Types of Memory - SRAM and DRAM

An excellent article on memory hierarchy is here. An excerpt from it about DRAM and SRAM...

RAM comes in two varieties - SRAM (static RAM) which is very expensive and very fast and which is mainly used for the local cache both on and off the CPU chip and DRAM (dynamic RAM) which is cheaper and slower and which is used for the main memory and graphics frame buffer (a frame buffer is just a dedicated memory chip which holds the colour information for each pixel to be displayed on the screen).

SRAM or static RAM is physically built with up to 6 transistors to store each bit or voltage signal and is a stable memory module whole signal is retained as long as it is supplied with power. SRAM is not affected by voltage fluctuations or noise. However, it is also more expensive and consumes more power than DRAM and is only used for the caches (both on and off the CPU chip).

DRAM or dynamic RAM is a simpler, slower and less stable memory module. Each cell in DRAM is stored as a charge in a small capacitor coupled with a single transistor. The charge in the capacitor decays rapidly within 10 to 100 ms and hence every bit of DRAM has to be refreshed (read and written back) within 10 ms! However, as the CPU operates in nanoseconds, this is not as bad as it sounds. Sometimes, DRAM has additional bits to encode error checking bits and error-checking circuitry (does this free the CPU of the responsibility of refreshing the bits then?). DRAM is also very sensitive to electrical noise, voltage fluctuations and even light. In fact, this last property is used to make digital cameras where the film is nothing but a bunch of super-sensitive DRAM cells.