Will Intel support PCIe 4.0 in Z490 Motherboards?

Will Intel Support PCIe 4.0 in Z490 motherboards

And what is up with this mess? 

Ok, it's no secret now, that Intel is releasing their 10th gen Comet Lake CPUs and corresponding LGA1200 Z490 motherboards, and to the surprise of many, Intel announced that they were sticking with the old PCIe 3.0 standard for these CPUs.

Note: As always, feel free to use the headers to navigate past the parts of the article that may be review for you.
Note2: I refer to PCIe storage throughout this article. Most fast storage uses an M.2 port and the NVMe interface to connect to the PCIe lanes. I simply refer to it as PCIe storage for simplicity, though it's commonly referred to as NVMe SSDs.
Note3: This is a pretty technical article that may be kind of dry, but it builds to a somewhat interesting conclusion. 

What is PCIe 3.0 and 4.0? 

In short, PCIe (PCI Express) is simply a high speed data bus, conceptually similar to USB, but is designed for parts inside the machine to communicate with each other. Its advantage is that it is fast, extremely low latency, and has multiple lanes that work in parallel. Sata is another low-latency connection for parts within the machine, but is not only slower, but does not run multiple lanes in parallel like PCIe. Here's a nice chart:

Technology	Bandwidth per lane	Theoretical Max Throughput
USB 2.0	480Mbps	60MB/s
USB 3.0 (3.2 gen 1)	5Gbps	625MB/s
SATA 3	6Gbps	600MB/s (due to encoding)
PCIe 3.0 x1	8Gbps	1GB/s
PCIe 3.0 x4	8Gbps	4GB/s
PCIe 4.0 x1	16Gbps	2GB/s
PCIe 4.0 x16	16Gbps	32GB/s

As you can see, the benefit of PCIe is you can run multiple lanes together. I just showed a couple configurations each of PCIe 3.0 and 4.0, but they can each run in 1x, 2x, 4x, 8x, or 16x. You can probably do the math. 

The downside of PCIe lanes is that there needs to be a sizeable PCIe controller to run it, either on the Chipset, or the CPU.

What is a chipset. 

A chipset is a chip that sits on the motherboard that handles a lot of the I/O for the CPU. It exists because in order to support more PCIe lanes, and USBs, and SATA ports, you have to have a bit of silicon on the chip that candles data to and from it. Increasing the size of a CPU increases its manufacturing cost exponentially, so it's cheaper to have the same total area split on two chips than on one single one. In an effort to reduce the size of the CPU, a lot of the responsibility for I/O gets offloaded to the chipset with the CPU only handling the main necessary components, such as the PCIe lanes to the GPU and the memory channels. The chipset then communicates to the CPU via a PCIe x4 bus. Intel handles this communication via a proprietary connection called DMI, and all you need to know about DMI is every time you see DMI, just replace it in your mind with PCIe. It's not technically accurate but DMI and PCIe are so similar, and DMI is ONLY used for Intel CPU <-> chipset communication (AMD uses PCIe for this connection). It even has a corresponding number, with DMI 3.0 indicating that chipset supports PCIe 3.0 x4. 

Chipsets can handle many I/O devices. For example, you might have multiple PCIe 3.0 x4 SSDs hooked up to the chipset, because that chipset will have maybe 16 or even 24 lanes coming off of it thus supporting 4 or 6 separate x4 devices. You might be thinking though "How can the Chipset have more lanes for other deviced than it has going to the CPU? Won't the bus between the Chipset and CPU present a bottleneck?" If you were, you would be absolutely correct. Since the chipset is connected to the CPU via a x4 bus, its total throughput across all of its connected devices cannot exceed the bandwidth of its CPU bus. However, say you have 2 PCIe SSDs, a SATA hard drive, and a pile of USB devices all going through the chipset. Usually not all of those devices are all running at the same time, and when it comes their time to run, the x4 bus to the CPU will usually be clear. If, however, you had multiple drives connected to the chipset all active at once, you might see a decrease in their performance as a result. That's still a lot better than only being able to connect one x4 device to the CPU though. 

NOTE: You may have heard some people refer to the Northbridge or the Southbridge. Over a decade ago, there used to be a set of chips (that's actually where the term chipset came from) the southbridge was effectively what the 'chipset' is today, and the northbridge handled the PCIe lanes to the GPU and the memory channels. The northbridge has since been moved onto the CPU. AMD recently halfway revived the northbridge in the I/O die that can be seen on modern Ryzen 3rd gen CPUs, though it is present on the CPU package itself, not the motherboard as the northbridge was. 

So why did I say that Z490 was a mess? 

Understanding this image

I understand that this image is kind of hard to look at, but I'll help you through the important parts. This is the chipset diagram for Intel 10th gen CPUs and the corresponding Z490 chipset. The two big outlined sections describe the CPU and the chipset, each with their own set of IO. Here, you can see the DMI 3.0 link mentioned earlier (remember, that's effectively just a PCIe 3.0 x4 connection as far as we care). Off the CPU, there are 16 PCIe 3.0 lanes, that can be in x16, 2 x8, x8 and 2 x4 as well as a pair of memory channels. Off the chipset, there's a whole pile of nonsense, like all the SATA ports, all the USB, the audio, the Ethernet, etc, and 24 PCIe lanes. That's a lot; that's enough for 6 separate PCIe 3.0 x4 drives. 

What it means for PCIe 4.0

Currently, there is no PCIe 4.0 anywhere to be seen on that image. However, the difference between PCIe 3.0 and 4.0 exists mainly in the controllers. The only difference on the board itself between the two is that with PCIe 4.0, you need a higher quality connection, which means either more insulation or a shorter path. 

Famously, when AMD released their Zen2 CPUs and X570 boards which brought the first mainstream support for PCIe 4.0, motherboards excitedly were announcing that many -- but not all -- of their X470 boards (which do not support PCIe 4.0; only 3.0) were actually compatible with 4.0 simply because they were well built. It would be impossible to get PCIe 3.0 off the chipset, but AMD actually has accessible 20 PCIe lanes directly to the CPU where Intel only has 16. This means that in theory, you could have on certain X470 boards one GPU running at PCIe 4.0 x16 and an M.2 port that supported PCIe 4.0 x4. However, AMD didn't like the product segmentation with some boards supporting it and some boards not supporting it, and they forced all board makers to disable PCIe 4.0 on all X470 boards. 

Intel, however, announced that PCIe 4.0 was not supporting PCIe 4.0 with their 10th generation CPUs, but popular opinion is that they will with their coming 11th gen Rocket Lake, which are expected to work on the same boards. So, motherboard makers all got together and have announced that all their Z490 boards will be compatible with PCIe 4.0 when it releases. 

But remember the chipset Z490 cannot be upgraded for PCIe 4.0 support

PCIe 4.0 support, if Intel were even to allow it, would always be limited to the PCIe lanes directly bound to the CPU, on this platform, limited to 16 lanes. 

Not only that, but at least on the Z490 Aorus Xtreme WF (a very high-end motherboard) there are only two PCIe ports that are even wired to the CPU, with the third x16 slot (wired only for x4, but physically as long as a x16 slot shown here as the bottom of the three PCIe x16 slots) actually wired through the chipset, not to the CPU. 

So what would the benefit of getting PCIe 4.0 on Z490 though a CPU upgrade be?

Frankly, limited. Not as limited as you might first guess though. PCIe lanes are mainly used for GPUs and storage. I'll split them up here and discuss each. 

Benefits of a GPU on PCIe 4.0 vs 3.0

There are some niche cases in which a GPU can benefit from a PCIe 4.0 bus, and it's likely not where you're thinking. In this Hardware Unboxed video we see that if a GPU is limited itself to 8 lanes, and is running in 3.0 mode (which will happen if it is installed on any PCIe 3.0 bus) AND if it is limited in memory thus using system RAM as a backup for its VRAM, we will see a performance increase on PCIe 4.0. While this feels like an extremely niche use case with a lot of caveats, it will happen any time you install a Radeon RX 5500XT 4GB on a motherboard without PCIe 3.0. Since only high-end AMD motherboards right now (until B550 comes out) support PCIe 4.0, it will likely be anytime anybody uses that GPU since it's a lower tier card most likely paired with a lower end motherboard. A high-end 2080ti actually has less need of bandwidth because it has ample RAM on the card, so it need not make nearly as many accesses to system RAM. 

Even so, most cards, even the 8GB version of the 5500XT, or any cards running on x16 will not benefit at all from PCIe 4.0 at this time. (this could change in the future as an increase in storage use leads to the cards making higher bandwidth)

Benefits of PCIe 4.0 on storage

Today, it means nothing for gaming. It's well documented that games hardly if at all load or run better on PCIe storage compared to slower storage because that's simply not where the bottleneck is. There is benefit in many professional applications where large amounts of data are streamed, or even in simply moving large files around. However for the most part, there is currently very littel mainstream use for this speed. 

But that will be changing over the next few years, as I discuss in my blog post here. However, while PCs may be making use of PCIe 4.0 in a few years, it's likely that in order for that to happen, your CPU will require some hardware acceleration for storage I/O that's not present on Intel 10th gen, and at least I've heard no indications that it may be present on Intel 11th gen. 

What does this mean for Z490 and PCIe 4.0 support

If you buy a Z490 motherboard for its PCIe 4.0 support, you're likely making a rather questionable decision to put it as gently as possible. First of all, if you're buying this tier of motherboard, the chances of you using a GPU that will benefit from the bandwidth is nil to none, and you won't benefit from the storage in gaming. 

But let's just say, hypothetically, that you're upgrading to an Intel platform now for whatever reason (and can't wait to buy the next generation of motherboards that will properly support PCIe 4.0 at the chipset), and you know that you will benefit from 4.0 when it comes out, and you're okay with upgrading your CPU in a year. (niche, odd use-case, but humor me.) In order to even get a PCIe 4.0 storage device working on a Z490 board, it has to be connected to one of the top two PCIe ports. This means running the GPU in x8 (not a problem with PCIe 4.0, but most GPUs still only support PCIe 3.0, so even if that port is now a 4.0 port, that GPU is likely running at 3.0 x8 potentially causing issues). It also means that since there are currently no PCIe 4.0 SSD cards (similar to this one but PCIe 4.0) you have to spend more money to buy a PCIe 4.0 card onto which you can install a PCIe 4.0 SSD. This is jest extra headache for very niche benefits. 

Will Intel allow PCIe 4.0 support on Z490 motherboards? 

I don't know, and honestly it doesn't matter. If they do, the only reason for them to do so is for good publicity, because the benefits to it are extremely few and niche. I find it more likely that Intel will want to avoid the confusion of people buying a Z490 board assuming more of the ports, such as an M.2 port on the board will support PCIe 4.0 when that won't be the case. 

If I had to guess, I would say I would expect Intel not to allow PCIe 4.0 on Z490. Combined with all the issues and limitations, Intel also has a tendency to want people to upgrade the motherboard whenever possible and they could see this as an opportunity to get people to buy a more expensive Z590 over Z490 later on. 

Thank you for reading! 

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