The Skylake CPU Architecture

As with any new Intel architecture, the devil is in the details. Previously at AnandTech we have been able to provide deep dives into what exactly is going on in the belly of the beast, although the launch of Skylake has posed a fair share of problems.

Nominally we rely on a certain amount of openness from the processor/SoC manufacturer in providing low level details that we can verify and/or explain. In the past, this information has typically been provided in advance of the launch by way of several meetings/consultations with discussions talking to the engineers. There are some things we can probe, but others are like a black box. The black box nature of some elements, such as Qualcomm’s Adreno graphics, means that it will remain a mystery until Pandora’s box is opened.

In the lead up to the launch of Intel’s Skylake platform, architecture details have been both thin on the ground and thin in the air, even when it comes down to fundamental details about the EU counts of the integrated graphics, or explanations regarding the change in processor naming scheme. In almost all circumstances, we’ve been told to wait until Intel’s Developer Forum in mid-August for the main reason that the launch today is not the full stack Skylake launch, which will take place later in the quarter. Both Ryan and I will be at IDF taking fastidious notes and asking questions for everyone, but at this point in time a good portion of our analysis comes from information provided by sources other than Intel, and while we trust it, we can't fully verify it as we normally would.

As a result, the details on the following few pages have been formed through investigation, discussion and collaboration outside the normal channels, and may be updated as more information is discovered or confirmed. Some of this information is mirrored in our other coverage in order to offer a complete picture in each article as well. After IDF we plan to put together a more detailed architecture piece as a fundamental block in analyzing our end results.

The CPU

As bad as it sounds, the best image of the underlying processor architecture is the block diagram:

From a CPU connectivity standpoint, we discussed the DDR3L/DDR4 dual memory controller design on the previous page so we won’t go over it again here. On the PCI-Express Graphics allocation side, the Skylake processors will have sixteen PCIe 3.0 lanes to use for directly attached devices to the processor, similar to Intel's previous generation processors. These can be split into a single PCIe 3.0 x16, x8/x8 or x8/x4/x4 with basic motherboard design. (Note that this is different to early reports of Skylake having 20 PCIe 3.0 lanes for GPUs. It does not.)

With this, SLI will work up to x8/x8. If a motherboard supports x8/x4/x4 and a PCIe card is placed into that bottom slot, SLI will not work because only one GPU will have eight lanes. NVIDIA requires a minimum of PCIe x8 in order to enable SLI. Crossfire has no such limitation, which makes the possible configurations interesting. Below we discuss that the chipset has 20 (!) PCIe 3.0 lanes to use in five sets of four lanes, and these could be used for graphics cards as well. That means a motherboard can support x8/x8 from the CPU and PCIe 3.0 x4 from the chipset and end up with either dual-SLI or tri-CFX enabled when all the slots are populated.

DMI 3.0

The processor is connected to the chipset by the four-lane DMI 3.0 interface. The DMI 3.0 protocol is an upgrade over the previous generation which used DMI 2.0 – this upgrade boosts the speed from 5.0 GT/s (2GB/sec) to 8.0 GT/s (~3.93GB/sec), essentially upgrading DMI from PCIe 2 to PCIe 3, but requires the motherboard traces between the CPU and chipset to be shorter (7 inches rather than 8 inches) in order to maintain signal speed and integrity. This also allows one of the biggest upgrades to the system, chipset connectivity, as shown below in the HSIO section.

CPU Power Arrangements

Moving on to power arrangements, with Skylake the situation changes as compared to Haswell. Prior to Haswell, voltage regulation was performed by the motherboard and the right voltages were then put into the processor. This was deemed inefficient for power consumption, and for the Haswell/Broadwell processors Intel decided to create a fully integrated voltage regulator (FIVR) in order to reduce motherboard cost and reduce power consumption. This had an unintended side-effect – while it was more efficient (good for mobile platforms), it also acted as a source of heat generation inside the CPU with high frequencies. As a result, overclocking was limited by temperatures and the quality of the FIVR led to a large variation in results. For Skylake on the desktop, the voltage regulation is moved back into the hands of the motherboard manufacturers. This should allow for cooler processors depending on how the silicon works, but it will result in slightly more expensive motherboards.

A slight indication of this will be that some motherboards will go back to having a large amount of multiplexed phases on the motherboard, and it will allow some manufacturers to use this as a differentiating point, although the usefulness of such a design is sometimes questionable.

Also Launching Today: Z170 Motherboards, Dual Channel DDR4 Kits Skylake's iGPU: Intel Gen9
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  • bischofs - Thursday, August 6, 2015 - link

    Absolutely agree, My overclocked 920 still runs like a watch after 8 years. Not sure what Intel is doing these days, but lack of competition is really impacting this market. Reply
  • stux - Friday, August 7, 2015 - link

    I upgraded my 920 to a 990x, it runs at about 4.4ghz on air in an XPC chassis! and has 6/12 cores.

    I bought it off ebay cheap, and with an SSD on a SATA3 card I see no reason to upgrade. It works fantastically well, and is pretty much as fast as any modern 4 core machine.
    Reply
  • Samus - Sunday, October 25, 2015 - link

    If you single GPU and don't go ultra-high-end then gaming is still relevant on x58, but it really isn't capable of SLI due to PCIe 2.0 and the lanes being reduced to 8x electrical when more than one 16x length slot is used. QPI also isn't very efficient by todays standards and at the time, AMD still had a better on-die memory controller, but Intel's first attempt was commendable, but completely overhauled with Sandy Bridge which offered virtually the same performance from 2 channels. Anybody who has run dual channel on X58 knows how bad it actually is and why triple channel is needed to keep it competitive with todays platforms.

    I loved X58. It is undoubtedly the most stable platform I'd had since the 440BX. But as I said, by todays standards, it makes Sandy Bridge seem groundbreaking, not because of the IPC, but because of the chipset platform. The reduced power consumption, simplicity and overall smaller-size and lower cost of 60/70 series chipsets, then the incredibly simplified VRM layout in 80/90 chipsets (due to the ondie FIVR of Haswell) makes X58 "look" ancient, but as I said, still relevant.

    Just don't load up the PCIe bus. A GPU, sound card and USB 3.0 controller is about as far as you want to go, and for the most part, as far as you need too!
    Reply
  • vdek - Thursday, August 6, 2015 - link

    Get a Xeon 5650, 6 core CPU, 32nm, will run at 4-4.2ghz all day on air. I upgraded my i7 920 the X5650 and I couldn't be happier. They go for about $70-80 on amazon or ebay. I'm planning on keeping my desktop for another 2-3 years, I upgraded the GPU to a GTX970 and it maxes out most of what I can throw at it. I don't really see my CPU as a bottleneck here. Reply
  • mdw9604 - Tuesday, August 11, 2015 - link

    Can you OC a Xeon 5650? Reply
  • mapesdhs - Wednesday, August 12, 2015 - link

    Of course, back then the main oc'ing method was still bclk-based based, though X58 was a little more involved than that compared to P55 (uncore, etc.) Reply
  • LCTR - Saturday, August 15, 2015 - link

    I'd been pondering the 6700K until I saw these posts from 920 users :)
    I use mine for gaming / video editing, it's running non-hyperthreaded at 4.2GHz on air (about 4Ghz with HT on)

    I also upgraded my GPU to a 970 and have seen decent gaming performance - if I could jump to a X5650 and stretch things for 1-2 years that'd be great...

    What sort of performance do you see from the X5650? Would it win 4GHz with HT enabled?
    The Xeon 5650's don't need any special mobo support or anything, do they? I have a gigabyte GA-EX58-UD5

    Reply
  • Nfarce - Wednesday, August 5, 2015 - link

    Well sadly, ever since SB (which I have one that's 4 years old, a 2500K, alongside a newer Haswell 4690K, each new tick/tock has not been much. The days of getting 50% boost in performance between a few generations are long gone, let alone 100% boost, or doubling performance. Also keep in mind that there is a reason for this decrease in increased performance: as dies shrink, physics with electrons start becoming an issue. Intel has been focusing more on decreased power usage. At some point CPU manufacturers will need to look at an entirely different manufacturing material and design as silicon and traditional PCB design is coming to its limit. Reply
  • Mr Perfect - Wednesday, August 5, 2015 - link

    It's not even 30% in high-end gaming. There is a clear improvement between SB and Skylake, but why should I build a whole new PC for 5FPS? I can't justify that expense.

    I'd be curious to see the high-end gaming benchmarks rerun with the next generation of GPUs. Will next gen GPUs care more about the CPU, or does DX12 eliminate the difference altogether?
    Reply
  • mkozakewich - Thursday, August 6, 2015 - link

    It's unlikely you'll be seeing doubles and doubles anymore. If you look at what's been going on for the past several years, we're moving to more efficient processes instead of improving performance. I'm sure Intel's end goal is to make powerful CPUs for devices that fit into people's pockets. At that point you might see more start going into raw performance. Reply

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