[SOLVED] Understanding 12th gen CPUs memory controller limitations ?

[Yanitwei]

Greetings and happy new year to all 🥳🎄 !

This is a theoretical question, please consider I'm not building a new rig and don't need any advices like "there won't be performance benefit" or "best available options right now to go with".
Now I really hope someone will answer this question, at least let's try to figure out together 😅💪

I want to understand how does the memory controllers inside CPU and the one on RAM stick actually works. let's take an abstract build for my question (components not necessarily need to be these).
CPU: Intel Core i7-12700K
Mobo 1: MSI Z690-A WIFI DDR4
Mobo 2: MSI Z690-A WIFI (DDR5)
here I'm trying to create "if others equal" conditions
(basically any motherboard that supports higher than CPU memory clock speed)
RAM 1: Corsair Vengeance RGB PRO 16GB (2 x 8GB) DDR4 @4600MHz
RAM 2: Corsair Vengeance 32GB (2x16GB) DDR5 @5200MHz
(any RAM sticks that works on higher than compatible by CPU memory clock speed)

Now theoretically
In case 1 - Mobo 1 and RAM 1, we're gonna have setup as 1DPC 2R memory @4600MHz clock speed
In case 2 - Mobo 2 and RAM 2, we're gonna have setup as 1DPC 2R memory @5200MHz clock speed
While in CPU by JEDEC
the DDR4 memory speed Is limited @3200MHz
and DDR5 memory speed is limited @4800MHz

How does the memory controller work inside CPU ??
Besides XMP overclocking capabilities of RAM stick and mobos I want to understand does it determines what type of memory is installed (with all it's characteristics)
by the memory controller located on DIMM ? and then limits the clock speed to the one set by default ? Why so ??

How does it determines memory type and limits the clock speed ??
Are there two different memory controllers inside this CPU ? the one for each type of memory ?
I assume there should be one single memory controller for desktop CPUs with only 2 memory channels, or am I missing something ?

Why is there a limitation difference between DDR4 and DDR5 ?
If this CPU is able to work with memory sticks running at 4800MHz - well we have pretty fast DDR4 memory sticks out there, what exactly makes it different from DDR4 to DDR5 ??

What causes the actual limitation for DDR4, and for DDR5 memory working speeds
OK In both cases I understand that IF the setup is overclocked and tweaked properly, it'll work at 4600MHz on DDR4 and 5200MHz on DDR5 but will it be struggling ? unstable ? why so ? if mobos support higher RAM speeds and there are such memory sticks available, why the default limit inside CPU is so little then ?

 

[Koekieezz]

First of all, Greetings and happy new year 😀!

How does the memory controller work inside CPU ??
Besides XMP overclocking capabilities of RAM stick and mobos I want to understand does it determines what type of memory is installed (with all it's characteristics)
by the memory controller located on DIMM ? and then limits the clock speed to the one set by default ? Why so ??

Memory controller in the past is located at the northbridge, separate from cpu. nowdays memory controller is merged as 1 along with the cpu I/O part (or unified on amd as I/O die), leaving the southbridge only as the other part for processing i/o outside the cpu. for your cpu case, the limit is on JEDEC specs, meaning that if the motherboard allows for overclocking ram, it could go above the speed limit. And why it is limited? because it's the chip design and the decision of their company to limit on that speed.

How does it determines memory type and limits the clock speed ??
Are there two different memory controllers inside this CPU ? the one for each type of memory ?
I assume there should be one single memory controller for desktop CPUs with only 2 memory channels, or am I missing something ?

Yes, intel usually does this on newer platform that have newer ddr capability, like back to core 2 era which got ddr2 and ddr3 controller each other, skylake till coffee lake refresh got ddr3 to ddr4 controller each other, and now alder lake that got ddr4 and ddr5 controller each other. Per memory controller does 2 channel, for ddr5 it's 4 Sub Channel (which could be said as 2 full channel)

Why is there a limitation difference between DDR4 and DDR5 ?
If this CPU is able to work with memory sticks running at 4800MHz - well we have pretty fast DDR4 memory sticks out there, what exactly makes it different from DDR4 to DDR5 ??

Because of how DDR4 and DDR5 memory works differently, where as DDR4 voltage are still supplied on the motherboard, and how the architechture of the ic in ddr4 itself, it limits the capability of ddr4 itself, hence why ddr5 is more like a buffed DDR4, but with advanced architecture for the IC itself and how they handle the data throughput is different from ddr4 + the voltage regulator for the ram is on the pcb itself, reason why ddr5 speeds are high and low voltage for jedec specs. you could see how the throughput different here.

What causes the actual limitation for DDR4, and for DDR5 memory working speeds
OK In both cases I understand that IF the setup is overclocked and tweaked properly, it'll work at 4600MHz on DDR4 and 5200MHz on DDR5 but will it be struggling ? unstable ? why so ? if mobos support higher RAM speeds and there are such memory sticks available, why the default limit inside CPU is so little then ?

Why they limit the jedec speeds: chip maker decision to do so, the "jedec standard" have the native capability to run the jedec stock speed. for example, you have a generic ddr4 3200mhz cl22, no xmp options, which means that the memory have the capability of 3200mhz Natively without the need to touch xmp, or overclocking, so as long the imc is able to run 3200mhz jedec speed, it will automatically use the speed.

For overclocking: YMMV, if your ram sticks is good enough, the motherboard is good enough, and the IMC itself on the cpu is good enough, you could do high clocks + lower CL, or high clocks at stock jedec speed (not changing the timings but could keep overclock the ram). the factor for overclocking is variable, such as how good is the imc, how good is the pcb design of the ram, how good is the memory IC, and how good is the signal integrity for both ram and motherboard. thats why there is such an enthusiast motherboard that only got 2 ram sticks like X570S unify x for example, because afaik it got the best signal integrity in theory since 1 slot 1 channel hence it handles the signal integrity traces on the motherboard better and to the max potential, while also giving high gurantee it would run at high speed and low cl, or any of those overclocker do as they wish.

CMMIW, i hope this answer helps you 😀

 

[Yanitwei]

CMMIW, i hope this answer helps you 😀

That helped a lot, thanks!