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If computer RAM was to be non volatile like other persistent storages, then there would be no such thing as bootup time. Then why is it not feasible to have a non volatile ram module? Thank you.

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This question deserves a full answer but T think non-volatile memory is much slower. –  Kwaio Aug 30 '13 at 10:06
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What made you think it does have to be volatile?? It wasn't 40 years ago. –  Daniel R Hicks Aug 30 '13 at 11:30
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RAM is volatile not because it have to be volatile, it's because the technology it used is volatile. –  Alvin Wong Aug 30 '13 at 13:32
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@jhocking because no non-volatile technology of comparable performance is available. –  Dan Neely Aug 30 '13 at 20:33
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Assume I ask this 2 years from now: Why can't you remember what the last flavor of soda that you drank was before you asked this question? –  Erik Reppen Aug 31 '13 at 2:51

9 Answers 9

up vote 107 down vote accepted

When most people read or hear "RAM", they think of these things:

Two SDRAM sticks, courtesy of Wikipedia

Actually these are made of DRAM chips, and it's controversial if DRAM is a kind of RAM. (It used to be "real" RAM, but technology had changed and it's more of a religious belief if it's RAM or not, see discussion in the comments.)

RAM is a broad term. It stands for "random access memory", that is any kind of memory that can be accessed in any order (where by "accessed" I mean read or written, but some kinds of RAM may be read-only).

For example HDD isn't a random access memory, because when you try to read two bits that aren't adjacent (or you're reading them in reverse order for whatever reason) you have to wait for the platters to rotate and the header to move. Only sequential bits can be read without additional operations in between. That's also why DRAM can be considered non-RAM - it's read in blocks.

There are many kinds of random access memory. Some of them aren't volatile and there are even read-only ones too, for example ROM. So non-volatile RAM exists.

Why don't we use it? Speed isn't the biggest problem as for example NOR Flash memory can be read as fast as DRAM (at least that's what Wikipedia says, but without citation). Write speeds are worse, but the most important issue is:

Because of the inner architecture of non-volatile memory, they have to wear out. The number of write-and-erase cycles is limited to 100,000-1,000,000. It looks like a great number and it's usually sufficient for non-volatile storage (pendrives don't break that often, right?), but it's an issue that already had to be addressed in SSD drives. RAM is written way more often than SSD drives, so it would be more prone to wearing.

DRAM doesn't wear out, it's fast and relatively cheap. SRAM is even faster, but it's also more expensive. Right now it is used in CPUs for caching. (and it's truly RAM without any doubt ;) )

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+1 for being with the 0.1% of people rightly stating ROM is also RAM ! (stating D-RAM is not RAM is a little extreme though ...) –  jlliagre Aug 30 '13 at 11:04
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But the original disk drives were referred to as "RAM" (since the other alternative was tape). If history determines precedence, DASD (what you young'ins refer to as HDD) is definitely RAM. –  Daniel R Hicks Aug 30 '13 at 11:33
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@DanielRHicks That's interesting. Maybe "RAMiness" isn't binary: DRAM is less random than SRAM, HDDs are less random than DRAM and so on. –  gronostaj Aug 30 '13 at 11:44
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if you call random access any memory where accessing a random spot takes only O(1) time in terms of size regardless of the current state then DRAM is random access, a HDD has access in O(#tracks+rotation_time) which varies for size –  ratchet freak Aug 30 '13 at 14:45
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"RAM" was I believe (I can't find a good reference) derived in opposition to sequential memory (magnetic or paper tape; mercury delay lines) which could only be accessed in order. Meanwhile, I found a digression on terms for "RAM" in other languages: smo.uhi.ac.uk/~oduibhin/tearmai/etymology.htm which emphasise different aspects of the RAM/ROM difference. –  pjc50 Aug 30 '13 at 15:35

DRAM is fast, can be built cheaply to extremely high densities (low $/MB and cm2/MB), but loses its state unless refreshed very frequently. Its very small size is part of the problem; electrons leak out through thin walls.

SRAM is very fast, less cheap (high $/MB) and less dense, and does not require refreshing, but loses its state once the power is cut. The SRAM construction is used for "NVRAM", which is RAM attached to a small battery. I have some Sega and Nintendo cartridges which have decades-old save states stored in NVRAM.

EEPROM (usually in the form of "Flash") is non-volatile, slow to write, but cheap and dense.

FRAM (ferroelectric RAM) is one of the new generation storage technologies that's becoming available that does what you want: fast, cheap, nonvolatile...but not yet dense. You can get a TI microcontroller that uses it and delivers the behaviour you want. Cutting power and restoring it allows you to resume where you left off. But it only has 64kbytes of the stuff. Or you could get 2Mbit serial FRAM.

"Memristor" technology is being researched to deliver similar properties to FRAM, but is not yet really a commercial product.


Edit: note that if you have a RAM-persistent system, you either need to work out how to apply updates to it while it's running or accept the need for the occasional restart without losing all your work. There were a number of pre-smartphone PDAs which stored all their data in NVRAM, giving you both instant-on and the potential instant loss of all your data if the battery went flat.

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Yay memristor technology, it will be at least 10 yrs or more before we see cool products based on these "new" devices. But they should hold a ton of promise for memory implementations. –  Chris O Aug 30 '13 at 16:14
    
DRUM is fast, but not very dense, and the cost per character is high. (What?? DRAM??? Never mind.) –  Daniel R Hicks Aug 30 '13 at 17:22
    
NVRAM is not the same as battery backed SRAM. NVRAM has a capacitor per bit that can be sufficiently insulated that any charge does not leak away, but can also be sensed, and programmed. The bit cell structure is fairly large, and in some technologies involved more exotic fab steps, so NVRAM is a low density high cost technology. But it also has very long storage lifetime. CMOS SRAM draws very little power when idle, and so backing it up with a battery is cost effective. The once common PC "CMOS" device is one example. –  RBerteig Aug 30 '13 at 19:24
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SRAM+battery assembly is not a true NVRAM. True NVRAM built on EEPROM. –  user539484 Aug 30 '13 at 23:13
    
@RBerteig: My understanding is that an NVRAM is a marriage of an SRAM with a non-volatile store and a large enough energy storage medium to allow the SRAM to be copied to the non-volatile store without extermal power. If the SRAM and non-volatile store were in separate chips, transferring one to the other would take awhile (and consume a lot of energy). Marrying them together allows the transfer to occur much faster. –  supercat Sep 1 '13 at 20:27

To answer the question- It does not!

Non-volatile random-access memory From Wikipedia, the free encyclopedia Non-volatile random-access memory (NVRAM) is random-access memory that retains its information when power is turned off (non-volatile). This is in contrast to dynamic random-access memory (DRAM) and static random-access memory (SRAM), which both maintain data only for as long as power is applied. The best-known form of NVRAM memory today is flash memory. Some drawbacks to flash memory include the requirement to write it in larger blocks than many computers can automatically address, and the relatively limited longevity of flash memory due to its finite number of write-erase cycles (most consumer flash products at the time of writing can withstand only around 100,000 rewrites before memory begins to deteriorate). Another drawback is the performance limitations preventing flash from matching the response times and, in some cases, the random addressability offered by traditional forms of RAM. Several newer technologies are attempting to replace flash in certain roles, and some even claim to be a truly universal memory, offering the performance of the best SRAM devices with the non-volatility of flash. To date these alternatives have not yet become mainstream.

Source: NVRAM wiki page

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Actually, RAM doesn't, strictly speaking, NEED to be volatile, but for the sake of convenience we generally make it that way. See Magnetic Ram on Wikipedia (http://en.wikipedia.org/wiki/Magnetoresistive_random-access_memory) for one potential non-volatile RAM technology, though one still in need of further development for practical use.

Basically, DRAM's advantage is size. It's a tremendously simple technology which has very fast read-write characteristics, but as a consequence, is volatile. Flash Memory has OK read characteristics, but is TREMENDOUSLY SLOW compared to what's needed for RAM.

Static RAM has extremely favourable read-write characteristics, and is quite low power, but has a large component count compared with DRAM, and is hence much more expensive. (Bigger footprint on silicon = more failures + lower chip counts per die = more cost) It's also volatile, but even a small battery could power it for some time, making it a kind of psudo-NVRAM if it weren't for the cost issue.

Whether it's MRAM or some other technology, it's likely that at some point in the future, we will find a way around the current need for tiered memory structures which slow down computers, we're just not there yet. Even once that era arrives however, it's likely we'll still need some variety of long term reliable (read: SLOW) storage medium to archive data.

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Deep down it's due to physics.

Any non-volatile memory must store its bits in two states which have a large energy barrier between them, or else the smallest influence would change the bit. But when writing to that memory, we must actively overcome that energy barrier.

Designer have quite some freedom in setting those energy barriers. Set it low 0 . 1, and you get memory which can be rewritten a lot without generating a lot of heat: fast and volatile. Set the energy barrier high 0 | 1 and the bits will stay put almost forever, or until you expend serious energy.

DRAM uses small capacitors which leak. Bigger capacitors would leak less, be less volatile, but take longer to charge.

Flash uses electrons which are shot at high voltage into an isolator. The energy barrier is so high that you can't get them out in a controlled way; the only way is to clean out an entire block of bits.

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Great answer! You actually answered the why of it and in an easy to understand way no less. –  Synetech Aug 30 '13 at 16:12
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The accepted answer doesn't actually answer the question, whereas this one does. –  Mark Adler Aug 31 '13 at 17:01
    
You probably avoid mentioning this because it's too "deep down in physics", but I'd like to say that the barrier is less about energy than entropy. SRAM has even smaller capacitors than DRAM and yet doesn't leak, because it uses field-effect transistors instead of resistors – which, vaguely speaking, bypass interference from thermal noise via an externally supplied voltage threshold. Only a few die shrinks into the future will we reach another type on interference – quantum tunnelling – where an actual energy barrier will be the only way to preserve classical information. –  leftaroundabout Sep 2 '13 at 21:44
    
@leftaroundabout: SRAM doesn't have capacitors at all, except parasitic and perhaps some research designs. –  MSalters Sep 3 '13 at 6:50
    
@MSalters parasitic or not, any FET has a capacitance. My point is, it's not actually necessary to store much energy in a capacitor, or an inductor or anything, to preserve information over a long time. –  leftaroundabout Sep 3 '13 at 7:03

It should be noted that the first commonly-used "main store" in computers was "core" -- tiny toroids of ferrite material arranged in an array, with wire running through them in 3 directions.

To write a 1 you'd send equal strength pulses through the corresponding X and Y wires, to "flip" the core. (To write a zero you wouldn't.) You'd have to erase the location before writing.

To read you'd try to write a 1 and see if a corresponding pulse was generated on the "sense" wire -- if so the location used to be a zero. Then you'd of course have to write the data back, since you'd just erased it.

(This is a slightly simplified description, of course.)

But the stuff was non-volatile. You could shut down the computer, start it up a week later, and the data would still be there. And it was most definitely "RAM".

(Before "core" most computers operated directly off a magnetic "drum", with only a few registers of CPU memory, and a few used stuff like storage CRTs.)

So, the answer as to why RAM (in it's current, most common form) is volatile is simply that that form is cheap and fast. (Intel, interestingly enough, was the early leader in developing semiconductor RAM, and only got into the CPU business to generate a market for their RAM.)

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Were core-based computers typically designed so that after an unexpected power failure they could (when power was re-applied) resume operation where they left off? My conjecture would be that if one performed a "shutdown" procedure one could have a system save everything of interest into core and then start executing NOPs until power was removed; if one used the proper procedure when restarting, one could then restore the system state. Do you know if systems typically had a means of autonomously triggering a shutdown procedure if external power was lost? If a core-based system were... –  supercat Nov 2 at 16:08
    
...to cease functioning due to power failure and didn't get a chance to finish up any operations that were in progress before power was lost completely, I would expect that whatever unit of memory was being acted upon would be lost; further, since I would expect that program counters, sequencers, etc. would not be kept in core memory, the contents of those would be lost as well. –  supercat Nov 2 at 16:12
    
@supercat - There were a wide variety of designs. Mainly the effort centered around maintaining the integrity of the file system, so crash recovery was most likely to try to find file operations that were in progress and complete those. But I'm remembering that it was fairly common to detect a power failure and stash the CPU registers. –  Daniel R Hicks Nov 2 at 19:33
    
If the memory is being used as a file system, I would expect that code could ensure that it would always be a in a valid state, such that any interrupted operation could be either rolled back or run to completion. On the other hand, by my understanding core memory was often used not because it was non-volatile, but rather because it was cheaper than any alternatives, so I'm curious to what extent designers took advantage of non-volatility or just ignored it. –  supercat Nov 2 at 19:37
    
@supercat - They took advantage of it quite often (and hence, eg, file systems were less robust than one would have liked for volatile RAM). Not that it was a big "selling point", but it was there, so why not? –  Daniel R Hicks Nov 2 at 19:43

Mainly because of catch-22. If your DRAM (as said already, RAM is very broad term. What you are talking about is called DRAM, with D for Dynamic) suddenly become non-volatile, people will call it NVRAM which is very different type of storage.

There is also a practical reason, currently no NVRAM (I mean true EEPROM-based NVRAM, with no power source required) types exists which allows an unlimited number of writes without hardware degradation.


Regarding DRAM-based mass storage devices: take a look at Gigabyte i-RAM (note the rechargeable Li-Ion battery, which makes it non-volatile for a while)

a

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  • Large capacity memories need small individual memory cells. A simple capacitor, which holds a 1 charge or a 0 charge can be me much smaller than complex logic in non volatile ram & faster.

  • Refilling the amount leaked is a hardware independent cycle. This logic is made in such a way that the processor is normally unhindered.

  • Power down on the other hand stops the refreshing. So yes, a total reload is needed, on boot or hibernation.

  • Larger capacity for the same size, wins the vote.

8GB ram = 8.589.934.592 bytes x 8 bits = 68.719.476.736 bits (cells - no parity)

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IMO the main problem here is indeed volatility. To write fast, writing has to be easy (i.e. not require extended periods of time). This contradicts what you'd like to see when selecting RAM: It has to be fast.

Everyday analogy: - Writing something on a whiteboard is very easy and takes little to no effort. Therefore it's fast and you can sketch all over the board within seconds. - However, your sketches on the whiteboard are very volatile. Some wrong movement and everything is gone. - Take some stone plate and engrave your sketch there - like The Flintstones style - and your sketch will stay there for years, decades or possibly centuries to come. Writing this takes a lot longer though.

Back to computers: The technology to use fast chips to store persistent data is already there (like flash drives), but speeds are still a lot lower compared to volatile RAM. Have a look at some flash drive and compare the data. You'll find something like "reading at 200 MB/s" and "writing at 50 MB/s". This is quite a difference. Of course, product price has some play here, however, general access time might improve spending more money, but reading will still be faster than writing.

"But how about flashing BIOS? That's built in and fast!" you might ask. You're right, but have you ever flashed a BIOS image? Booting through BIOS takes just moments - most time is wasted waiting for external hardware - but the actual flashing might take minutes, even if it's just a few KByte to burn/write.

However, there are workarounds for this issue, e.g. Windows' Hybernate feature. RAM contents are written to a non-volatile storage (like HDD) and later on read back. Some BIOS on netbooks provide similar features for general BIOS configuration and settings using a hidden HDD partition (so you essentially skip the BIOS stuff even on cold boots).

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