What does it mean when Dynamic RAM needs to be refreshed and why does it need to be refreshed?

[NonTechSavvySheep]

What sort of things DRAM do when refreshing? How does it work?

Also, what are the main reasons that DRAM is slower and far less expensive than SRAM?

 

[shrapnel_indie]

What sort of things DRAM do when refreshing? How does it work?

Also, what are the main reasons that DRAM is slower and far less expensive than SRAM?

Dynamic RAM works like a capacitor in a way (simplified view.) It discharges over time. Due to this process it needs refreshed every so often to recharge a bit value of 1 before it drops too low for it to be recognized as such. To be efficient the refresh process will access multiple memory cells at a time, otherwise it would never be able to keep up and also get in the way of normal operation.

For a more detailed explanation: http://computer.howstuffworks.com/ram1.htm

DRAM is slower because it takes time to access those cells. Unlike static RAM which is organized a bit differently and is thus faster to access at the cost of using more power. The makeup of the two is what also keeps SRAM more expensive than DRAM, SRAM uses all transistors in such a way that when a bit is a 1 or a 0, it doesn't change as long as it has power. The use of both capacitors and transistors in DRAM makes it cheaper, but as I already said, needs refreshed every so often to keep its 1s and 0s as 1s and 0s respectively. If the voltage of the RAM enters a certain level range, the RAM doesn't know if it is a 1 or a 0, and therefore is considered corrupted.

More info on SRAM: http://computer.howstuffworks.com/ram2.htm

Another comparison: http://computer.howstuffworks.com/question452.htm


For a larger comparison, in a summary form: http://computer.howstuffworks.com/ram3.htm