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Intel's Core i7-6700K 'Skylake' processor reviewed


Enthusiasts get the first taste of a new architecture
— 7:00 AM on August 5, 2015

Well, this is certainly something. As you may know, Intel has been focused like mad on mobile computing for the past few years, attempting to insert itself into a growing market against established rivals like ARM. Desktop computing has kind of been on the backburner as a result. But a funny thing happened on the way to the death of the PC: yet another revival at the high end of the market. PC gaming is more alive and vibrant than ever, and folks are pioneering new applications like virtual reality on the PC, as well.

Intel has decided to acknowledge the thriving PC gaming market by throwing us a big, juicy bone. The first-ever version of the Skylake, its next-generation CPU architecture, is making its debut today in a pair of socketed processors for desktop PCs. The Core i7-6700K and Core i5-6600K are the first ever Skylake parts available to the public, and they're arriving alongside an armada of motherboards based on the new Z170 chipset.


It's a lake in the sky!
2015 has been a busy year in PC hardware, but it's been full of strange product introductions. We've covered a number of product unveilings that have involved big architecture reveals and great fanfare but very little actual hardware to review. Heck, Intel announced the desktop version of its Broadwell CPUs back in June, but you still can't buy them in North America. Skylake is the opposite situation. We have a Core i7-6700K chip in our grubby hands, but we don't yet know the details of this new CPU microarchitecture. Intel says it's planning to reveal those in a couple of weeks, at its Intel Developer Forum event in San Francisco. So we can show you how Skylake performs, but we can't yet tell you exactly why.

We also don't yet know the exact shape of the entire lineup of Skylake-based products. Intel says it will be releasing the rest of the family later in the third quarter of this year, after IDF. For now, these two desktop CPUs will stand alone.


The Core i7-6700K processor

Truth be told, though, we really do know quite a bit about Skylake already. The desktop CPU we're reviewing is a quad-core, eight-threaded processor with 8MB of L3 cache, much like its predecessors. Skylake parts are manufactured on Intel's 14-nm fabrication process with tri-gate transistors, like the Broadwell parts of the prior generation. However, Skylake is a "tock" in Intel's familiar "tick-tock" development cadence; it brings with it a new CPU microarchitecture.

As usual, this architectural refresh is meant to improve clock-for-clock performance through various clever enhancements to the processor's efficiency and throughput. Intel claims the 6700K is "up to 10% faster" than its predecessor, the Haswell-based Core i7-4790K.


In fact, the Core i7-6700K replaces the 4790K at the exact same price, just as the 6600K replaces the 4690K. The 6700K's peak clock speed is down a little bit, at 4.2GHz instead of the 4.4GHz Turbo peak on the 4790K, but the 6700K can run all four cores at 4.2GHz under load. Since the 6600K and 6700K are both K-series parts, they have unlocked multipliers for easy overclocking, too.

Intel has pushed a little on the power front in order to deliver higher performance in recent generations. The top Ivy Bridge quad core topped out at 77W, while Haswell moved up to 88W. Now, Skylake nudges the limit up to 91W.

That added CPU power draw could be offset at a platform level by the transition to a new memory type, DDR4. Skylake supports both DDR4 and DDR3L type memories, for lower voltage operation and power savings. Bog-standard DDR3 isn't officially supported at its usual 1.5V, although we may see some motherboard makers hack their way around that limitation. Most of the market will likely embrace DDR4 as the new standard, since it promises higher throughput and more headroom for transfer rates going forward. Intel's high-end Haswell-E platform made the transition to DDR4 last year, as did the dual-socket Haswell-EP Xeons.

Like those platforms, Skylake desktop parts are getting a conservative start with DDR4. The first products only officially support 2133 MT/s operating speeds. Running your memory any faster is technically overclocking. That said, our test rigs are outfitted on day one with Corsair Vengeance LPX DIMMs rated for 2666 MT/s operation, and much higher speeds are possible.


The LGA 1151 socket

As you might expect given the new memory type, Skylake processors are not socket-compatible with prior generations. They require a motherboard with a new socket type known as LGA1151. Although the pinouts and plastic retention tabs are different, this socket is virtually the same size and shape as the one that came before it. As a result, any CPU coolers meant for Haswell CPUs ought to be compatible with LGA1151-based motherboards.

The new socket brings another change of note: the death of FIVR, the fully integrated voltage regulator first introduced and hailed as progress on Haswell processors. I expect we'll hear more about the reasons behind the decision to spike the integrated VR at IDF, but we already know FIVR caused some complications for the mobile Broadwell chips. The inductors required by FIVR increased the Z-height of the processor, and Intel had to cut a hole in the motherboard in order to shoehorn Broadwell CPUs into extra-thin devices. Furthermore, FIVR wasn't optimally efficient at low voltages, and that fact prompted Intel to build a bypass mechanism into Broadwell called LVR. As we wrote back then, "The need for 3DL and LVR makes one wonder whether the level of VR integration in Broadwell makes sense for future generations of Intel SoCs." I suppose we don't have to wonder anymore.

The question now is what exact arrangement has replaced FIVR. Presumably, Intel hasn't taken a step back on things like independent supply rails for the CPU cores and the chip's internal I/O ring.

Whatever the case there, Skylake does bring progress on other fronts, including tweakability. Intel says this CPU is its first ever to have features included expressly for overclocking. For one thing, the CPU's base clock or BCLK has been liberated. The Asus Z170 Deluxe motherboard in my test rig offers options from 40 to 500MHz for BCLK speeds in 1MHz increments. Folks tweaking the BCLK don't need to worry about the DMI and PCI Express ratios, either; a new PEG/DMI domain has its own isolated 100MHz clock. That should take a lot of the pain out of BCLK-based overclocking. Finally, Skylake's memory controller supports a ton of different DDR4 speeds, up to 4133 MT/s in increments of 100 and 133MHz.

As a guy who just likes to overclock his stuff and not a dude with a professional overclocking career, I'm not sure what to make of these changes. I mean, I'm quite happy with the unlocked multipliers on K-series processors, which will let you squeeze a little extra out of a processor using conventional cooling. And I'm down with the memory clock flexibility. But those BCLK-oriented tuning knobs are probably meant for the folks with liquid nitrogen pots. I mean, I'm happy that they get these features, but I think it's pretty obvious these capabilities aren't for everyone. I seriously doubt Intel will allow lots of BCLK tuning leeway on the non-K variants of Skylake, for instance. That prospect seems very unlikely.