells are designed around tiny capacitors that store electrical charges. These capacitors work like very tiny batteries and will gradually lose their stored charges if they are not re-energized.

SRAM (Static RAM) also uses memory cells laid out in rows and columns to store data, but SRAM is about five times faster, twice as expensive, and twice as big as DRAM.

SRAM is also volatile (it must have power to retain its data) but SRAM does not need to be constantly refreshed like DRAM. SRAM designs use pretzel-like flip-flop circuits that allow electricity to flow through one side or the other depending on which one of two transistors is activated. This "flow through" design is faster than the "stored charge" design of DRAM, but it consumes more power. Because of its lower cost and smaller size, DRAM is preferred for use in computer main memory, while SRAM, because of its speed, is used primarily for cache memory.

Cache RAM is a small block of high-speed memory (usually SRAM) located between the CPU and main memory that is used to store frequently requested data and instructions. When the processor needs data, it will check in high-speed cache first to see if the data is there. If not, then the processor will retrieve the data from slower main memory.

Cache works much like a home refrigerator. A refrigerator can be considered a "cache" for groceries. Instead of going to the grocery store (main memory) every time you're hungry, you can go to the refrigerator (cache) first to see if the food you want is there. If it is, then you've saved a lot of time. If not, then you have to spend the extra time to get it from the store.

SGRAM (Synchronous Graphics RAM) an extension of SDRAM that includes graphics-specific read/write features. SGRAM allows data to be retrieved and modified in blocks, instead of individually. This reduces the number of reads and writes that the memory must perform, and increases the performance of the graphics controller by making the process more efficient.

VRAM (Video RAM), AKA graphics memory, must work very quickly to update, or refresh, the screen (60-70 times a second) in order to prevent screen "flicker." At the same time, graphics memory must respond very quickly to the CPU or graphics controller in order to change the image on screen. With ordinary DRAM, the CRT and CPU must compete for a single data port, causing a bottleneck of data traffic.

VRAM is a "dual-ported" memory that solves this problem by using two separate data ports. One port is dedicated to the CRT, for refreshing and updating the image on the screen. The second port is dedicated for use by the CPU or graphics controller, for changing the image data stored in memory.

VRAM works much like a fast food drive-through that uses two windows. After you place an order, you pay at one window, then drive up and get your food at the next window. This makes the process faster and more efficient.

WRAM (Window RAM) is another type of dual-ported memory also used in graphics intensive systems. It is slightly different from VRAM in that its dedicated display port is smaller, and that it supports EDO features.