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In this article:
- What is RAM
- History of RAM
- Types of RAM
What is RAM
RAM is an acronym for Random access memory which is a type of computer memory. The term ‘random access’ signifies that one can access any byte stored in RAM without having to go through the bytes stored before it (as in sequential access).
RAM is the exact opposite of serial access memory or SAM under which data is stored as a series of memory cells that can only be accessed sequentially (as in a cassette). If the data being searched for is not in the current location, each memory cell needs to be checked till the desired data is found. SAM is apt for use in case of memory buffers, where the data is stored in the order of its intended usage.
A RAM is essentially an integrated circuit or IC, similar to a microprocessor. It is made up of millions of transistors and capacitors integrated onto a silicon base (the chip).
RAM is basically available in two avatars. These are:
- Dynamic Random Access Memory
This is the most widely found computer memory and is called DRAM for short. It has a transistor-capacitor pair that makes a memory cell which represents a single bit of data. The capacitor holds the bit of information which is either a 0 or a 1. The transistor works as a switch that allows the control circuitry present on the memory chip read the capacitor or change its state.
The capacitors on a DRAM chip have a tendency to loose their data as they continue to discharge. Therefore, either the CPU or the controller of the memory has to recharge all of the capacitors holding a 1 before they discharge. For achieving this, the memory controller reads the memory and then writes back. This refresh operation happens automatically thousands of times per second and this is where dynamic RAM gets its name. It is here where the problem with DRAM lies. The tendency to loose its data and the need to refresh it takes time and makes this type of RAM slow.
- Static Random Access Memory
In Static RAM, a form of flip-flop holds each bit of memory. Such flip-flops do not need to be refreshed. This is what makes static RAM faster than dynamic RAM but the problem is that a single static memory cell takes up more space than a dynamic memory cell on a chip. Hence, static RAM is more expensive. It is used to make a CPU’s cache.
A single chip of RAM is often called a RAM module.
History of RAM
Originally, each computer manufacturer developed unique modules that could work only with a certain type of systems. After this, the single in-line memory modules or SIMMs were developed. This memory board made use of a 30-pin connector and was nearly 3.5 x .75 inches in size (about 9 x 2 cm). It was required that SIMMs be installed in pairs of equal capacity and speed. The maximum allowed memory with SIMM was 256MB.
A new standard came after SIMM, aptly named dual in-line memory module or DIMM. It had a 168-pin or 184-pin connector and had a size of 5.4 x 1 inch (about 14 x 2.5 cm). DIMM allowed up to 1 GB per module and could be installed singly instead of in pairs. Rambus in-line memory module or RIMM was quite similar to DIMM but used a special memory bus to greatly increase speed.
Most memory available today is highly reliable and most systems rely on the memory controller to check for errors at start-up.
Types of RAM modules
The following are some common types of RAM modules:
- SRAM
Static random access memory, as explained above, uses multiple transistors and is used primarily for cache.
- DRAM
Dynamic random access memory, as discussed, has memory cells with a paired transistor and capacitor that need constant refreshing.
- SDRAM
Synchronous dynamic random access memory takes advantage of the burst mode concept to improve performance it stays on the row containing the requested bit and moving rapidly through the columns, reading each bit as it goes.
- DDR SDRAM (or DDR RAM)
Double data rate synchronous dynamic RAM is simliar to SDRAM except that is has higher bandwidth, meaning greater speed.
- VRAM
VideoRAM or multi-port dynamic random access memory (MPDRAM) is a type of RAM used in video adapters or 3-D accelerators. It has two independent access ports that allow the CPU and graphics processor to access the RAM at the same time.
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