RAM memory SPD, what it is and how your PC can read it

When we go to purchase a memory module we all know its traits by the truth that the producer tells us within the specs, however how does the CPU know? It doesn’t perceive the identical language as we do and solely understands a specific dialect of the binary. Well, it does it by means of a bit of {hardware} that provides it this info.

What is Serial Presence Detect or SPD?

Serial Presence Detect

First of all we have now to remember that we’re speaking a couple of element that It is discovered within the RAM reminiscences of PC, and it is a EEPROM that shops the DRAM info. Often it is normally situated on a small chip which normally accompanies all DIMMs or SO-DIMMs as proven within the image above.

By utilizing a programmable ROM by means of electrical impulses, its content material is written to the SPD as soon as the memory chip has been manufactured and subsequently assessments have been carried out to find out its working velocity. When instruments like HWinfo or CPU-Z give the specs of the RAM memory, what they ask the CPU is to ask for its info within the SPD.

The SPD is additionally present in different sorts of memory such because the VRAM utilized in graphics playing cards. In this case, the SPD is not on a separate chip because the memory is soldered onto the board, however on every of the memory chips within the case that it is GDDR-type memory or within the base the place the logic of every HBM stack is .

What does the Serial Presence Detect do on the PC?

RAM Memory Module Tempo

When beginning a PC the very first thing the processor does is entry the SPD, and the latter informs you of entry occasions and memory velocity. This is essential attributable to the truth that the compass on the time of accessing the RAM, not solely by the processor however for all the system, is an entry carried out on the improper time that finally ends up sending the communication with the memory to the fret, for which it is key.

Today there are instances resembling AMD Ryzen processors that entry the RAM of the system with variable clock velocity, in order that they should know the specs of the RAM to which they’re linked with a purpose to function in situations and that don’t trigger issues on the time of mentioned entry. And it is that you must bear in mind that there is a window to entry the RAM restricted in time.

If the data despatched by the CPU to the RAM arrives exterior the precise interval, then the RAM can return misguided knowledge or write in an element that is not the handle of the RAM memory that we needed to entry for it.

How does the CPU talk to the RAM SPD?

Serial Presence Detect SPD Pins

It is apparent that it does this by means of the interface that is situated on the perimeter of the processor and that is wired to the RAM of the system.

Access to the EEPROM on the SSD nonetheless is not finished by means of the information pins and addressing with the RAM. To do that, they add extra communications which can be assigned to the SMBus, which is a variant of the I²C that makes use of solely 4 poles: one is used as a clock, the second transmits knowledge and the third and fourth report which of the 4 modules it is. To end, the SPD shares the bottom pin with the remainder of the DIMM, however makes use of its personal pin for energy provide.

What info can the SPD comprise inside?


The direct and easy reply is one which requires the CPU to operate, we can’t overlook that it is a ROM memory of a really small dimension and subsequently it will need to have its knowledge organized in essentially the most compact method potential.

That is why the way in which by which such info is saved inside the EEPROM of the SPD is at all times the identical, which helps to save lots of area and facilitates not solely communication with the CPU, but additionally with the motherboard chipset. And what is the order and who dictates it? Well, the committee that is the JEDEC does it, which is the one which units the memory requirements for each NAND Flash and RAM that we use in our computing units.

In every JEDEC customary the content material of the SPD varies, however the knowledge it incorporates is not solely about velocity, but additionally info like now:

  • The sort of RAM it is: DDR, LPDDR, and many others.
  • How many chips are there and the storage capability of every one.
  • Serial quantity and producer.
  • Voltages supported by the DIMM for overclocking.
  • And so on.

What occurs once we overclock the RAM?

RAM modules

Memories which have the power to fluctuate their clock velocity use a special Serial Presence Detect than customary modules. Fortunately, the JEDEC means that you can add extensions on the finish of the Serial Presence Detect.

The most well-known of all is Intel’s XMP, which was designed first for DDR3 and later ported to DDR4. The reply to this was the AMD AMP that they used of their failed line of RAM memory modules, though over time they’ve ended up adopting the extension of their rival, which has grow to be common within the overwhelming majority of DIMMs and motherboards.

So with out Serial Presence Detect then memory overclocking would not even work. The motive for this is that they rely on your info in order that there aren’t any communication errors and additionally in eventualities resembling the very fact of accelerating the velocity of the RAM (and with it the voltage) which could possibly be deadly for the well being of the memory.