The platform: Socket 2066 and X299 chipset
The Skylake-X and Kaby Lake-X CPUs use a new socket with the name Socket R4 otherwise known as Socket 2066. Because of this, these CPUs are not compatible with existing motherboards.
Although we have not counted all of them, Socket 2066 uses 2066 pins as the name suggests. In terms of size, Socket 2066 does not differ all that much compared with Socket 2011 of Intel’s current high-end desktop processors. This means that every CPU-cooler that is suited for Socket 2011 is also suited for Socket 2066.
The new Socket 2066 motherboards use the new X299 chipset. This is connected to the processors via DMI 3.0. This is basically a PCI-Express 3.0 x4 connection, which means that CPU and chipset communicate with each other at a speed of 4 GB/s.
The X299 sounds new, but is actually an old acquaintance. After all it is the exact same chip as the Z270 as it is used on motherboards for Socket 1151 processors. This means that the functionality that the X299 offers is identical to that of Z270. However, this does not mean that the motherboards are equal; because the Skylake-X processor offers a lot more PCI-Express lanes (44 or 28 instead of 16 with Socket 1151 CPUs), the X299 motherboards normally offer more PCI-Express x16 slots for graphics cards as well as more M.2-slots for SSDs compared with Z270 motherboards.
The X299, like the Z270, has a lot of I/O-ports that are very flexible. In total the chipset offers 30 I/O-lanes, that can be used for PCI-Express 3.0, Serial ATA or USB 3.0. Not all lanes can be used for all three protocols. In total, a maximum of 24 lanes can be used as PCIe 3.0, a maximum of eight as SATA 600 and a maximum of 10 as USB 3.0. When a motherboard manufacturer chooses to offer the maximum amount of SATA 600 and USB 3.0 connectors, six PCIe 3.0 lanes remain. However, the flexibility also allows the motherboard manufacturer to place multiple M.2-slots for modern SSDs (which need four PCIe lanes each); these are turned off as soon as they are used for one or more Serial ATA-connectors. How these different I/O-lanes are utilized on every X299 motherboard is quite the puzzle and we will try to find this out as well as possible in our motherboard reviews.
Again, do not forget that the Skylake-X processors themselves also have a PCI-Express 3.0 controller with 28 or even 44 lanes. In practice the motherboard manufacturers choose to use 16/32 of these for expansion slots for graphics cards and use the remaining 12 for, for example, M.2-slots. This means you can find motherboards with three M.2-slots, that are each directly connected to the CPU. In that case you do have a problem if you use a Kaby Lake-X CPU: the M.2-slots or other expansion slots that are directly connected to the CPU will not work.
Intel states that X299 motherboards are suited for Optane memory, but in our opinion it is at least as important – and a critical aspect – that modern interfaces such as USB 3.1 and Thunderbolt 3.0 are not integrated into the chipset. It remains remarkable that AMD can provide in integrated USB 3.1, while Intel is one of the inventors of the USB-interface. USB 3.1 will be found on (nearly) every Socket 2066 motherboard, although motherboard manufacturers will have to use extra controller chips for this. We did not find Thunderbolt 3.0 on Intel X299 boards during Computex. Reportedly this was because Intel “did not have the time yet” to validate their most high-end interface for their most high-end CPUs. If anyone could give an explanation for this…