The "Kaby Lake-X" Silicon

As we mentioned in the introduction, the Core i5-7640X and its sibling, the i7-7740X, are essentially quad-core "Kaby Lake" dies placed on LGA2066 packages, which means that apart from being for a different socket and losing out on integrated graphics, they are practically identical to 7th generation Core "Kaby Lake" processors. The "Kaby Lake-X" die in itself is optically identical to the "Skylake-S" quad-core die, except that is built on the newer 14 nm+ process, which allows higher transistor drive current, letting Intel increase clock speeds by up to 15 percent at minimal power/thermal cost.

The "Kaby Lake-X" die physically features just four CPU cores, with the same essential cache hierarchy as older generations of Core processors, where the core has an arbitrary 256 KB of faster dedicated L2 cache, while the die itself either has 1.5 MB/core or 2 MB/core of shared L3 cache. This is in contrast to the other LGA2066 "Skylake-X" processors, which implement Intel's biggest change in cache-organization in close to a decade, featuring 1 MB of faster dedicated L2 cache per core, and 1.375 MB/core of shared L3 cache.



Another major "Skylake-X" feature not found on the "Kaby Lake-X" die is the new mesh-topology, where the various components of the processor are interconnected at multiple points that make up a "mesh" as opposed to the "ring-bus" topology Intel has been using since "Nehalem" (circa 2008). The mesh topology is designed to improve inter-core communication as the ring-bus approach can't keep up with increasing core counts (ring stops) without adding an intolerable amount of latency. Since "Kaby Lake-X" only features four physical cores, this won't be much of a deal. Since the LGA2066 platform lacks onboard graphics as a feature, the integrated Gen 9.5 graphics core of the "Kaby Lake-X" silicon is dead weight.



Intel's "Kaby Lake" die only features a dual-channel DDR4-integrated memory controller, and the Core i5-7640X will only support up to 64 GB of dual-channel memory; only half the memory slots on your motherboard will work (the manual will tell you which ones). What's more, the "Kaby Lake" die has a narrower PCI-Express gen 3.0 root complex with only 16 lanes allocated to PEG (PCI-Express Graphics) and four to the DMI 3.0 chipset bus. Certain PCIe and M.2 slots on your motherboard will outright not work. Those that do might do so at a lower-than-expected bandwidth. If you only have one graphics card, install it in the topmost slot, as that one tends to have the full x16 wiring to the CPU. When in doubt, consult your motherboard manual.

For the Core i5-7640X, Intel continues to keep HyperThreading disabled, making it the first Intel HEDT chip since 2008 to lack it. The L3 cache amount is 6 MB, which is on par with its LGA1151 siblings. The only on-paper difference between this chip and the i5-7600K is its slightly higher clock speed, which is 4.00 GHz instead of the 3.80 GHz clock of the i5-7600K. The Turbo Boost clock remains unchanged at 4.20 GHz, and the unlocked base-clock multiplier lets you overclock the chip. The "Kaby Lake-X" quad-core chips also lose out on the Turbo Boost Max 3.0 feature of the "Skylake-X" chips, which enables automatic overclocking beyond the max Turbo Boost frequency and depends on your cooling setup's effectiveness. And yet, the TDP of these chips has been bumped up to 112W for some reason. It had better indicate higher overclocking headroom.

The LGA2066 Platform

People pick HEDT platforms not only for higher core-counts, but increased platform connectivity, in the form of more PCI-Express lanes, support for more memory, more storage options, etc. The LGA2066 platform in itself covers all the bases with connectivity. You can run up to four graphics cards for multi-GPU greatness, up to eight DDR4 memory modules for a total of 128 GB of quad-channel memory, and a plethora of other onboard devices, thanks to the higher PCI-Express budget. Unfortunately, the Core i5-7640X and Core i7-7740X neither deliver on the promise of more cores or connectivity than a mainstream desktop. When it comes to platform connectivity, they bring nothing new to the table since their PCI-Express lane budget is the same as the LGA1151 platform.

The Intel HEDT platform originally gave you a high PCI-Express lane budget across the lineup. The company then began segmenting its processors by giving the cheaper processors fewer PCI-Express lanes (28 vs. 44), which still ended up being higher than the 16 lanes put out by mainstream-desktop Core processors. The Core i5-7640X is essentially a mainstream-desktop processor in high-end desktop clothing, and hence gives you that very limited lane budget. LGA2066 motherboards come with PCI-Express auto-switching to either cope with 28-lane or 44-lane root complexes, but when a 16-lane processor such as the i5-7640X is installed, certain slots, onboard devices, and M.2 slots will be disabled because those 16 lanes are wired to the PCI-Express x16 slots.