Skylake (microarchitecture)

Skylake is the codename used by Intel for a processor microarchitecture that was launched in August 2015 succeeding the Broadwell microarchitecture. Skylake is a microarchitecture redesign using the same 14 nm manufacturing process technology as its predecessor, serving as a "tock" in Intel's "tick–tock" manufacturing and design model. According to Intel, the redesign brings greater CPU and GPU performance and reduced power consumption. Skylake CPUs share their microarchitecture with Kaby Lake, Coffee Lake, Cannon Lake, Whiskey Lake, and Comet Lake CPUs.
Skylake is the last Intel platform on which Windows earlier than Windows 10 will be officially supported by Microsoft, although enthusiast-created modifications exist that allow Windows 8.1 and earlier to continue to receive Windows Updates on later platforms.
Some of the processors based on the Skylake microarchitecture are marketed as "6th-generation Core".
Intel officially declared end of life and discontinued Skylake LGA 1151 CPUs on March 4, 2019.
Development history.
Skylake's development, as with processors such as Banias, Dothan, Conroe, Sandy Bridge and Ivy Bridge, was primarily undertaken by Intel Israel at its engineering research center in Haifa, Israel. The Haifa development team worked on the project for four years, and faced many challenges: "But by re-writing the microarchitecture and developing new concepts such as the Speed Shift Technology, we created a processor for 4.5 W to 45 W mobile devices, and up to 91 W for desktop devices." The Skylake processors are used to power a wide range of devices, from fanless laptops, all the way to desktops. "Because of Skylake's features, companies will be able to release laptop PCs that are half as thick and half as heavy as those from five years ago," according to Intel.
In September 2014, Intel announced the Skylake microarchitecture at the Intel Developer Forum in San Francisco, and that volume shipments of Skylake CPUs were scheduled for the second half of 2015. Also, the Skylake development platform was announced to be available in Q1 2015. During the announcement, Intel also demonstrated two computers with desktop and mobile Skylake prototypes: the first was a desktop testbed system, running the latest version of 3DMark, while the second computer was a fully functional laptop, playing 4K video.
An initial batch of Skylake CPU models (6600K and 6700K) was announced for immediate availability during the Gamescom on August 5, 2015, unusually soon after the release of its predecessor, Broadwell, which had suffered from launch delays. Intel acknowledged in 2014 that moving from 22 nm (Haswell) to 14 nm (Broadwell) had been its most difficult process to develop yet, causing Broadwell's planned launch to slip by several months; yet, the 14 nm production was back on track and in full production as of Q3 2014. Industry observers had initially believed that the issues affecting Broadwell would also cause Skylake to slip to 2016, but Intel was able to bring forward Skylake's release and shorten Broadwell's release cycle instead. As a result, the Broadwell architecture had an unusually short run.
Overclocking of unsupported processors.
Officially Intel supported overclocking of only the "K" and "X" versions of Skylake processors. However, it was later discovered that other "non-K" chips could be overclocked by modifying the base clock value – a process made feasible by the base clock applying only to the CPU, RAM, and integrated graphics on Skylake. Through beta UEFI firmware updates, some motherboard vendors, such as ASRock (which prominently promoted it under the name "Sky OC") allowed the base clock to be modified in this manner.
When overclocking unsupported processors using these UEFI firmware updates, several issues arise:
These issues are partly caused by the power management of the processor needing to be disabled for base clock overclocking to work.
In February 2016, however, an ASRock firmware update removed the feature. On February 9, 2016, Intel announced that it would no longer allow such overclocking of non-K processors, and that it had issued a CPU microcode update that removes the function. In April 2016, ASRock started selling motherboards that allow overclocking of unsupported CPUs using an external clock generator.
Operating system support.
In January 2016, Microsoft announced that it would end support of Windows 7 and Windows 8.1 on Skylake processors effective July 17, 2017; after this date, only the "most critical" updates for the two operating systems would be released for Skylake users if they have been judged not to affect the reliability of the OS on older hardware, and Windows 10 would be the only Microsoft Windows platform officially supported on Skylake, as well as all future Intel CPU microarchitectures beginning with Skylake's successor Kaby Lake. Terry Myerson stated that Microsoft had to make a "large investment" in order to reliably support Skylake on older versions of Windows, and that future generations of processors would require further investments. Microsoft also stated that due to the age of the platform, it would be "challenging" for newer hardware, firmware, and device driver combinations to properly run under Windows 7.
On March 18, 2016, in response to criticism over the move, primarily from enterprise customers, Microsoft announced revisions to the support policy, changing the cutoff for support and non-critical updates to July 17, 2018 and stating that Skylake users would receive all critical security updates for Windows 7 and 8.1 through the end of extended support. In August 2016, citing a "strong partnership with our OEM partners and Intel", Microsoft stated that it would continue to fully support 7 and 8.1 on Skylake through the end of their respective lifecycles. In addition, an enthusiast-created modification was released that disabled the Windows Update check and allowed Windows 8.1 and earlier to continue to be updated on this and later platforms.
As of Linux kernel 4.10, Skylake mobile power management is in reasonably good shape with most Package C states supported seeing some use. If this is not the case, then the cause is likely bugs in the system firmware of the particular computer, which might be resolved by updating the BIOS. The user can easily optimize power management beyond the Linux default settings with the PowerTOP utility; for those who use Lennart Poettering's systemd, PowerTOP provides a service which will start up with the computer and auto-tune various settings to reduce power usage. Linux 4.11 enables Frame-Buffer Compression for the integrated graphics chipset by default, which lowers power consumption. Battery runtime should be similar to Windows 10 and possibly better, but further improvements can still be made.
Skylake is fully supported on OpenBSD 6.2 and later, including accelerated graphics.
Features.
Like its predecessor, Broadwell, Skylake is available in five variants, identified by the suffixes "S" ("SKL-S"), "X" ("SKL-X"), "H" ("SKL-H"), "U" ("SKL-U"), and "Y" ("SKL-Y"). SKL-S and SKL-X contain overclockable "K" and "X" variants with unlocked multipliers. The H, U and Y variants are manufactured in ball grid array (BGA) packaging, while the S and X variants are manufactured in land grid array (LGA) packaging using a new socket, LGA 1151 (LGA 2066 for Skylake X). Skylake is used in conjunction with Intel 100 Series chipsets, also known as "Sunrise Point".
The major changes between the Haswell and Skylake architectures include the removal of the fully integrated voltage regulator (FIVR) introduced with Haswell. On the variants that will use a discrete Platform Controller Hub (PCH), Direct Media Interface (DMI) 2.0 is replaced by DMI 3.0, which allows speeds of up to 8 GT/s.
Skylake's U and Y variants support one DIMM slot per channel, while H and S variants support two DIMM slots per channel. Skylake's launch and sales lifespan occur at the same time as the ongoing SDRAM market transition, with DDR3 SDRAM memory gradually being replaced by DDR4 memory. Rather than working exclusively with DDR4, the Skylake microarchitecture remains backward compatible by interoperating with both types of memory. Accompanying the microarchitecture's support for both memory standards, a new SO-DIMM type capable of carrying either DDR3 or DDR4 memory chips, called UniDIMM, was also announced.
Skylake's few P variants have a reduced on-die graphics unit (12 execution units enabled instead of 24 execution units) over their direct counterparts; see the table below. In contrast, with Ivy Bridge CPUs the P suffix was used for CPUs with completely disabled on-die video chipset.
Other enhancements include Thunderbolt 3.0, SATA Express, Iris Pro graphics with Direct3D feature level 12_1 with up to 128 MB of L4 eDRAM cache on certain SKUs. The Skylake line of processors retires VGA support, while supporting up to five monitors connected via HDMI 1.4, DisplayPort 1.2 or Embedded DisplayPort (eDP) interfaces. HDMI 2.0 (4K@60 Hz) is only supported on motherboards equipped with Intel's Alpine Ridge Thunderbolt controller.
The Skylake instruction set changes include Intel MPX (Memory Protection Extensions) and Intel SGX (Software Guard Extensions). Future Xeon variants will also have Advanced Vector Extensions 3.2 ("AVX-512F").
Skylake-based laptops were predicted to use wireless technology called Rezence for charging, and other wireless technologies for communication with peripherals. Many major PC vendors agreed to use this technology in Skylake-based laptops; however, no laptops were released with the technology as of 2019.
The integrated GPU of Skylake's S variant supports on Windows DirectX 12 Feature Level 12_1, OpenGL 4.6 with latest Windows 10 driver update (OpenGL 4.5 on Linux) and OpenCL 3.0 standards, as well as some modern hardware video encoding/decoding formats such as VP9 (GPU accelerated decode only), VP8 and HEVC (hardware accelerated 8-bit encode/decode and GPU accelerated 10-bit decode).
Intel also released unlocked (capable of overclocking) mobile Skylake CPUs.
Unlike previous generations, Skylake-based Xeon E3 no longer works with a desktop chipset that supports the same socket, and requires either the C232 or the C236 chipset to operate.
Known issues.
Short loops with a specific combination of instruction use may cause unpredictable system behavior on CPUs with hyperthreading. A microcode update was issued to fix the issue.
Skylake is vulnerable to Spectre attacks.
In fact, it is more vulnerable than other processors because it uses indirect branch speculation not just on indirect branches but also when the return prediction stack underflows.
The latency for the spinlock instruction has been increased dramatically (from the usual 10 cycles to 141 cycles in Skylake), which can cause performance issues with older programs or libraries using pause instructions. Intel documents the increased latency as a feature that improves power efficiency.
Configurations.
Skylake processors are produced in five main families: Y, U, H, S, and X. Multiple configurations are available within each family:
List of Skylake processor models.
Mainstream desktop processors.
Common features of the mainstream desktop Skylake CPUs:
High-end desktop processors (Skylake-X).
Common features of the high performance Skylake-X CPUs:
Mobile processors.
See also "Server, Mobile" below for mobile workstation processors.
Server processors.
E3 series server chips all consist of System Bus 9 GT/s, max. memory bandwidth of 34.1 GB/s dual channel memory. Unlike its predecessor, the Skylake Xeon CPUs require C230 series (C232/C236) or C240 series (C242/C246) chipset to operate, with integrated graphics working only with C236 and C246 chipsets. Mobile counterparts uses CM230 and CM240 series chipsets.