Memory errors in consumer devices such as PCs and phones are not something you hear much about, yet they are probably one of the most common ways these machines fail.
I'll use this thread to explain how this happens, how it affects you and what you can do about it. But I'll also talk about how the industry failed to address it and how we must force them to, for the sake of sustainability. 🧵 1/17
First of all let's talk briefly about how memory works. What you have in your PC or phone is what we call dynamic random access memory. That is memory that stores bits by putting a minuscule amount of charge into vanishingly small capacitors (or not putting it in if we're storing a zero).
These capacitors continuously leak this charge, so it needs to be refreshed periodically - every few milliseconds - which is why it's called "dynamic". 2/17
Diagnosing it is hard. Windows has a memory diagnostic tool which will catch the worst offenders and is easy to use: https://www.microsoft.com/en-us/surface/do-more-with-surface/how-to-use-windows-memory-diagnostic
It's not enough though, some errors can only be caught with more extensive testing. I recommend the open-source memtest86+ (https://memtest.org/) tool or the closed source memtest86 one (https://www.memtest86.com/) 8/17
@gabrielesvelto FYI, this seems to be a nonnegligible cause of death of Nintendo 3DS units. some units either exhibit strange behavior, corruption in text characters (which turn out to be single bitflips), or just straight up refuse to boot
it's possible to demonstrate these are indeed DRAM* errors by using the boot9strap jailbreak (with ntrboot if not installed beforehand), as these run from SoC-internal SRAM instead of DRAM. booting the OS then typically fails, and it can also be used as a point to run a memtest
problem is that replacing the DRAM chips is *very* difficult. not only would it require BGA rework (because it's so small they couldn't not solder it on the mobo), Nintendo also used epoxy 'underfill' to glue the DRAM chip stuck to the PCB to deter RAM probing attacks (as those were used against the DSi, the 3DS' predecessor), see the "white glue" here: https://giltesa.com/wp-content/uploads/2013/12/Nintendo_3DS_PCB-Top.jpg
*: DRAM is more often called FCRAM on the 3DS because that's the type of DRAM by fujitsu it uses
@gabrielesvelto though, when running such a memtest, in most cases the errors seem to have the same pattern throughout the entire DRAM.
this means it's probably a solderball crack under the DRAM or SoC, rather than actual semiconductor failure, though the latter is also sometimes observed
Gabriele Svelto [moved]
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