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 previous 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 final design was largely an evolution of Haswell, with minor improvements to performance and several power-saving features being added. A major priority of Skylake's design was to design a microarchitecture for envelopes as low as 4.5W to embed within tablet computers and notebooks in addition to higher-power desktop computers and servers.

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. 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:

• C-states are disabled, therefore the CPU will constantly run at its highest frequency and voltage


• Turbo-boost is disabled


• Integrated graphics are disabled


• AVX2 instruction performance is poor, approximately 4-5 times slower due to the upper 128-bit half of the execution units and data buses not being taken out of their power saving states


• CPU core temperature readings are incorrect

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 supported with most Package C states supported seeing some use. Linux 4.11 enables Frame-Buffer Compression for the integrated graphics chipset by default, which lowers power consumption.

Skylake is fully supported on OpenBSD 6.2 and later, including accelerated graphics.

For Windows 11, only the high-end Skylake-X processors are officially listed as compatible. All other Skylake processors are not officially supported due to security concerns. However, it is still possible to manually upgrade using an ISO image (as Windows 10 users on those processors won't be offered to upgrade to Windows 11 via Windows Update), or perform a clean installation as long as the system has Trusted Platform Module (TPM) 2.0 enabled, but the user must accept that they will not be entitled to receive updates, and that damage caused by using Windows 11 on an unsupported configuration are not covered by the manufacturer's warranty.

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. The Quick Sync video engine now includes support for VP9 (GPU accelerated decode only), VP8 and HEVC (hardware accelerated 8-bit encode/decode and GPU accelerated 10-bit decode), and supports for resolutions up to 40962048.

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.

Architecture changes compared to Broadwell microarchitecture

CPU

* Improved front-end, deeper out-of-order buffers, improved execution units, more execution units (third vector integer ALU(VALU)) for five ALUs in total, more load/store bandwidth, improved hyper-threading (wider retirement), speedup of AES-GCM and AES-CBC by 17% and 33% accordingly.
* Up to four cores as the default mainstream configuration and up to 18 cores for X-series
*AVX-512: F, CD, VL, BW, and DQ for some future Xeon variants, but not Xeon E3
* Intel MPX (Memory Protection Extensions)
* Intel SGX (Software Guard Extensions)
* Intel Speed Shift
*Larger Re-order buffer (224 entries, up from 192)
* L1 cache size unchanged at 32 KB instruction and 32 KB data cache per core.
*L2 cache was changed from 8-way to 4-way set associative
*Voltage regulator module ( FIVR) is moved back to the motherboard
*Enhancements of Intel Processor Trace: fine grained timing through CYC packets (cycle-accurate mode) and support for IP (Instruction Pointer) address filtering.
* 64 to 128 MB L4 eDRAM cache on certain SKUs

GPU

* Skylake's integrated Gen9 GPU supports Direct3D 12 at the feature level 12_1
* Full fixed function HEVC Main/8bit encoding/decoding acceleration. Hybrid/Partial HEVC Main10/10bit decoding acceleration. JPEG encoding acceleration for resolutions up to 16,000×16,000 pixels. Partial VP9 encoding/decoding acceleration.

I/O

* LGA 1151