2013: Advent Computing: Memory: what matters

by on December 15, 2013

We know our computers store numbers, but the only way we’ve talked about how to store those numbers was that infinitely long piece of tape the Turing machine had. I want to spend a bit of time talking about how computers actually store data.

Before we can compare the different methods, it’s worth first talking about the many different things it’s worth comparing.

First is permanence. If my computer writes a number to some memory, will it still be there some time later? The normal reason why this might not be the case is that some types of memory require constant power to keep their number stores. One example of this is the RAM in your home PC, another is anything stored in the old vacuum tubes.

Another is writability: some kinds of memory can only be written to once, after which they can never be changed again. CDs and DVDs are the obvious examples, but storing data with holes punched in card is also a special case, since you can always add more holes but removing them again is a lot more tricky.

Next up is relatively obvious: price. How much does it cost me per byte of storage, or per megabyte or per petabyte. Price can be exceedingly variable, because we’re constantly finding new ways to manufacture drives more cheaply.

One we don’t think of a lot for personal computers but is utterly vital for more industrial data storage is density: how much physical space does each byte take up? The progress we’ve made on this is quite remarkable; only a few years ago the idea of storing several terabytes of data in something the size of a laptop was unthinkable; now it’s reasonably trivial.

We also need to consider reliability. If my computer writes a “1” to some bit of memory, what are the chances that when it reads that memory at a later date, it’ll mistakenly read a “0”? Will it at least be able to tell me that the memory has changed? Magnetic disk failures are one of the most common ways for PCs to fail, so finding something more reliable is an easy way to improve the life of your computer.

Finally, it can be useful to think about speed. For some applications, such as your computer’s store for calculations it’s performing at the time, the computer needs to be able to read any part of the memory as fast as it is performing those calculations – billions of times per second. For long term data backup, it may be acceptable for data to take hours to be read or written.

Tomorrow, we can start talking about the different kinds of memory and the different advantages they offer us.

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