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Static RAM (SRAM) and Dynamic RAM (DRAM)

Introduction
There are two common types of RAM, each with their own characterisitcs and advantages and disadvantages.

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DRAM needs the data to be refreshed periodically to retain the data. SRAM doesn't need to do this. As long as there is power on the SRAM, it will hold the data. DRAM has to have extra circuitry to allow the refreshing of data to take place and to get the timing to do this correct and this has implications for the amount of power a DRAM needs to have to run compared to the much simpler SRAM unit. All of this means that data can be read to and from SRAM a lot faster than DRAM.

Another implication of the simplicity of SRAM is that it is a lot easier for people to design hardware that uses it compared to the more complicated DRAM. If you are trying to design an application, you want to use hardware that is easy to use and integrate with your other components.

It isn't all rosy for SRAM, however! Each bit that you need to store in DRAM requires 1 transisitor and 1 capacitor. SRAM requires six of each for every bit that is stored. If SRAM needs six times the amount of transistors and capacitors then it clearly is going to be a more expensive design and therefore the cost of buying 8 Gbytes of SRAM is going to be a lot more than the cost of buying 8 Gbytes of DRAM.

The relatively cheaper cost of DRAM is the reason why it is the most commonly found type of RAM in computer systems, despite being slower and needing more power than SRAM. It is used as general purpose memory, to hold the operating system, the applications and files you are actually using at any particular moment in time.

In devices where speed is crucial, SRAM is the memory of choice. The typical use for this is in cache. Programs are made up of instructions. These are fetched from DRAM by the CPU and executed. This is done quickly but still takes time. However, some instructions in DRAM are needed again and again. If you put these instructions in 'super-fast memory' instead (in other words, into SRAM), you can speed up processing and make your computer go much faster. That's what cache is - super-fast memory that holds instructions and data you keep needing. You only get limited amounts of it in a computer system, though, because as we have seen, SRAM is much more expensive than DRAM.

One question that should always be asked when buying a new computer is "How much cache has it got"? It is always worth getting as much cache for a computer as you can afford because it speeds up processing - but at a price!

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