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Since some people run into problems with four RAM modules on modern MSI mainboards, i wanted to explain the reasons behind that, and why two modules are often superior. The main reason lies in the way the memory slots are connected to the memory controller, which is inside the CPU. So the first explanation is about:
1) RAM slot layout
All regular mainboards and desktop CPU models have a dual-channel memory system. Since a lot of boards offer four RAM slots, a pair of two slots have to each form a RAM channel. So the four RAM slots are not individually addressed, but in pairs, as two channels. The different ways to connect the RAM slot pairs on the board are either "Daisy chain" or "T-Topology". This RAM slot layout decision - the way the slots are connected - has a big influence on how many modules (two or four) the board works best with.
Here is a slide from an MSI presentation, showing that almost all of today's boards have a "daisy chain" memory slot layout. This layout heavily prefers two-module-operation. The presentation is a bit older, but it's safe to say that the the vast majority of recent mainboards (B550, Z590, Z690 etc...) also have a daisy chain layout, and it's confirmed in several reviews. Especially MSI are known to use this layout on almost all their modern boards. For other mainboard makers, it depends on the board model, but they will also tend to prefer this layout.
Daisy chain means that the slot pairs are connected one after the other, and therefore optimized for two modules total. The right slot of each channel is the end point.
Using two RAM modules, they are to be inserted into slot 2 and 4 counted from the left as per the mainboard manual. Meaning, into the second slot of each channel and thus the end point.
The reason is, this puts them at the very end of the PCB traces coming from the CPU, which is important for the electrical properties.
PCB (printed circuit board) traces are the thin signal lines that are visible on the mainboard, especially between the CPU and the RAM slots.
Why is this important? The PCB traces, going from the memory controller contacts of the CPU, to each contact of the RAM slots, are optimized to result in exactly the same distance between all those points. They are essentially "zig-zagging" across the board for an electrically ideal layout, making a few extra turns if a direct line would lead to an uneven distance.
This is done so that, with two modules, a) each RAM module is at the very end of the electrical traces coming from the CPU's memory controller, and b) each module has exactly the same distance to the memory controller across all contacts. We are dealing with nanosecond-exact timings, so all this matters.
On a mainboard with a daisy-chain RAM slot layout, this optimization is done with only two modules in mind, which are in slot 2 and 4 (on the board, those slots are called A2 and B2). This is the configuration that most buyers would use, and it also results in the best overclocking potential. This way, the mainboard makers can boast with higher RAM overclocking frequencies when advertising the board, and the majority of buyers will have the ideal solution with two RAM modules.
Note: Never populate slots 1 and 3 first. When putting the modules into slot 1 and 3, the empty slots 2 and 4 would be like having some loose wires hanging from the end of each RAM contact, creating unwanted signal reflections and so on. So with two modules, they need to go into the second slot (slot 2+4, or A2 and B2) of each memory channel, to not have "loose ends" after the RAM module.
Now the interesting question. What happens when we populate all four slots on a mainboard with a daisy-chain slot layout? Well, the module in the second and fourth slot become "daisy-chained" after the modules in the first and third slot. This completely worsens the electrical properties of the whole memory system.
With four modules, there are now two modules per channel, and the two pairs of a channel don't have the same distance from the memory controller anymore. That's because the PCB traces go to the first slot, and then over to the second slot. This daisy-chaining - with the signal lines going to the first and then to the second module of a memory channel - introduces a lot of unwanted electrical handicaps when using four modules. The signal quality worsens considerably in this case.
Only with a "T-Topology" slot layout, the PCB traces have exactly the same length across all four slots, which would provide much better properties for four-module operation. But mainboards with T-Topology have gone a bit out of fashion, since most people use just two modules. Plus the memory OC numbers look much better with a daisy chain layout and two modules. So if the mainboard makers were to use T-topology on a board, they couldn't advertise with such high overclocking numbers, and people would think the board is worse (and it actually would be, for only two modules).
Example of an ASUS board with the rare T-Topology layout, advertising the fact that it works better with four modules compared to the much more common boards using the daisy-chain layout.
2) Single-rank vs. dual-rank
Another consideration is single-rank vs. dual-rank modules. This is about how a RAM module is organized, meaning, how the individual memory chips on the module are addressed. To put it simply, most (if not all) 8 GB modules are single-rank nowadays, as well as some 16 GB modules. A single-rank module puts less stress on the memory system. There's also a bunch of 16 GB modules that are dual-rank, and all bigger modules are always dual-rank. Dual-rank is slightly faster performance-wise (up to 4%), but also loads the memory controller more. One dual-rank module puts almost as much stress on the memory system as two single-rank modules!
What is the memory system? It consists of the CPU's integrated memory controller (IMC), the mainboard and its BIOS, and the RAM itself.
Every modern mainboard will be the happiest with two single-rank modules (for dual-channel operation), because this causes the least stress on the memory system, and is electrically the most ideal, considering that the memory slots are connected as "daisy chain". This fact is reflected in the maximum DDR frequencies that the mainboards are advertised with.
Here is an example from the highest MSI DDR4 board model using newest Intel Z690 chipset, which should therefore be quite advanced.
Specifications of MPG Z690 EDGE WIFI DDR4, under "Detail".
Max. overclocking frequency:
1DPC 1R Max speed up to 5333+ MHz
1DPC 2R Max speed up to 4800+ MHz
2DPC 1R Max speed up to 4400+ MHz
2DPC 2R Max speed up to 4000+ MHz
"DPC" means DIMM (=module) per channel, 1R means single-rank, 2R means dual-rank.
With 1DPC 1R = two single-rank modules (so, 2x 8 GB or 2x 16 GB single-rank), the highest frequencies can be reached.
With 1DPC 2R = two dual-rank modules (like 2x 16 GB dual-rank or 2x 32 GB), the maximum attainable frequency is lower, since the memory system is under more stress.
With 2DPC 1R = four single-rank modules (4x 8 GB or 4x 16 GB single-rank), the maximum frequency drops again, because four modules are even more challenging than two dual-rank modules.
And 2DPC 2R = four dual-rank modules (like 4x 16 GB dual-rank or 4x 32 GB) combines the downsides of the highest possible load on the memory controller with the electrical handicap of using four slots on a daisy-chain-mainboard.
The last configuration can already be difficult to get stable at DDR4-3200 sometimes, let alone DDR4-3600. One could consider themselves lucky to get DDR4-3600 working with four dual-rank modules, maybe having to use more relaxed timings for example. The 16 GB and 32 GB modules also often don't have particularly tight XMP timings to begin with, like DDR4-3600 18-22-22-42.
By the way: The second timing (tRCD) is more telling and important than the first one (tCL) to determine the module quality, but most people only look at tCL = CAS Latency.
With the new DDR5 standard, this drop in attainable frequency is even more pronounced. This is from the specs of one of the top MSI Z690 boards.
Specifications of MEG Z690 ACE, under "Detail".
Max. overclocking frequency:
1DPC 1R Max speed up to 6666+ MHz
1DPC 2R Max speed up to 5600+ MHz
2DPC 1R Max speed up to 4000+ MHz
2DPC 2R Max speed up to 4000+ MHz
When going from two modules (first and second entry) to four modules, the attainable frequency drops drastically. With two single-rank modules, DDR5-6000 and above is possible according to MSI. With two dual-rank modules, that goes down a little already. But with four modules, the memory system is under a lot more stress, and MSI are quite open about the result. This seems to be a limitation of the DDR5 memory system, which relies even more on a very clean signal quality. Using four DDR5 modules on a board with a daisy-chain layout clearly is not good in that regard.
This deterioration with four DDR5 modules is so drastic that the conclusion could be: DDR5 motherboards should come with only 2 dimm slots as standard (Youtube)
Now, with the 13th gen "Raptor Lake" Intel CPUs being available (13600K and up) which come with an improved memory controller, as well as newer BIOS versions containing some memory code optimizations, MSI have revised the frequency numbers for the boards a bit. Again looking at the Z690 ACE, the revised numbers are:
The main takeaway from all this for DDR5:
Whatever total RAM size needed, it's better to reach it with two modules if decent speed is required. There's a reason why four-module kits are an absolute rarity (only a handful kits being sold by Corsair, but none by any other brand), and why those few kits sold are all below DDR5-6000. Combining two kits (of two high-speed modules each) simply has a low likelihood of working. As mentioned, with four modules, the maximum frequency that the memory system can reach drops down considerably, which makes XMP/EXPO speeds not work anymore. With DDR5 it's always better to use two modules only (and even with DDR4 that is advised, but four modules can at least work quite decently there).
This also means that DDR4 is actually better for high-capacity memory configurations such as 128 GB total, because:
- It doesn't experience such a large drop in the electrical properties of the memory system when using four modules
- Four-module high-capacity kits are readily available (and at a lower price)
- Four-module kits are actually certified on the memory QVL at MSI
- They will most likely outperform their DDR5 equivalent due to DDR4's lower latencies, when compared to DDR5's necessary low required frequencies at this configuration.
The overall higher DDR5 latencies just can't be compensated for by higher RAM frequencies anymore, since using four DDR5 modules requires lower frequencies to be stable.
See also RAM performance scaling.
Generally, in case of RAM problems, no matter the technology, there are three possibilities, which can also be used in combination:
- Lower the frequency
- Loosen the timings
- Raise the voltage(s)
But in some cases, buying different RAM might be the best solution.
3) Amount of RAM
32 or 64 GB RAM can be justified for large video editing projects, rendering, heavy photoshop use and such cases. But if it's gaming for example, modern games very rarely use more than 16 GB RAM. There are just the first games coming out, like Flight Simulator 2020, who will use a little more than 16 GB RAM, but still basically run at the same speed. So, 32 GB would definitely be a generous amount, but it can be befitting for a high-end gaming system. However, 64 GB amounts to a waste of money for gaming, no matter what. Before any game will ever touch more than 32 GB, the whole PC will be long outdated, because it could take years. Why are games satisfied with 16 GB nowadays? Consoles. A lot of games are developed with the lucrative console market in mind, and even the PlayStation 5 only has 16 GB of RAM. So games are designed from the ground up not to need more RAM, which then also applies to the PC versions of those games.
Recommendations for use on modern consumer mainboards:
8 GB RAM: Use 2x 4 GB, or even 1x 8 GB if RAM performance isn't critical anyway (entry-level systems, office work etc.)
16 GB RAM: Use 2x 8 GB (good mid-range / gaming systems).
32 GB RAM: Use 2x 16 GB (high-end gaming systems - when all other bottlenecks are removed - and uses beyond gaming, such as video processing etc.)
64 GB RAM: Use 2x 32 GB ("beyond gaming" - only necessary for professional use).
128 GB RAM total : Use 4x 32 GB ("beyond gaming" - only necessary for professional use).
This last configuration - using four dual-rank high-capacity modules - is maximally stressing the memory system, so stay at DDR4-3200 or lower. DDR4-3600 might already cause trouble. Also, for 128 GB total, i recommend DDR4, not DDR5. DDR5 really doesn't run well with 4x 32 GB, it would be restricted to quite low frequencies, pretty much negating the DDR5 advantage. With DDR5 RAM, i would actually never recommend using four modules, not even 4x 8 GB (the 8 GB modules are slower and 2x 16 GB work better).
For all the DDR4 configurations up to 64 GB total, i usually recommend DDR4-3600 speed (see chapter 4).
3b) How to increase the RAM size when you have 2x 4 GB or 2x 8 GB RAM?
First choice: Replace the 2x 4 GB with 2x 8 GB, or the 2x 8 GB with 2x16 GB. The new RAM should be a kit of matched modules. This will ensure the best performance and the least problems, because there's only two modules again in the end.
Second choice: Add a kit of two matching modules to your two existing modules. But you might not be able to get the same modules again. Even if they are the same model, something internally might have changed. Or you might toy with the idea of adding completely different modules (for example, adding 2x 8 GB to your existing 2x 4 GB). This can all cause problems. The least problems can be expected when you add two modules that are identical to your old ones. But then there's still this: You are now stressing the memory system more with four modules instead of two, so the attainable RAM frequency might drop a little. Also, it's electrically worse on a mainboard with daisy-chain layout, as explained under 1).
Lastly, adding just one more module (to have three modules total) is by far the worst choice for several reasons. Every desktop platform has a dual-channel memory setup. This means it works best with two modules, and it can work decently with four modules. And if you only use the PC for light office work, even a single 4GB or a single 8GB module would do. But in a PC where performance matters, for example for gaming, getting a single RAM module to upgrade when you have two existing modules is not good at all. The third module will be addressed in single-channel mode, while simultaneously ruining the memory system's electrical properties and making everything work at whatever the slowest module's specification is.
Note: When upgrading the RAM, it's always good to check for BIOS updates, they often improve compatibility with newer RAM modules (even if it's not explicitly mentioned in the changelog).
4) Today's sweet spot of DDR4-3600 with the latest CPUs
On AMD, DDR4-3600 has been the sweet spot for quite a while. But now, Intel introduced new memory controllers in their 11th gen and 12th gen CPUs which also require a divider above a certain RAM frequency. Only up to DDR4-3600 (but that pretty much guaranteed), the RAM and the CPU's memory controller (IMC) run at the same frequency (Intel calls this "Gear1 mode"). Somewhere above that RAM frequency, depending on the IMC's capabilities, the IMC has to resort to Gear2 mode, which introduces a divider for it and makes it run at half the RAM frequency. This costs a lot of performance.
An example on Intel Z590 with a kit of DDR4-3200: The IMC doesn't require a divider and can comfortably run in 1:1 mode (Gear1), which has the best performance.
The Gear2 mode that becomes necessary at high RAM frequencies has a substantial performance penalty, because the latencies increase (everything takes a little longer). This basically leads to the same situation that we already know from AMD: RAM frequencies that are considerably above DDR4-3600 are almost useless, because of the divider being introduced for the IMC (memory controller). The performance loss with a divider is just too significant.
For the RAM performance to be on the same level again as DDR4-3600 without a divider (Gear1 mode on Intel), it requires something like DDR4-4400 (!) with the divider in place (Gear2 mode).
Looking at the high prices for DDR4-4400 kits or what it takes to overclock a normal kit of RAM to that, it's not practical. So with Rocket Lake (Core i-11000) and Alder Lake (Core i-12000) CPUs, and of course recent AMD CPUs, the "sweet spot" is usually at DDR4-3600. This frequency is known to not require a divider for the memory controller and thus gives the best performance and bang-for-buck.
Some more recent AMD CPUs, as well as 12th gen Intel "Alder Lake" CPUs, can sometimes go a bit above DDR4-3600 without requiring a divider for the memory controller.
But DDR4-3600 almost always runs well in 1:1 mode and has a better price/performance than RAM with higher specs, so it's still the top pick.
Here's an example of an AMD system (X570 with Ryzen 3900X). The tool HWinfo64 can show those frequencies in the "Sensors" window.
DDR4-3866 is too much to run in 1:1 mode, so the divider for the memory controller is active and performance is worse. DDR4-3600 manages to run in 1:1 mode and the performance is better.
The best thing on both platforms nowadays is to run DDR4-3600 without a divider and with some nice low timings if possible. Something like DDR4-4000 will usually make the BIOS enable the divider for the memory controller and it will be slower overall than DDR4-3600, despite the higher RAM frequency. This is because the latencies are effectively increased when the memory controller has to work at a lower frequency. With a DDR4-4000 kit of RAM for example, i would enable XMP, but then manually set a DRAM frequency of DDR4-3600. This should make the BIOS remove the divider for the memory controller and the performance will immediately be better.
Here's a page from an MSI presentation about 11th gen Rocket Lake CPUs, showing the increased latencies when the divider comes into play:
And here's from an AMD presentation about the Ryzen 3000-series, showing similarly increased latencies once the divider is active:
With higher DDR5 speeds, a divider is used, because it's not feasible to run the memory controller at the same speed anymore. But with DDR5, the divider for the memory controller has less of a penalty than with DDR4, because DDR5 can access a module via two seperate sub-channels of 2x 32 bits (instead of one 64 bit channel like on DDR4). This allows for higher/better interleaving of memory accesses on DDR5 and alleviates most of the latency penalties. On AMD the FCLK can be left at 2000 MHz with DDR5, it seems to be the new "sweet spot".
5) RAM stability testing
Memtest86 Free from https://www.memtest86.com/
I use this as a basic stability test on a new system before i update the BIOS to the newest version (which is always one of the first things to do, as the factory BIOS will already be quite outdated). Also, since it runs from a USB stick/drive, i use it as a first check before booting Windows, when something has significantly changed with the RAM or its settings. One or two passes of this give me a good idea if the system is generally stable enough to start installing Windows (or boot it).
It's a good first test if you are completely unsure about stability, as well as a good "finisher" if you want to be extra sure that everything is ok with your memory system after doing other testing. The main advantage is that it runs from USB. The main disadvantage is that RAM tests in Windows are more thorough in catching errors.
Launch the included ImageUSB program to prepare a USB drive with it, then boot from that drive (press F11 during POST for the boot menu).
Some people may also know "Memtest86+", a fork of Memtest86 which was better for a while. But by now, the regular Memtest86 is more current and the one to use.
Once in Windows, a quick way for detecting RAM instabilities is TestMem5 or TM5 for short: https://testmem.tz.ru/tm5.rar
Used with this config file put into the TM5\bin folder. Note that some antivirus scanners don't like ".cfg"-files, but it's just a harmless text file, you can check in any text editor.
TM5 delivers a good and relatively quick indication of RAM stability. Run as admin. A full run of three passes with the config file from the link should take 10 to 40 minutes.
The precise time it takes depends on the amount of RAM and the RAM/CPU performance.
Example of unstable RAM, with TM5 detecting six errors during testing:
Those weird letters on the bottom left are shown because that font doesn't contain the proper Cyrillic letters (but even if it did, most people couldn't read it).
Those lines say at which test (0, 8, 2, 2, 0...) the errors were found. But just make sure there are no errors.
If the RAM passes all tests, TM5 will say "of errors is not detected" at the end.
One of the best tools to thoroughly test RAM stability is from Google, and it's called GSAT (Google stressapptest). It has been specifically developed by Google to detect memory errors, because they use ordinary PCs instead of specialized servers for a lot of things. The only downside, it takes a bit of time to set up. To run GSAT, you first have to enable the "Windows Subsystem for Linux":
After the necessary reboot, open the Microsoft Store app and install "Ubuntu", then run Ubuntu from the start menu.
It will ask for a username and password, they are not important, just enter a short password that you remember, you need to enter it for the update commands.
Then run the following commands one after the other (copy each line, then right-click into the Ubuntu window to paste it, then press enter):
sudo apt-get update
sudo apt full-upgrade -y
sudo apt-get install stressapptest
Then you can start GSAT with the command:
stressapptest -W -M 12000 -s 3600
This example tests 12 GB of RAM (in case of 16 GB total, because you need to leave some for Windows), for 3600 seconds (one hour). You can also enter -s 7200 for two hours.
If you have more RAM, always leave 4 GB for Windows, so with 32 GB, you would use "-M 28000".
GSAT looks unspectacular, just some text scrolling through, but don't let that fool you, that tool is pretty stressful on your RAM (as it should be).
At the end, it has to say Status: PASS, and there should be no so-called "hardware incidents". Otherwise it's not stable.
Then, HCI Memtest is quite good. There is a useful tool for it, called MemTestHelper: https://github.com/integralfx/MemTestHelper/releases/tag/v2.2.0
It requires Memtest 6.4, which can be downloaded here: https://www.3dfxzone.it/programs/?objid=18508
(Because in the newest Memtest 7.0, they made a change so that MemTestHelper doesn't work anymore and you should be forced to buy Memtest Pro).
Put both tools in the same folder. Start MemTestHelper, and with 16 GB RAM, you can test up to 12000 MB (the rest is for Windows).
Let it run until 400% are passed. That's a good indicator that your RAM is stable. If you want to make really sure, let it run to 800%.
Another popular tool among serious RAM overclockers is Karhu from https://www.karhusoftware.com/ramtest/
But it costs 10€ to register, so i would just use the other free programs (unless RAM OC is your hobby).
A stability test which also challenges the memory controller a lot, and therefore definitely useful to round out the RAM-related testing:
Linpack Xtreme from https://www.techpowerup.com/download/linpack-xtreme/
Run Linpack, select 2 (Stress test), 5 (10 GB), set at least 10 times/trials, press Y to use all threads, 2x N, and let it do its thing.
It's one of the best tools to detect instability, but warning, this also generates a lot of heat in the CPU. So i would watch the temperatures using HWinfo64 Sensors.
Each trial has to say "pass", and it has to say "checks passed" at the end.
It also puts out a "GFlops" number, that one is actually a decent performance metric to quickly judge if a certain RAM tuning (lowering timings) has performance benefits.
An important note about RAM and heat: Higher ambient temperatures are not good for RAM stability. The RAM might be perfectly stable in a RAM-specific stress test, but depending on the graphics card (its power consumption and cooling design), once that dumps its heat into the case very close to the RAM slots during gaming, there can be RAM-related crashes. Simple because it heats up the RAM a lot and makes it lose stability.
So to be absolutely sure that the RAM is stable even when it's hot, it can be good to run something like FurMark alongside the RAM stability test. Not for hours, because FurMark creates extreme GPU load, but just for 20 minutes or so, to really heat things up. A lot of times, the fins of the cooler are oriented towards the mainboard and the side panel, so the heat comes out from the sides of the card, and the RAM sits right above that.
If your RAM is fine in isolated RAM stress tests, but you have crashes in games (or when otherwise loading the GPU) with the same RAM settings, then you need to loosen up those settings a bit to add more headroom for those circumstances. Go by the three principles of RAM instability: Loosen timings and/or lower frequency and/or raise voltage.
Deep-diving a bit more into RAM:
It can quickly become a bit complicated, but if there are any questions, feel free to ask.
My other guides:
Guide: How to find a good PSU
Guide: How to set up a fan curve in the BIOS
1) RAM slot layout
All regular mainboards and desktop CPU models have a dual-channel memory system. Since a lot of boards offer four RAM slots, a pair of two slots have to each form a RAM channel. So the four RAM slots are not individually addressed, but in pairs, as two channels. The different ways to connect the RAM slot pairs on the board are either "Daisy chain" or "T-Topology". This RAM slot layout decision - the way the slots are connected - has a big influence on how many modules (two or four) the board works best with.
Here is a slide from an MSI presentation, showing that almost all of today's boards have a "daisy chain" memory slot layout. This layout heavily prefers two-module-operation. The presentation is a bit older, but it's safe to say that the the vast majority of recent mainboards (B550, Z590, Z690 etc...) also have a daisy chain layout, and it's confirmed in several reviews. Especially MSI are known to use this layout on almost all their modern boards. For other mainboard makers, it depends on the board model, but they will also tend to prefer this layout.
Daisy chain means that the slot pairs are connected one after the other, and therefore optimized for two modules total. The right slot of each channel is the end point.
Using two RAM modules, they are to be inserted into slot 2 and 4 counted from the left as per the mainboard manual. Meaning, into the second slot of each channel and thus the end point.
The reason is, this puts them at the very end of the PCB traces coming from the CPU, which is important for the electrical properties.
PCB (printed circuit board) traces are the thin signal lines that are visible on the mainboard, especially between the CPU and the RAM slots.
Why is this important? The PCB traces, going from the memory controller contacts of the CPU, to each contact of the RAM slots, are optimized to result in exactly the same distance between all those points. They are essentially "zig-zagging" across the board for an electrically ideal layout, making a few extra turns if a direct line would lead to an uneven distance.
This is done so that, with two modules, a) each RAM module is at the very end of the electrical traces coming from the CPU's memory controller, and b) each module has exactly the same distance to the memory controller across all contacts. We are dealing with nanosecond-exact timings, so all this matters.
On a mainboard with a daisy-chain RAM slot layout, this optimization is done with only two modules in mind, which are in slot 2 and 4 (on the board, those slots are called A2 and B2). This is the configuration that most buyers would use, and it also results in the best overclocking potential. This way, the mainboard makers can boast with higher RAM overclocking frequencies when advertising the board, and the majority of buyers will have the ideal solution with two RAM modules.
Note: Never populate slots 1 and 3 first. When putting the modules into slot 1 and 3, the empty slots 2 and 4 would be like having some loose wires hanging from the end of each RAM contact, creating unwanted signal reflections and so on. So with two modules, they need to go into the second slot (slot 2+4, or A2 and B2) of each memory channel, to not have "loose ends" after the RAM module.
Now the interesting question. What happens when we populate all four slots on a mainboard with a daisy-chain slot layout? Well, the module in the second and fourth slot become "daisy-chained" after the modules in the first and third slot. This completely worsens the electrical properties of the whole memory system.
With four modules, there are now two modules per channel, and the two pairs of a channel don't have the same distance from the memory controller anymore. That's because the PCB traces go to the first slot, and then over to the second slot. This daisy-chaining - with the signal lines going to the first and then to the second module of a memory channel - introduces a lot of unwanted electrical handicaps when using four modules. The signal quality worsens considerably in this case.
Only with a "T-Topology" slot layout, the PCB traces have exactly the same length across all four slots, which would provide much better properties for four-module operation. But mainboards with T-Topology have gone a bit out of fashion, since most people use just two modules. Plus the memory OC numbers look much better with a daisy chain layout and two modules. So if the mainboard makers were to use T-topology on a board, they couldn't advertise with such high overclocking numbers, and people would think the board is worse (and it actually would be, for only two modules).
Example of an ASUS board with the rare T-Topology layout, advertising the fact that it works better with four modules compared to the much more common boards using the daisy-chain layout.
2) Single-rank vs. dual-rank
Another consideration is single-rank vs. dual-rank modules. This is about how a RAM module is organized, meaning, how the individual memory chips on the module are addressed. To put it simply, most (if not all) 8 GB modules are single-rank nowadays, as well as some 16 GB modules. A single-rank module puts less stress on the memory system. There's also a bunch of 16 GB modules that are dual-rank, and all bigger modules are always dual-rank. Dual-rank is slightly faster performance-wise (up to 4%), but also loads the memory controller more. One dual-rank module puts almost as much stress on the memory system as two single-rank modules!
What is the memory system? It consists of the CPU's integrated memory controller (IMC), the mainboard and its BIOS, and the RAM itself.
Every modern mainboard will be the happiest with two single-rank modules (for dual-channel operation), because this causes the least stress on the memory system, and is electrically the most ideal, considering that the memory slots are connected as "daisy chain". This fact is reflected in the maximum DDR frequencies that the mainboards are advertised with.
Here is an example from the highest MSI DDR4 board model using newest Intel Z690 chipset, which should therefore be quite advanced.
Specifications of MPG Z690 EDGE WIFI DDR4, under "Detail".
Max. overclocking frequency:
1DPC 1R Max speed up to 5333+ MHz
1DPC 2R Max speed up to 4800+ MHz
2DPC 1R Max speed up to 4400+ MHz
2DPC 2R Max speed up to 4000+ MHz
"DPC" means DIMM (=module) per channel, 1R means single-rank, 2R means dual-rank.
With 1DPC 1R = two single-rank modules (so, 2x 8 GB or 2x 16 GB single-rank), the highest frequencies can be reached.
With 1DPC 2R = two dual-rank modules (like 2x 16 GB dual-rank or 2x 32 GB), the maximum attainable frequency is lower, since the memory system is under more stress.
With 2DPC 1R = four single-rank modules (4x 8 GB or 4x 16 GB single-rank), the maximum frequency drops again, because four modules are even more challenging than two dual-rank modules.
And 2DPC 2R = four dual-rank modules (like 4x 16 GB dual-rank or 4x 32 GB) combines the downsides of the highest possible load on the memory controller with the electrical handicap of using four slots on a daisy-chain-mainboard.
The last configuration can already be difficult to get stable at DDR4-3200 sometimes, let alone DDR4-3600. One could consider themselves lucky to get DDR4-3600 working with four dual-rank modules, maybe having to use more relaxed timings for example. The 16 GB and 32 GB modules also often don't have particularly tight XMP timings to begin with, like DDR4-3600 18-22-22-42.
By the way: The second timing (tRCD) is more telling and important than the first one (tCL) to determine the module quality, but most people only look at tCL = CAS Latency.
With the new DDR5 standard, this drop in attainable frequency is even more pronounced. This is from the specs of one of the top MSI Z690 boards.
Specifications of MEG Z690 ACE, under "Detail".
Max. overclocking frequency:
1DPC 1R Max speed up to 6666+ MHz
1DPC 2R Max speed up to 5600+ MHz
2DPC 1R Max speed up to 4000+ MHz
2DPC 2R Max speed up to 4000+ MHz
When going from two modules (first and second entry) to four modules, the attainable frequency drops drastically. With two single-rank modules, DDR5-6000 and above is possible according to MSI. With two dual-rank modules, that goes down a little already. But with four modules, the memory system is under a lot more stress, and MSI are quite open about the result. This seems to be a limitation of the DDR5 memory system, which relies even more on a very clean signal quality. Using four DDR5 modules on a board with a daisy-chain layout clearly is not good in that regard.
This deterioration with four DDR5 modules is so drastic that the conclusion could be: DDR5 motherboards should come with only 2 dimm slots as standard (Youtube)
Now, with the 13th gen "Raptor Lake" Intel CPUs being available (13600K and up) which come with an improved memory controller, as well as newer BIOS versions containing some memory code optimizations, MSI have revised the frequency numbers for the boards a bit. Again looking at the Z690 ACE, the revised numbers are:
- 1DPC 1R Max speed up to 6666+ MHz
- 1DPC 2R Max speed up to 6000+ MHz
- 2DPC 1R Max speed up to 6000+ MHz
- 2DPC 2R Max speed up to 5600+ MHz
The main takeaway from all this for DDR5:
Whatever total RAM size needed, it's better to reach it with two modules if decent speed is required. There's a reason why four-module kits are an absolute rarity (only a handful kits being sold by Corsair, but none by any other brand), and why those few kits sold are all below DDR5-6000. Combining two kits (of two high-speed modules each) simply has a low likelihood of working. As mentioned, with four modules, the maximum frequency that the memory system can reach drops down considerably, which makes XMP/EXPO speeds not work anymore. With DDR5 it's always better to use two modules only (and even with DDR4 that is advised, but four modules can at least work quite decently there).
This also means that DDR4 is actually better for high-capacity memory configurations such as 128 GB total, because:
- It doesn't experience such a large drop in the electrical properties of the memory system when using four modules
- Four-module high-capacity kits are readily available (and at a lower price)
- Four-module kits are actually certified on the memory QVL at MSI
- They will most likely outperform their DDR5 equivalent due to DDR4's lower latencies, when compared to DDR5's necessary low required frequencies at this configuration.
The overall higher DDR5 latencies just can't be compensated for by higher RAM frequencies anymore, since using four DDR5 modules requires lower frequencies to be stable.
See also RAM performance scaling.
Generally, in case of RAM problems, no matter the technology, there are three possibilities, which can also be used in combination:
- Lower the frequency
- Loosen the timings
- Raise the voltage(s)
But in some cases, buying different RAM might be the best solution.
3) Amount of RAM
32 or 64 GB RAM can be justified for large video editing projects, rendering, heavy photoshop use and such cases. But if it's gaming for example, modern games very rarely use more than 16 GB RAM. There are just the first games coming out, like Flight Simulator 2020, who will use a little more than 16 GB RAM, but still basically run at the same speed. So, 32 GB would definitely be a generous amount, but it can be befitting for a high-end gaming system. However, 64 GB amounts to a waste of money for gaming, no matter what. Before any game will ever touch more than 32 GB, the whole PC will be long outdated, because it could take years. Why are games satisfied with 16 GB nowadays? Consoles. A lot of games are developed with the lucrative console market in mind, and even the PlayStation 5 only has 16 GB of RAM. So games are designed from the ground up not to need more RAM, which then also applies to the PC versions of those games.
Recommendations for use on modern consumer mainboards:
8 GB RAM: Use 2x 4 GB, or even 1x 8 GB if RAM performance isn't critical anyway (entry-level systems, office work etc.)
16 GB RAM: Use 2x 8 GB (good mid-range / gaming systems).
32 GB RAM: Use 2x 16 GB (high-end gaming systems - when all other bottlenecks are removed - and uses beyond gaming, such as video processing etc.)
64 GB RAM: Use 2x 32 GB ("beyond gaming" - only necessary for professional use).
128 GB RAM total : Use 4x 32 GB ("beyond gaming" - only necessary for professional use).
This last configuration - using four dual-rank high-capacity modules - is maximally stressing the memory system, so stay at DDR4-3200 or lower. DDR4-3600 might already cause trouble. Also, for 128 GB total, i recommend DDR4, not DDR5. DDR5 really doesn't run well with 4x 32 GB, it would be restricted to quite low frequencies, pretty much negating the DDR5 advantage. With DDR5 RAM, i would actually never recommend using four modules, not even 4x 8 GB (the 8 GB modules are slower and 2x 16 GB work better).
For all the DDR4 configurations up to 64 GB total, i usually recommend DDR4-3600 speed (see chapter 4).
3b) How to increase the RAM size when you have 2x 4 GB or 2x 8 GB RAM?
First choice: Replace the 2x 4 GB with 2x 8 GB, or the 2x 8 GB with 2x16 GB. The new RAM should be a kit of matched modules. This will ensure the best performance and the least problems, because there's only two modules again in the end.
Second choice: Add a kit of two matching modules to your two existing modules. But you might not be able to get the same modules again. Even if they are the same model, something internally might have changed. Or you might toy with the idea of adding completely different modules (for example, adding 2x 8 GB to your existing 2x 4 GB). This can all cause problems. The least problems can be expected when you add two modules that are identical to your old ones. But then there's still this: You are now stressing the memory system more with four modules instead of two, so the attainable RAM frequency might drop a little. Also, it's electrically worse on a mainboard with daisy-chain layout, as explained under 1).
Lastly, adding just one more module (to have three modules total) is by far the worst choice for several reasons. Every desktop platform has a dual-channel memory setup. This means it works best with two modules, and it can work decently with four modules. And if you only use the PC for light office work, even a single 4GB or a single 8GB module would do. But in a PC where performance matters, for example for gaming, getting a single RAM module to upgrade when you have two existing modules is not good at all. The third module will be addressed in single-channel mode, while simultaneously ruining the memory system's electrical properties and making everything work at whatever the slowest module's specification is.
Note: When upgrading the RAM, it's always good to check for BIOS updates, they often improve compatibility with newer RAM modules (even if it's not explicitly mentioned in the changelog).
4) Today's sweet spot of DDR4-3600 with the latest CPUs
On AMD, DDR4-3600 has been the sweet spot for quite a while. But now, Intel introduced new memory controllers in their 11th gen and 12th gen CPUs which also require a divider above a certain RAM frequency. Only up to DDR4-3600 (but that pretty much guaranteed), the RAM and the CPU's memory controller (IMC) run at the same frequency (Intel calls this "Gear1 mode"). Somewhere above that RAM frequency, depending on the IMC's capabilities, the IMC has to resort to Gear2 mode, which introduces a divider for it and makes it run at half the RAM frequency. This costs a lot of performance.
An example on Intel Z590 with a kit of DDR4-3200: The IMC doesn't require a divider and can comfortably run in 1:1 mode (Gear1), which has the best performance.
The Gear2 mode that becomes necessary at high RAM frequencies has a substantial performance penalty, because the latencies increase (everything takes a little longer). This basically leads to the same situation that we already know from AMD: RAM frequencies that are considerably above DDR4-3600 are almost useless, because of the divider being introduced for the IMC (memory controller). The performance loss with a divider is just too significant.
For the RAM performance to be on the same level again as DDR4-3600 without a divider (Gear1 mode on Intel), it requires something like DDR4-4400 (!) with the divider in place (Gear2 mode).
Looking at the high prices for DDR4-4400 kits or what it takes to overclock a normal kit of RAM to that, it's not practical. So with Rocket Lake (Core i-11000) and Alder Lake (Core i-12000) CPUs, and of course recent AMD CPUs, the "sweet spot" is usually at DDR4-3600. This frequency is known to not require a divider for the memory controller and thus gives the best performance and bang-for-buck.
Some more recent AMD CPUs, as well as 12th gen Intel "Alder Lake" CPUs, can sometimes go a bit above DDR4-3600 without requiring a divider for the memory controller.
But DDR4-3600 almost always runs well in 1:1 mode and has a better price/performance than RAM with higher specs, so it's still the top pick.
Here's an example of an AMD system (X570 with Ryzen 3900X). The tool HWinfo64 can show those frequencies in the "Sensors" window.
DDR4-3866 is too much to run in 1:1 mode, so the divider for the memory controller is active and performance is worse. DDR4-3600 manages to run in 1:1 mode and the performance is better.
The best thing on both platforms nowadays is to run DDR4-3600 without a divider and with some nice low timings if possible. Something like DDR4-4000 will usually make the BIOS enable the divider for the memory controller and it will be slower overall than DDR4-3600, despite the higher RAM frequency. This is because the latencies are effectively increased when the memory controller has to work at a lower frequency. With a DDR4-4000 kit of RAM for example, i would enable XMP, but then manually set a DRAM frequency of DDR4-3600. This should make the BIOS remove the divider for the memory controller and the performance will immediately be better.
Here's a page from an MSI presentation about 11th gen Rocket Lake CPUs, showing the increased latencies when the divider comes into play:
And here's from an AMD presentation about the Ryzen 3000-series, showing similarly increased latencies once the divider is active:
With higher DDR5 speeds, a divider is used, because it's not feasible to run the memory controller at the same speed anymore. But with DDR5, the divider for the memory controller has less of a penalty than with DDR4, because DDR5 can access a module via two seperate sub-channels of 2x 32 bits (instead of one 64 bit channel like on DDR4). This allows for higher/better interleaving of memory accesses on DDR5 and alleviates most of the latency penalties. On AMD the FCLK can be left at 2000 MHz with DDR5, it seems to be the new "sweet spot".
5) RAM stability testing
Memtest86 Free from https://www.memtest86.com/
I use this as a basic stability test on a new system before i update the BIOS to the newest version (which is always one of the first things to do, as the factory BIOS will already be quite outdated). Also, since it runs from a USB stick/drive, i use it as a first check before booting Windows, when something has significantly changed with the RAM or its settings. One or two passes of this give me a good idea if the system is generally stable enough to start installing Windows (or boot it).
It's a good first test if you are completely unsure about stability, as well as a good "finisher" if you want to be extra sure that everything is ok with your memory system after doing other testing. The main advantage is that it runs from USB. The main disadvantage is that RAM tests in Windows are more thorough in catching errors.
Launch the included ImageUSB program to prepare a USB drive with it, then boot from that drive (press F11 during POST for the boot menu).
Some people may also know "Memtest86+", a fork of Memtest86 which was better for a while. But by now, the regular Memtest86 is more current and the one to use.
Once in Windows, a quick way for detecting RAM instabilities is TestMem5 or TM5 for short: https://testmem.tz.ru/tm5.rar
Used with this config file put into the TM5\bin folder. Note that some antivirus scanners don't like ".cfg"-files, but it's just a harmless text file, you can check in any text editor.
TM5 delivers a good and relatively quick indication of RAM stability. Run as admin. A full run of three passes with the config file from the link should take 10 to 40 minutes.
The precise time it takes depends on the amount of RAM and the RAM/CPU performance.
Example of unstable RAM, with TM5 detecting six errors during testing:
Those weird letters on the bottom left are shown because that font doesn't contain the proper Cyrillic letters (but even if it did, most people couldn't read it).
Those lines say at which test (0, 8, 2, 2, 0...) the errors were found. But just make sure there are no errors.
If the RAM passes all tests, TM5 will say "of errors is not detected" at the end.
One of the best tools to thoroughly test RAM stability is from Google, and it's called GSAT (Google stressapptest). It has been specifically developed by Google to detect memory errors, because they use ordinary PCs instead of specialized servers for a lot of things. The only downside, it takes a bit of time to set up. To run GSAT, you first have to enable the "Windows Subsystem for Linux":
After the necessary reboot, open the Microsoft Store app and install "Ubuntu", then run Ubuntu from the start menu.
It will ask for a username and password, they are not important, just enter a short password that you remember, you need to enter it for the update commands.
Then run the following commands one after the other (copy each line, then right-click into the Ubuntu window to paste it, then press enter):
sudo apt-get update
sudo apt full-upgrade -y
sudo apt-get install stressapptest
Then you can start GSAT with the command:
stressapptest -W -M 12000 -s 3600
This example tests 12 GB of RAM (in case of 16 GB total, because you need to leave some for Windows), for 3600 seconds (one hour). You can also enter -s 7200 for two hours.
If you have more RAM, always leave 4 GB for Windows, so with 32 GB, you would use "-M 28000".
GSAT looks unspectacular, just some text scrolling through, but don't let that fool you, that tool is pretty stressful on your RAM (as it should be).
At the end, it has to say Status: PASS, and there should be no so-called "hardware incidents". Otherwise it's not stable.
Then, HCI Memtest is quite good. There is a useful tool for it, called MemTestHelper: https://github.com/integralfx/MemTestHelper/releases/tag/v2.2.0
It requires Memtest 6.4, which can be downloaded here: https://www.3dfxzone.it/programs/?objid=18508
(Because in the newest Memtest 7.0, they made a change so that MemTestHelper doesn't work anymore and you should be forced to buy Memtest Pro).
Put both tools in the same folder. Start MemTestHelper, and with 16 GB RAM, you can test up to 12000 MB (the rest is for Windows).
Let it run until 400% are passed. That's a good indicator that your RAM is stable. If you want to make really sure, let it run to 800%.
Another popular tool among serious RAM overclockers is Karhu from https://www.karhusoftware.com/ramtest/
But it costs 10€ to register, so i would just use the other free programs (unless RAM OC is your hobby).
A stability test which also challenges the memory controller a lot, and therefore definitely useful to round out the RAM-related testing:
Linpack Xtreme from https://www.techpowerup.com/download/linpack-xtreme/
Run Linpack, select 2 (Stress test), 5 (10 GB), set at least 10 times/trials, press Y to use all threads, 2x N, and let it do its thing.
It's one of the best tools to detect instability, but warning, this also generates a lot of heat in the CPU. So i would watch the temperatures using HWinfo64 Sensors.
Each trial has to say "pass", and it has to say "checks passed" at the end.
It also puts out a "GFlops" number, that one is actually a decent performance metric to quickly judge if a certain RAM tuning (lowering timings) has performance benefits.
An important note about RAM and heat: Higher ambient temperatures are not good for RAM stability. The RAM might be perfectly stable in a RAM-specific stress test, but depending on the graphics card (its power consumption and cooling design), once that dumps its heat into the case very close to the RAM slots during gaming, there can be RAM-related crashes. Simple because it heats up the RAM a lot and makes it lose stability.
So to be absolutely sure that the RAM is stable even when it's hot, it can be good to run something like FurMark alongside the RAM stability test. Not for hours, because FurMark creates extreme GPU load, but just for 20 minutes or so, to really heat things up. A lot of times, the fins of the cooler are oriented towards the mainboard and the side panel, so the heat comes out from the sides of the card, and the RAM sits right above that.
If your RAM is fine in isolated RAM stress tests, but you have crashes in games (or when otherwise loading the GPU) with the same RAM settings, then you need to loosen up those settings a bit to add more headroom for those circumstances. Go by the three principles of RAM instability: Loosen timings and/or lower frequency and/or raise voltage.
Deep-diving a bit more into RAM:
It can quickly become a bit complicated, but if there are any questions, feel free to ask.
My other guides:
Guide: How to find a good PSU
Guide: How to set up a fan curve in the BIOS
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