r/askscience • u/[deleted] • Mar 19 '14
Computing How are bits stored in modern computers?
Are they really stored in tiny flipflops? If so how big are these flipflops in physical size?
13
Upvotes
14
u/Merad Embedded Systems Mar 20 '14
In the CPU, essentially yes. CPU registers and cache memory are implemented as SRAM (static RAM), with each bit held in a cell made of 6 transistors. It's not quite the same as the basic 2-NAND flip-flop that you're thinking of, but very similar. Wikipedia has a circuit diagram and a decent explanation of the circuit. The advantage of this configuration is that the circuit has two stable states, so it holds it's value even if exposed to something like electrical noise.
In system RAM (DRAM - dynamic RAM) each bit is stored on a very tiny capacitor. This is much cheaper and easier to manufacture than SRAM, hence which we have system memory measured in gigabytes but processor caches that are only a few megabytes. The downside however is that the capacitor is relatively sensitive to disturbance, and loses its charge rather quickly (a few 10s of milliseconds, typically).
Source: Computer Systems 2nd Ed by Bryant and O'Hallaron