Every 7.8μs your computer’s memory has a hiccup


Modern DDR3 SDRAM. Source: BY-SA/4.0 by Kjerish
During my recent visit to the Computer History Museum in Mountain View, I found myself staring at some ancient magnetic core memory.

Source: BY-SA/3.0 by Konstantin Lanzet
I promptly concluded I had absolutely no idea on how these things could ever work. I wondered if the rings rotate (they don't), and why each ring has three wires woven through it (and I still don’t understand exactly how these work). More importantly, I realized I have very little understanding on how the modern computer memory - dynamic RAM - works!

Source: Ulrich Drepper's series about memory
I was particularly interested in one of the consequences of how dynamic RAM works. You see, each bit of data is stored by the charge (or lack of it) on a tiny capacitor within the RAM chip. But these capacitors gradually lose their charge over time. To avoid losing the stored data, they must regularly get refreshed to restore the charge (if present) to its original level. This refresh process involves reading the value of every bit and then writing it back. During this "refresh" time, the memory is busy and it can't perform normal operations Continue reading


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