Mdthbs

So, I was just wondering about how volatile memory storage loses data when the power to them is cut off. But can we not solve that problem by using a battery in conjunction with a RAM and a ROM and when there's a power cut i.e. when the computer didn't receive a shutdown signal the battery kicks in and a controller transfers data from RAM to an EEPROM?

EEPROM - because I guess HDDs and SDDs consume more power. I mean if we could do this then we can maybe program it to explicitly store data in EEPROM, if not data then the OS(only a copy). That way we can also boot faster. Maybe, all of this is just dumb. There maybe very obvious reasons not to do this. But they are not clear to me. So can you tell me why is this not done?

Short answer: You might be able to "connect a battery to a RAM to prevent data loss during power outage", but this depends on the type of RAM.

Lithium "coin cell" primary (non-rechargeable) batteries, along with a diode, are commonly used for SRAM retention. SRAM is not DRAM, explained below. SRAMs are found pretty much everywhere else, such as in a modern (home) thermostat; it uses a battery to retain your settings if the power ever goes out. And in an alarm clock; uses a battery to keep the time (and to operate from for a short period), etc. SRAM is even present in the CPU as it's cache memory.

One caveat to battery-backing SRAM is, given the SRAM voltage from a battery of 3.00v a diode must be used to prevent charging the battery from the typical Vdd voltage of 3.3v or 5.0v. The supply current is very small (even nano-amps) so data can be retained for years.

The acronym SRAM means "Static Random Acccess Memory." Static, because its contents stay whatever state they are programmed, indefinitely (as long as power is applied.) SRAM is relatively expensive and bulky physically, but it is very fast, so only finds niche roles.

DRAM is a different beast altogether. DRAM needs to be "refreshed" continuously, else its contents are lost. Normal operation of the PC keeps the data in DRAM refreshed continuously, so that data is always available. But attempting to power DRAM from a battery alone would quickly (within thousandths of a second) cause the data to be lost. DRAM is relatively dense (capacity-wise) and inexpensive, so is used for computer main system memory. It is also slower.

Aside: to get around this slowness of DRAM, PC's use these in parallel, termed "banks". A whole stick is accessed at once (x8 speed since there are 8 chips on it), then if the motherboard supports dual-banking, two modules at once (2x8 = 16x). Triple-banking = 24x, etc.

Can we not simply connect a battery to a RAM to prevent data loss during power cuts? So can you tell me why is this not done?

Of course we do! It's called battery-backed SRAM, and it's widely used in embedded systems. These days, the cost of NVRAM technology (such as conventional EEPROM, or new FRAM) is low, they can do the same thing without power, so it's not as common as before, but it's still here. And it's very likely that your desktop computer still has one inside to keep the BIOS settings.

Applications include:

• Date/Time. The coin cell on your computer motherboard powers the real-time clock chip, which has SRAM inside that keeps the time. It's the reason that your desktop computer remembers the time even if power is removed.




• Calibration Parameters. A piece of test equipment such as an oscilloscope can save the current calibration data inside SRAM (A common problem of old test equipment is losing all calibration data when the battery is dead).



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  Runtime Parameters. A microcontroller can save important runtime data in a battery-backed SRAM, so that the data never loses even if the microcontroller is reset due to a power outage or crashes. Similarly, a handheld scientific calculator can save your variables.

  
  
  
  
  
  • Many desktop computers still use this technique to retain some BIOS settings, along with the date/time. It is kept because people found that it is easy to remove the battery and reset everything if the BIOS setting is bad. Early PCs used real dedicated SRAM chips, modern PCs use an integrated solution. Although modern UEFI firmware mostly uses NVRAM, but some still have a battery-backed SRAM in addition to NVRAM.
  
  
  
  



• Cryptography and Security. A security hardware can save the private key inside a battery-backed SRAM, with a temper-detection mechanism that erases and disconnects the SRAM if intrusion is detected, permanently destroys the private key. Similarly, in the extreme case, the program itself is saved in SRAM to frustrate any reverse-engineering efforts by competitors.

We don't do this on desktop computers. The large DRAM on computers use a lot of power. Also, most operating systems and software is not designed to continue working after a power loss anyway. You can implement something like hibernation, but if so, why don't you just use hibernation? Overall, it's not done, not because it's impossible, it's simply because nobody really wants to do it.

For a PC, that's more or less what "hibernate on low battery" is. The contents of RAM are written to disk in the hibernation file. Since you need the whole system to be on while doing this, it needs quite a lot of power, and is therefore only suitable for systems which would have a battery anyway like laptops.

Similarly it's possible to boot quickly from hibernation; Intel call this "rapid start" and it's been available on some systems for a long time.

The main limiting factor is that RAM is so large that it takes many seconds to write it all to Flash.