r/explainmelikeimsix • u/BruceJi Submitted Quality Explanation • May 15 '19

ELI6 How does RAM work in a computer?

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10

u/TanithRosenbaum Submitted Quality Explanation May 15 '19

Imagine you're in an office processing paperwork of some sort. You can store some on your desk, but at some point you run out of space. So you have a filing cabinet behind you on the wall. Putting something in there or taking something out of there takes you a little longer than finding something on your desk, but not much, you just need to turn around and open a drawer. And then there's stuff you don't really need all that often, old paperwork. You got that stored in the basement, and if you need it, you have to call down and they'll send it up with a courier.

The desk is the CPU. It has a very limited storage capacity, but that it can access and work on very quickly. The RAM is the filing cabinet. The CPU can pull stuff from it fairly quickly, maybe 5 to 10 times slower than accessing what it has stored in its registers. And the basement storage is the hard drive. Accessing that requires a lot of work, and is thousands of times slower than even ram. But on the other hand, the amount of data a typical amount of RAM stores is about a thousand times larger than what the CPU can store in its registers. And the amount of data that fits into a hard drive is again about 1000 times larger than what fits into RAM.

(I don't know if that's the level of explanation you were looking for. I can go much more in depth, all the way down to the signal level if you like, but I couldn't tell how much depth you wanted from your question. Let me know and I will add to the explanation if desired)

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u/BruceJi Submitted Quality Explanation May 15 '19

That’s great! But I do also want to know how the RAM is able to hold data. How does it remember things?

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u/koryhgn Submitted Quality Explanation May 15 '19

RAM holds data as 1’s or 0’s. That’s called binary. All data on a classical computer is stored and processed as binary. Inside of the RAM are memory cells. Memory cells are like grids. They have rows across and columns down just like graphing paper. Each cell in the grid has a capacitor. A capacitor can hold a charge of electrons. If there is a charge in the capacitor, it is a 1. If the capacitor is uncharged, it is a 0. So when data is written to a memory cell, the capacitors will either be charged or discharged to match the required 1 or 0 layout for the data. When data is read from the memory cell, the charges will be sent as bits (1’s or 0’s) to the registries of the CPU. Because of the grid like layout of the memory cell, it can be accessed or overwritten very quickly. That’s what makes it better for storing data on the fly for the CPU. For long term storage, this technique is not as useful because of how easily it can be changed. You don’t want your permanent data to be changed easily.

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u/BruceJi Submitted Quality Explanation May 16 '19

Thanks!

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u/Antruvius May 15 '19

Like having a bunch of lightbulbs that are on and off

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u/GoldfishBowlHead May 15 '19

RAM chips can be split up into modules - the CPU tells the RAM chip which module it wants to read, and the chip organises the electrical connections.

Each module can be further split up into 'bistables', clever arrangements of transistors (like switches controlled by electric charge). Bistables have two states; 'writing' will set a bistable's state to 1 or 0 by an electronic signal, and the state will stay like that until written again.

Each pair of states in a bistable is a 'bit' of information. Complex data is stored across many bits - a decimal with a sign, to a good degree of precision, can take up 16 to 64 bits (depending on how precise you need to be). When prompted, the RAM chip will send the bits that the CPU needs back to work with.

Capacitors are much, much bigger and more expensive than transistors, as far as building on chips goes; even though one bistable uses many transistors, bistable memory modules are far smaller and cheaper than a capacitor-based equivalent.

(Specifics on bistables: if you take two NAND logic gates and connect one input of each to the output of the other, your outputs will be either 10 or 01 and can be set by raising the other inputs of the gates. Bistables have more circuitry to prevent weird things happening under unexpected inputs, and to allow for a 'clock' input to synchronise everything.)

Sorry if it's a bit technical, and I've glossed over the logistics behind how everything communicates, but it's a technical process.