Why does RAM have to be volatile?

Question

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

Answer 1

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

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 ;) )

Answer 2

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.

Answer 3

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.)

Answer 4

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.

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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.

Answer 5

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).

Answer 6

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.

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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)

_{(source: techreport.com)}

Answer 7

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.

Answer 8

As many others have mentioned, modern RAM is only volatile by design - not by requirement. SDRAM and DDR-SDRAM have the added troubles of also requiring a refresh to remain reliable in operation. That's just the nature of Dynamic RAM modules. But, I couldn;t help but wonder if there is another option available. What types of memory exist that can fit the criteria? In this walk-through, I will only cover memory that can be read/written to at runtime. This kicks out ROM, PROM, and other one-time use chips - they're meant to be unchanging once programmed.

If we inch a bit closer to the non-volatile side of the spectrum, we do encounter SRAM along the way - but its non-volatility is quite limited. Actually, it's just data remanence. It doesn't require a refresh, but it sure will drop its data when the power is off for too long. In addition to this, it's also a bit faster than DRAM - until you reach GB size. Due to the increased size of memory cells (6 transistors per cell), when compared to DRAM, the viability of SRAM's speed advantage begins to fade as the size of the memory in use goes up.

Next up is BBSRAM - Battery Backed SRAM. This type of memory is a modified version of SRAM that uses a Battery to become non-volatile in case of a power failure. However, this introduces some issues. How do you dispose of a battery once its done for? And isn't SRAM by itself already big enough as it is? Adding a power-management circuit and battery to the mix only reduces the amount of space that can be used for actual memory cells. I also don't remember batteries playing nice with prolonged heat exposure...

Further to the non-volatile side of the spectrum, we now lay eyes on EPROM. 'But wait', you ask - 'isn't EPROM one-time use also?' Not if you have a UV light and the will to take high risks. EPROMs can be rewritten if exposed to UV light. However, they are usually packed in an opaque enclosure once programmed - that would have to come off first. Highly impractical, seeing that it can't be rewritten at runtime, in-circuit. And you wouldn't be able to target individual memory addresses/cells - only wipe. But, EEPROM might help...

The EE stands for Electrically-Erasable. That opens the door for write operations occurring in circuit for once (in comparison to ROM, PROM, and EPROM). However, EEPROMs use floating-gate transistors. This leads to a gradual accumulation of trapped electrons, which will eventually render the memory cells inoperable. Or, the memory cells could encounter charge loss. That leads to the cell being left in an erased state. It's a planned death sentence - not what you were looking for.

MRAM is next in the list. It uses a Magnetic Tunnel Junction, consisting of a permanent magnet paired with a changeable magnet (separated by thin insulation layer), as a bit. According to Wikipedia,

" The simplest method of reading is accomplished by measuring the electrical resistance of the cell. A particular cell is (typically) selected by powering an associated transistor that switches current from a supply line through the cell to ground. Due to the Tunnel magnetoresistance, the electrical resistance of the cell changes due to the relative orientation of the magnetization in the two plates. By measuring the resulting current, the resistance inside any particular cell can be determined, and from this the magnetization polarity of the writable plate. "

This form of memory is based upon differences in resistance and measuring voltage, rather than charges and currents. It doesn't need a charge pump, which aids in making its operation less power consuming than DRAM- especially for STT-based variants. MRAM has multiple advantages to its design, including memory density comparable to that of DRAM; performance and speed comparable to that of SRAM in limited test cases; power consumption much lower than DRAM; and lack of degradation due to repeated read/write operations. This has put MRAM in the spotlight for researchers and scientists alike, furthering its development. In fact, it's also being looked at as a possible candidate for "universal memory". However, fab costs for this type of memory are still very high, and popular manufacturers are more interested in other options - ones that look a bit unwieldy at this point.

I could go over Ferroelectric RAM, but it's a rather sad option. F-RAM is similar to DRAM in construction - simply replace the dielectric layer with ferroelectric material instead. It has lower power consumption, decent read/write endurance - but the advantages wane after this. It has much lower storage densities, an outright storage cap, a destructive read process (requiring changes to any IC to accommodate for it with write-after-read arch.), and higher overall cost. Not a pretty sight.

The last options on the spectrum are the SONOS, CBRAM, and Flash-RAM (NAND Flash, NOR-based, etc.). Common SSD-like storage won't cut it though, so we can't quite find any viable options at the end of this spectrum. SONOS and Flash-RAM both suffer the issues of limited read/write speeds (used primarily for permanent storage - not optimized for RAM-like operation speeds), the need to write in blocks, and limited numbers of read/write cycles before saying 'good night'. They may be good for paging, but they sure won't work for high-speed access. CBRAM is also a bit too slow for your purposes.

The future for this hunt looks bleak currently. But fear not - I left a few honorable mentions out for your personal reading. T-RAM (Thysistor-RAM), Z-RAM, and nvSRAM are possible candidates as well. While both T-RAM and Z-RAM need a refresh occasionally (in comparison to DRAM, SDRAM, and DDR-SDRAM), nvSRAM is free of such requirements. All three of these options have either better memory density, better read/write speeds, and/or better power consumption rates. They also don't need batteries - which is a big plus (BBSRAM is crying in a corner). With a closer look at nvSRAM, it appears as though we have found the viable candidate for the dreaded DDR-SDRAM replacement.

But soon (at least for those who chose to read this far), we will all be crying in our own separate corners - in addition to having the same size issues as SRAM, nvSRAM is also not available in large enough modules for use as a suitable DDR-SDRAM replacement. The option(s) are there - but either aren't yet ready for production (like MRAM), or simply never will be (nvSRAM). And before you ask, the Gigabyte i-RAM is out too - it only works via SATA interface, producing a performance bottleneck. It also has a battery. I guess we should all be looking at where memory may be going next? A bitter-sweet end, I suppose.

Answer 9

• 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)

Answer 10

Strictly speaking, RAM does not need to be volatile. Multiple forms of non-volatile RAM was used in computers. Ferrite core memory, for one, was the dominant form of RAM (acting as main storage, from which the processor took information directly) in the '50s up until the '70s, when transistorized, monolithic memory became prevalent.

I believe IBM also referred HDD as random-access storage, as it differed from sequential access storage, such as magnetic tape. The difference is comparable to a cassette tape and a vinyl record -- you have to wind through the entire tape before you can get to the last song, whereas you can simply reposition the pin on any location on the record to start listening from there.

Answer 11

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