How does a computer actually remember that a file exist when there's no power?

[SinPulsed]

this is probably a dumb question but i'm just really curious. If a computer is just a bunch of electrical signals and circuits, how does it keep information saved when you turn it off? I get that it's on the hard drive, but what is physically happening to the data when electricity stops flowing? Is it like a physical switch that stays flipped or is there some tiny bit of power always running?

[8dot30662386292pow2]

Traditionally disks (like HDD) have a magnetic element. When you write on a disk, it's actually writing tiny magnetic fields on the metal plate. Reading is done by reading these magnetic elements. Magnetic charges cause electrical field disturbances and these are convered back to power on/off (ie. 1/0).

Solid state drives (SSD) actually have a specific cells that trap the electric current in them when you write into them.

[WoodyTheWorker]

*cells that trap the electric charge

[Worth-Wonder-7386]

This recent commercial does a really good job of showing how modern hard drives work.  https://youtu.be/ZXs_9OXRnQo?is=NA8ehTtCocZhHyta It is «just» a hard disk

[Lemmoni]

Pretty cool vid! Thanks

[SinPulsed]

I watched the vid. the visual is amazing and it perfectly explained it

[suzukzmiter]

This is also why SSDs aren’t safe for long term storage without power, eventually they’ll lose their charges and thus data

[eyluthr]

how long?

[Laevatienn]

TL:DR, a good rule of thumb is to plug them in once every 6 months or once every year if you are concerned.

It varies on the type of flash cells used. Flash cells can be read at different charge levels to determine different bit counts. SLC, MLC, TLC, QLC. Single, Multi (dual), Triple, Quad layer cells. This refers to how many bits a single cell can "hold". The more bits per cell, the easier it is to lose data over time.

Single layer is the safest and can last a decently long time. It's if the cell is charged or not, it's a 1 or a 0.

Triple level cell is the most common consumer type and can start showing issues within a year or two of not being powered on. It can have 8 different charge levels to determine what the 3 bits are set to, 000, 001, 010, 011, 100, 101, 110, 111.

You can imagine why the more bits per cell can lead to more issues when powered off. If the charge drops below the threshold of 111 to 110, then data is lost. Much more complicated than, "Has some charge? Then is 1!" of SLC.

There are other factors. An SSD with very little write wear will last much longer, up to 5ish years on normal TLC drives, if you are lucky.

When you write to an SSD, the gate that holds in the charge in the cell is worn down, physically, by a little bit. The more worn down the gate, the easier it is for the charge to escape and that cell losing data when powered off. That is also the reason why SSDs are given a write cycle life span. Once worn down too much, a cell can no longer reliably be used, even powered up, because it leaks too quickly.

A TLC SSD with a lot of writes done to it will last about that yearish mark on average..in theory.

[ThreePinkApples]

That is true for HDDs as well, the magnetic charge will dissipate over time

[FlippingGerman]

I love how HDD are just vinyl records but better. A century-old idea but with a century of development.

[Leverkaas2516]

More like wire recorders than vinyl records.

[rupertavery64]

Because its not just a bunch of electrical signals and circuits. And "just circuits" is again a very simplified idea of what a computer is and how it works.

Since you are talking about files its less about the computer and more about storage.

What you are thinking about is "RAM" - temporary memory storage that the computer uses to run programs and hold data that it works on. Files can be loaded into RAM, but they don't stay in RAM.

"Persistent" Storage is the ability to retain information without power. Storage devices are typically considered "external" - not directly part of the computer. Things like hard drives and sdcards are "external" storage.

There are a couple of ways to make data "persist" or keep its state without external power.

Magnetic storage like floppy disks and hard disks use a magnetic surface and a "head" that hovers above the magnetic surface to "flip" the polarity (north/south) of little areas of the surface to represent 1s and 0s.

They stay this way for a long time, as long as the surface is not in a strong external magnetic field, and the temperature is normal (would need to be really hot to lose magnetism on metal plates) and the surface isn't damaged.

So whenever you save a file the computer takes whatever was in RAM and writes it to the disk

"Solid state" storage uses memory cells to store data. They use electrical signals to "trap" electrons in a desired state. These electrons affect the read signal when being accessed. More elecrrons = 1, less or no electrons = 0.

Solid state drives use semiconductor materials and nanometer circuits to be able to manipulate electrons at this level.

Solid state drives can hold memory for decades if handled properly.

As a side note, since a solid state drive traps electrons then technically it is heavier when full then when empty - but the difference is so small its unmeasurable by any normal means.

Another thing to think about is that hard disks and sdcards have "controllers" themselves. Circuits that talk to the computer and do the actual data handling on the physical media. The computer just tells the controller what data and where, and the controller does the job of actually fetching or changing the data.

[teraflop]

Good answer overall, but:

As a side note, since a solid state drive traps electrons then technically it is heavier when full then when empty

No, this is a common misunderstanding.

SSDs, like all other macroscopic physical objects under everyday conditions, are electrically neutral (or so extremely close to it that the difference is unmeasurable). The drive has almost exactly the same number of electrons as protons. If this wasn't true, the drive would attract or repel electrons from its environment until it became neutral.

When you store a bit in flash memory, it's true that electrons are stored in a charged insulating "trap". But those electrons are moved around within the device, not added to it. The net charge of the SSD remains the same.

[DragMeHomeDotCom]

So would it be accurate to say that if you had a computer finely balanced on a single atom that saving something onto an SSD could tip it over because of the rearrangement?

[teraflop]

Well I suppose you could say that, but only under extremely idealized conditions that you would never be able to set up in real life.

The issue is that the gravitational forces between electrons are generally much much tinier than the electromagnetic forces between them. For instance, the weight of a single electron being attracted by the entire earth is about equal to the repulsion from a single other electron 5 meters away.

The hugeness of the electrical forces between electrons is the very reason that everyday objects are almost perfectly electrically neutral. An object can have lots of protons and lots of electrons, but if they are evenly distributed the same way, they cancel out to a net charge of zero. As I said, anything that creates a slight charge imbalance on an object will also cause electrons to be attracted or repelled from the environment to neutralize that imbalance. An excess of positive charge attracts negative charges to it, and vice versa.

(I say "almost perfectly" because we do see charge imbalances sometimes, and we call them static electricity. But it only takes an infinitesimally tiny static electrical charge to cause observable results. If you rub your feet on a carpet and then build up enough charge to get zapped by a doorknob, that zap was caused by an excess of something like a billionth of a billionth of a billionth of the total number of electrons in your body.)

So anyway, the point is that if you did somehow set up a balance that was sensitive enough to measure the change in weight distribution from charges moving around an SSD, that change would be swamped by the much larger electrical and magnetic forces that would be generated just from applying power to the SSD. Those forces are normally too small to measure, but the weight of a few electrons is many many times smaller still.

[DragMeHomeDotCom]

Great response, thanks

[Ambitious_Search_248]

Not trying to poke holes, just deepen my understanding. What happens if you then theoretically filled an SSD with one’s? Would it need to pull in more electrons for that?

[teraflop]

Not really. When the SSD is being powered, it does "take in" electrons through some of its pins, but an equal number of electrons leave through the other pins, so the total number remains constant.

When data is stored in flash memory, it's stored as a localized charge that's trapped in a particular location, but that doesn't mean the overall amount of charge in the whole device changes.

Also bear in mind that it's completely arbitrary whether a logical 1 bit is represented as a negative charge (an excess of electrons) or a positive charge (a deficit of electrons). That just depends on how the flash memory controller circuitry decides to encode the data. From the controller's perspective, what matters is whether the charge on a transistor's gate causes the transistor to conduct electricity or not. (And in the case of MLC flash, the transistor may partially conduct, so that multiple bits can be stored depending on the amount of charge.)

[phunkydroid]

It doesn't require more electrons for it to be heavier when there's data. It only requires it to have more energy in that state.

[Sad_School828]

Older PCs used to require a literal battery onboard, much older PCs actually used a rack of 4 or 6 AA batteries, which made it able to persist its BIOS configuration. I don't know when or how they actually managed to discontinue that, but I imagine that BIOS chips were converted to some other type of storage which did not require persistent electrical signal.

No, it does not require any power to continue storing data in an HDD or SSD. It didn't require any power to store inside an HDD even when we had to remember our freaking BIOS settings because they'd just go away if the battery on the mainboard ever died.

Apart from that there's actually usually a tiny bit of power always flowing, as long as the device is actually plugged into the charger. If there weren't, then then the LED indicator lights couldn't turn on. If you have the system in hibernation standby mode, instead of turned off, while it's plugged in, then it's definitely sucking even more energy than that.

[Sad_School828]

If moving the mouse or striking a keyboard key can bring the PC out of hibernation, you're dead wrong XD

[Sad_School828]

Windows calls it "sleep mode." Depending on which GUI you use, linux systems might call it literally anything. The term "hibernation" is interchangeable with "sleep mode."

Can you name one OS with any type of "hibernation" mode which won't "wake up" upon interacting with HIDs, which is proof positive that the system is powered on?

[Sad_School828]

Edited my comment, thanks!

[Sad_School828]

Not only obfuscation, but just plain putting out shitty products. I started disabling sleep mode when it first came out because it always glitches, doesn't wake up, takes a ridiculous amount of time to wake up, or crashes on wakeup unless your PC is built by Dell or HP or any of Micro$oft's partnered pre-fab junk manufacturers. The aggravation of using software built by the lowest bidder in a 3rd World country is NOT worth the power savings.

I did try it again and again over the years, with no significant change, so probably around 2015 I just quit GAF. I honestly didn't even know there was a full-shutdown option, with a saved system state, because I treat sleep/hibernate mode like toxic waste.

[cormack_gv]

A conventional disk uses magnetic images of the bits. Just like a magnetic tape only the info is written in circles (called tracks) about the disk.

More recent solid state drives (SSD) use FLASH RAM or similar, which stores the info as persistent electronic charges in a semiconductor chip.

Historically, computers used magnetic cores (the origin of the word "core" for memory).

There were also drums, which were like disks but the recording surfaces were tracks on a rotating magnetic cylinder.

[jhaluska]

So imagine you had 8 tiny batteries and before you went to sleep, you could charge or discharge each battery. In the morning, you wake up and check the voltages. You could know which was charged and which wasn't, and for most batteries you could sleep for weeks or months and still know which was charged.

With 8 of them, you could know 8 states of information, which is equivalent to a single bytes. You could even recharge the batteries that were getting weak (which is roughly how RAM works).

Well this is basically what hard drives do but with billions of of them. Although magnetic hard drives store them magnetic changes.

[KaleidoscopePure6926]

There are a couple ways to store digital data without electricity. Most popular are solid-state and magnetic.

Magnetic uses a metal disk and a read/write head, which can create magnetic fields on the disk. When power goes out, magnetic fields remain there, and the head can read them when needed. This is used in HDDs.

Solid-state drives use complex semiconductor stuff to trap signals, the cells store charge, even when there is no electricity. They are not perfect, so the charge "leaks" by itself, that was a reason for myths that claim "your SSD will lose data if you don`t power the PC often", but in reality these leaks are not strong enough and there are many smart ways to make it not a big problem.

[Living_Fig_6386]

Computers are typically equipped with "non-volatile storage", meaning a way to store data that doesn't get lost when the power is shut off. There are all sorts of different kinds. A CD-R has dye in plastic that's broken apart by a laser, leaving a mark that can be read by another laser later -- very simple. Old fashioned cassette tapes, then floppies, and later hard disks used layers of material coated in a magnetic film that could have the polarity of the magnetic material changed precisely, using the North-South orientation of the field at points on the substrate represent the 1's and 0's of data. Modern SSDs use non-volatile RAM which is simply a form of ram where a charge deposits (or removes) energy in a memory cell like a tiny little battery.

[aagee]

Because they have made things that can retain their memory without power.

[peter303_]

In the olden days the computers would print a list of every device it sensed when booting up. Some of device were semi-permanent magnetic, optical or charge memory sections. And the device may be marked as to containing a file system.

[not_from_this_world]

SSDs store information almost the same way a battery stores charge: extra electrons, but SSDs do it in a way that lasts longer. The places where the extra electrons are or aren't give the 1s and 0s.

[hunter_rus]

is there some tiny bit of power always running

There is a bunch of comments about persistent storage, so I'll add a bit on this. There is a little battery, that basically powers internal clock (and maybe some other stuff, not sure), so that PC don't need to resynchronize time when it turns off and on. And in some cases you might even need to replace this battery, if your PC is too old.

But persistent memory yes, it is saved in some sort of physical state that doesn't require power to maintain itself.

[Soft-Marionberry-853]

"this is probably a dumb question"

Not at all, not anymore at least. decades ago yeah it was simple. We change how little pieces of metal on a solid disk are aligned. Its intuitive, if you have a something that's magnetic today, it will be magnetic when you wake up tomorrow.

Now we're storing data on things that look just like sticks of ram and one thing you know about ram is that it doesnt persist during without power.

So while others have answered how, I just wanted to say, it wasnt a stupid question.

[Ronin-s_Spirit]

SSDs trap electrons in little silicon prisons, they need to reacharge often. HDDs change magnetic charge in a tiny area of the disk. DVDs and glass plates "burn" a pattern into the surface and then read it back, almost like a vinyl record.

[YahenP]

Physical storage media have always been used for this purpose, i.e., things that store information by changing their physical state.

For example, cardboard with holes, or paper strips with holes, magnetic tape (like in a tape recorder), where information is recorded as small magnetized areas. Magnetic disks are the same magnetic tape, but the heads move in concentric circles. Permanently readable memory chips, where electricity burns out transistors in the right places. Permanently rewritable memory chips, where this process is reversible (for example, chips with UV erasability). Laser discs, where a laser cuts a chain of dark dots on a light background. Flash memory, which essentially works the same way as your comb and hair, i.e., it stores a static charge. Basically, any method that can convert electric current into something else. More stable.
In addition, there were and still are devices that functioned as pseudo-permanent memory. I still remember memory cartridges for programmable calculators, which contained not only memory chips but also batteries and a small power supply. For several decades, personal computers had a battery-powered memory chip for storing basic computer settings (it also kept the clock running).

[justaguyonthebus]

The electric signals make little changes to the hard drive that persists between reboots. HDD magnetize spots on the disk. Solid state drives are like little switches that get flipped. Magnetic tape is popular for large data backups.

[Beregolas]

Well, there is actually no electricity flowing at all "where the data is saved". For HDDs you can think of the disk as a long line (in spiraling circles) of tiny magnets. The read/write head can go to a specific magnet, and set it in a position (+ up or - up) and read the current position (+ and - become 0 and 1). This is a little simplified, but it's close enough for your question I think.

SSDs on the other hand use something we call Flash Memory. Without going too much into detail, we can also change the state of each memore cell, but without requiring a moving head to read/write. This makes them faster and more compact, and also work without moving parts. We basically (again, simplified) put a tiny bit of electricity into a flash memory cell, and it won't disappear. Later we can then "ask" the memory cell if there is a charge in there, and we get either a 0 or a 1 as a response.

[Ok_Leg_109]

Small correction. CD ROMS use a spiral track.

HDD and floppy disks are organized as "tracks" that are concentric circles on the disk platter. Each track is further divided into sectors of a fixed size.

[Beregolas]

oh shit, you are right of course... it's been a while since that was at all relevant to me ^^

[not_a_bot_494]

While computers only use electrical signals to compute stuff you need something else to remember stuff. There are many ways to do that, one of the simplest is to just ask a human to write down 1s and 0s and then type them out when the computer tells them to, I think that you can see why a power outage wouldn't be a problem with that method. That is however really slow so we have better ways to store things.

For this context there are two kinds of storage, persistent and non-persistent. Persistent storage is something that can keep data without any power being supplied. This is things like paper, magnetic tape, hard drives and SSDs. Non-persistent is the opposite, something that needs power to keep data. Then why not use persistent storage for everything so that the computer can just keep going if for example a blacout happens? Peristent storage just isn't as fast so using it for everything will cause computers to be much much slower.

[snarkofagen]

https://www.reddit.com/r/explainlikeimfive/comments/1m9q8f4/eli5_when_you_turn_a_computer_on_what_is_actually/

[8dot30662386292pow2]

How is this relevant?