Some understanding of memory hardware

The lower-level parts of computers General: Computer power consumption · Computer noises Memory: Some understanding of memory hardware · CPU cache · Flash memory · Virtual memory · Memory mapped IO and files · RAM disk · Memory limits on 32-bit and 64-bit machines Related: Network wiring notes - Power over Ethernet · 19" rack sizes Unsorted: GPU, GPGPU, OpenCL, CUDA notes · Computer booting

"What Every Programmer Should Know About Memory" is a good overview of memory architectures, RAM types, reasons bandwidth and access speeds vary.

RAM types

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

DRAM - Dynamic RAM

a fairly wide term, mostly for constructions that use a capacitor+transistor per cell

capacitor leakage means it forgets its state, so this has to be refreshed regularly,

where 'regularly' means on the rough order of milliseconds (there have been many generations, processes, die sizes, etc. of DRAM)

and this is a game of statistics - probably many cells are fine for maybe half a second, but we care about integrity of all, then we have to consider the worst case under the worst conditions

also meaning you need a DRAM controller, more complexity (not something you'd DIY), and not the lowest possible latency

(and also increasing latency somewhat, though this is less of an issue when you have multiple chips)

...but the lower component count per cell, so high-density, and cheaper per storage size

a variant of DRAM is most common as any temporary memory you want a lot of, due to cost

SDRAM - Synchronous DRAM

mostly a practical design consideration

...that of coordinating the DRAM via an external clock signal (earlier DRAM was asynchronous, manipulating state as soon as lines changed)

This allows the interface to that RAM to be a predictable state machine, which allows easier buffering, and easier interleaving of internal banks

which makes higher data rates a bunch simpler - though not necessarily at lower latency

SRAM - Static RAM

Has a higher component count per cell (6 transistors) than e.g. DRAM

Retains state as long as power is applied to the chip, no need for refresh, also making it a little lower-latency

no external controller, so simpler to use

the higher component count per cell makes it more expensive per storage size

e.g used in caches, due to speed, and acceptable cost for lower amounts

PSRAM - PseudoStatic RAM

A design tradeoff, somewhere between SRAM and DRAM

it's like DRAM with built-in refresh, so functionally it's a "don't think about it" as SRAM

(yes, DRAM technically can have built-in refresh, but that's often points a sleep mode that retains state without requiring an active DRAM controller, not something for active use)

it's slower than DRAM, and cheaper than SRAM

SRAM makes sense for small internal RAM, PSRAM makes sense for extended RAM in situations DRAM is not necessary

Non-volatile RAM

While the concept of Random Access Memory (RAM) only tells you that you can access any part of it with comparable ease (contasted with e.g. tape storage, where more distance meant more time, so more storage meant more time)...

...we tend to think about RAM as volatile, only useful as an intermediate scratchpad between storage and use, and will lose its contents very soon after it loses power. Probably because the commonly chosen designs all have that property.

Yet there are various designs that are both easily accessible and keep their state.

And there is a gliding scale of this, and some other properties.

We may well call it NVM (non-volatile memory) when we haven't yet gotten to some more specific properties, like how often we may read or write, or how difficult that is.

Say, some variants of EEPROM aren't the easiest to deal with. We like Flash more, even though it's basically a development from EEPROM. But both wear out.

NVRAM on the other hand tends to be easier, more reisable, like FRAM, MRAM, and PRAM, or nvSRAM or even BBSRAM.

nvSRAM - SRAM and EEPROM stuck on the same chip.

seems intended as a practical improvement on BBSRAM

and/or a "access common stuff quickly, occasionally write a chunk to EEPROM" style of data logging, black boxes, that sort of thing

https://en.wikipedia.org/wiki/NvSRAM

BBSRAM - Battery Backed SRAM

basically just SRAM alongside a lithium battery, so that it'll live a good while

is sort of cheating, but usefully so.

FRAM - Ferroelectric RAM

functions more like flash, also limited in amount of use (but with many more cycles)

read process is destructive (like e.g DRAM), so you need a write-after-read to keep data around

so it's great for things like round-robin logging (which would be pretty bad for Flash)

https://electronics.stackexchange.com/questions/58297/whats-the-catch-with-fram

PRAM

https://en.wikipedia.org/wiki/Phase-change_memory

DRAM stick types

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

ECC RAM ('Error correction code')

can detect many (and correct some) hardware errors in RAM

The rate of of bit-flips is low, but will happen. If your computations or data are very important to you, you want ECC rather than the regular, non-ECC type.

See also:

http://en.wikipedia.org/wiki/ECC_memory

DRAM Errors in the Wild: A Large-Scale Field Study

Registered RAM (sometimes buffered RAM) basically places a buffer on the DRAM modules (register as in hardware register)

offloads some electrical load from the main controller onto these buffers, making it easier to have designs more stably connect more individual memory sticks/chips.

...at a small latency hit

typical in servers, because they can accept more sticks

Must be supported by the memory controller, which means it is a motherboard design choice to go for registered RAM or not

pricier (more electronics, fewer units sold)

because of this correlation with server use, most registered RAM is specifically registered ECC RAM

yet there is also unregistered ECC, and registered non-ECC, which can be good options on specific designs of simpler servers and beefy workstations.

sometimes called RDIMM -- in the same context UDIMM is used to refer to unbuffered

https://en.wikipedia.org/wiki/Registered_memory

FB-DIMM, Fully Buffered DIMM

same intent as registered RAM - more stable sticks on one controller

the buffer is now between stick and controller [1] rather than on the stick

physically different pinout/notching

SO-DIMM (Small Outline DIMM)

Physically more compact. Used in laptops, some networking hardware, some Mini-ITX

EPP and XMP (Enhanced Performance Profile, Extreme Memory Profiles)

basically, one-click overclocking for RAM, by storing overclocked timing profiles

so you can configure faster timings (and V_dimm and such) according to the modules, rather than your trial and error

normally, memory timing is configured according to a table in the SPD, which are JEDEC-approved ratings and typically conservative.

EPP and XMP basically means running them as fast as they could go (and typically higher voltage)

In any case, the type of memory must be supported by the memory controller

DDR2/3/4 - physically won't fit

Note that while some controllers (e.g. those in CPUs) support two generations, a motherboard will typically have just one type of memory socket

registered or not

ECC or not

Historically, RAM controllers were a thing on the motherboard near the CPU, while there are now various cases where the controller is on the CPU.

More on...

DRAM versus SRAM

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

EPROM, EEPROM, and variants

PROM is Programmable ROM

can be written exactly once

EPROM is Erasable Programmable ROM.

often implies UV-EEPROM, erased with UV shone through a quartz window. You would e.g. tape that over to avoid it corrupting later.

EEPROM's extra E means Electrically Erasable

meaning it's now a command, and not a window.

early EEPROM read, wrote, and erased (verify) a single byte at a time. Modern EEPROM can work in larger chunks.

you only get a limited amount of erases (much like Flash. Flash is arguably just an evolution of EEPROM)

Flash memory (intro)

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Flash can be considered a refinement on EEPROM. (The name probably came from marketing, helping to distinguish it as its own thing, with distinct properties)

Where EEPROM had to be wiped completely before rewriting, "Flash memory" allows erasing a block that is generally much smaller than the whole thing (and larger than anything you erase a byte at a time), a tradeoff that helps write speed, longevity, use for random-storage needs, and eventually cost.

Something that did not change much between EEPROM and flash is that it has a limited number of erase cycles, and isn't the fastest to erase.

Most memory cards and USB stick have one flash chip and a simple controller, which is the cheapest setup, and why flash rarely broke 10MB/s twenty years ago and cheap ones still don't. The simplest setup also tends to not have enough wear leveling to last very long.

SSDs are made of flash, but are what you might call managed flash, which just means the controller you talk to is fancier.

They go faster partly because they have more Flash chips to interleave/parallelize between (RAID-like layout), it hides Flash's relatively slow erase speed (by postponing it, which usually works under home-use loads, but not under heavy load) and which lasts longer by avoiding reuse of the same cell.

On fake flash

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Fake flash refers to a scam where cards's controller reports a larger size than the actual storage has.

These seem to come in roughly two variants:

once you ask for what's not there, they will immediately error out

once you ask for what's not there, or it will wrap back on itself and write in existing area and corrupt

...which is mostly a technical but not a practical distinction, in that the latter case errors out a little later and behaves only slightly differently.

There are some tools to detect fake flash. You can e.g. read out what flash memory chips are in there and whether that adds up. Most scammers don't go so far to fake this.

The more thorough check is a write-and-verify test, see below.

Memory card health

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

While memory cards and USB sticks are made of flash memory, much like SSDs, memory cards and USB sticks usually lack wear leveling and health introspection like SMART.

So you should assume that they will fail completely, without warning, so never keep your only copy of something on them.

At lower level, even if they are still completely readable (and that's not a guarantee), filesystems are not made to deal with write failure, so you may need special tools, and/or a technical friend, for recovery.

You can check, non-destructively, whether it's all still readable with a tool that just read all disk surface (for chkdsk it's the 'scan for an attempt recovery of bad sectors" checkbox, but there are clearer ways with better summaries)

The only real test of whether it's fully writable is to write to all of it, but

that will actively wear it a little

it's destructive of the data still on there.

it only proves that it hasn't failed already, not that it won't do so soon

...this can still be a halfway decent way to inspect a pile of cards you don't know - to gauge speed, whether they haven't failed yet, and are fake flash.

One useful tool is H2testw, which creates a file in free space (if empty, then almost all the SD card)

It will also tell you actual average write and read speed, not the potential lie on the front.