CUDIMM Explained! Clock Driver RAM – DRAM IS CHANGING!

You’re going to hear a lot about CUDIMMs or Clock Driver RAM next week with the review launch of Intel’s Arrow Lake CPUs, so preempting that, I thought I’d take a second to explain what the hell that means, and why you might be buying CUDIMM RAM real soon. In short, RAM is changing, and generally I’d say it’s for the better. Our RAM has functionally been the same for over two decades – maybe three actually. The Double-Data-Rate modules we’ve been installing in our systems have functioned basically the same, with DDR5 actually ushering in the first big change in a while with the introduction of the power management circuitry on the modules themselves, but it seems we’re back at it again as DDR5 is taking another leap forward with these CUDIMMs. 

But what’s the problem with the current standard, why do we need these things? Well, as the speed of the memory marches onwards and upwards, it gets increasingly difficult to drive the RAM at those ever-escalating speeds. When you’re talking about driving a 3.2GHz clock signal over the physical distance between the CPU and the RAM – that’s 6,400 MT/s by the way, since these are Double-Data-Rate modules – you start to run into instability. Now a lot of overclocked modules can do that sort of speed just fine, although you normally have to push the voltage higher to reach that sort of stability, and at a point you’ll run into a wall where the built in memory controller just can’t handle pushing a high enough speed, even with a higher voltage, so, the industry came up with a solution.

CUDIMMs – or Clock-Driver unbuffered DIMMs – differ from the UDIMMs you’re likely already familiar with all thanks to this little chip on here. This is a clock redriver chip, and it basically takes the relatively weak signal from the memory controller onboard the CPU and re-drives it at full speed. You might have seen this sort of diagram from Linus’ HDMI cable testing a while ago now, but basically a good clean clock signal is a square wave – either on or off, and instantly one or the other. Unfortunately in the real world you can’t just instantly turn it on and off, and especially at ridiculously high speeds, that gets even more difficult too, so the signal gets smoothed out, and then it gets difficult to know what counts as a pulse. So, the clock redriver chip takes that weak signal in, and cleans it up to send to the memory modules themselves, meaning the memory controller on the CPU has to work a lot less hard to achieve ludicrously high speeds. 

The main benefit here is higher speeds, at lower voltages. Take Corsair’s 6400MT/s regular UDIMM modules – those require a whopping 1.4 volts to achieve that speed stably. Now take Crucial’s new CUDIMM 6400MT/s modules which run at just 1.1V and you see how big a difference that can be. That’s huge! For the time being anyway, Crucial is only launching this kit of CUDIMMs – their basic heat-spreader-less modules – meaning the CAS latency isn’t exactly the tightest. This is a CL52 kit – specifically 52-52-52-103, which compared to a gaming kit is really quite slow. For comparison, that Corsair 6400MT/s kit is CL32 – specifically 32-40-40-84. That’s a fair bit faster – although I have no doubt Crucial will be launching more gaming oriented CUDIMMs soon. 

You might be concerned that with this new standard, if you accidentally buy CUDIMM RAM for, say, your Ryzen 9800X3D based system, that you’ll be left with a bricked machine. Luckily, at least for this Crucial kit (although I’m sure all CUDIMM makers will do the same) this has a fallback mode, so should the board and chip not support CUDIMMs, which at least at the time of filming means everything except Arrow Lake chips, this will enter the passthrough mode and work just like a regular UDIMM, but obviously at a reduced speed. In theory you could pump the voltage back up to match a regular UDIMM to get it to run at it’s rated 6400MT/s, but still, the point is it should at least post. Crucial wanted to make it clear that the memory selector tool they’ve got on their website is there to help you pick the right RAM for your system, so if in doubt, you can check there. You just stick your motherboard in and it’ll tell you if it’s compatible, and if not what options you’ve got instead. Pretty handy!

So, CUDIMMs are likely the future of system memory, although the modules available at launch aren’t quite what us gamers are likely to be after. You can of course get laptop memory with clock drivers – those are CSODIMMs – although the newer LPCAMM2 modules don’t as of yet have a clock driver option. I’m sure at some point once the LP memory they use hit high enough speeds you might see that, but anyway, that’s clock-driver RAM explained! If you’ve got any questions, feel free to leave them in the comments below, and if you want to see these things in action, check out the Arrow Lake videos starting next Thursday!