Skylake

Intel Core i7-6700K with four physical cores
General information
Launched	August 5, 2015; 10 years ago (2015-08-05)
Discontinued	March 4, 2019; 6 years ago (2019-03-04) (desktop processors)
Marketed by	Intel
Designed by	Intel
Common manufacturer	Intel
CPUID code	0406e3h, 0506e3h
Product code	80662 (mainstream and mobile Xeon E3) 80673 (enthusiast and server)
Performance
Max.CPUclock rate	Up to 5.0 GHz
Cache
L1cache	64 KB per core (32 KB instructions + 32 KB data)
L2 cache	256 KB per core (1 MB per core for Skylake-X, SP, and W)
L3 cache	Up to 38.5 MB shared
L4 cache	128 MB ofeDRAM (on Iris Pro models)
Architecture and classification
Technology node	14 nm bulk silicon 3D transistors (Tri-Gate)
Microarchitecture	Skylake
Instruction set	x86-16,IA-32,x86-64
Extensions	AES-NI,CLMUL,RDRAND,MPX,TXT,SGX[1] MMX,SSE,SSE2,SSE3,SSSE3,SSE4,SSE4.1,SSE4.2,ADX AVX,AVX2,AVX-512 (Skylake-SP, Skylake-W & Skylake-X[2][3][4]),TSX,FMA3 VT-x,VT-d
Physical specifications
Cores	2–28
Sockets	Desktop LGA 1151 LGA 2066 Server & Workstation LGA 3647 LGA 2066 Mobile BGA 1168 BGA 1356 BGA 1515 BGA 1440[5]
Products, models, variants
Product code name	SKL
Brand name	Core i3 Core i5 Core i7 Core i9 Core X-Series Core m3 Core m5 Core m7 Xeon Celeron Pentium
History
Predecessor	Broadwell (tick/process)
Successors	Kaby Lake (optimization, desktop, laptop, low-end server, and mobile workstation) Cascade Lake (HEDT, workstation, and mid- to high-end server) Palm Cove (process)
Support status
Client: Unsupported as of December 30, 2022 for iGPU Xeon E3 v5: Unsupported as of December 30, 2022 for iGPU Other Xeon: supported

Skylake^[6][7] isIntel's codename for its sixth generationCoremicroprocessor family that was launched on August 5, 2015,^[8] succeeding theBroadwell microarchitecture.^[9] Skylake is a microarchitecture redesign using the same14 nmmanufacturing process technology^[10] as its predecessor, serving as a tock in Intel'stick–tock manufacturing and design model. According to Intel, the redesign brings greater CPU andGPU performance and reduced power consumption. Skylake CPUs share their microarchitecture withKaby Lake,Coffee Lake,Whiskey Lake, andComet Lake CPUs.

Skylake is the last Intel platform on which Windows earlier thanWindows 10 are officially supported byMicrosoft,^[11] although enthusiast-createdmodifications are available that disabled theWindows Update check and allowedWindows 8.1 and earlier to continue to receiveWindows Updates on this and later platforms.^[12][13][14]

Some of the processors based on the Skylake microarchitecture are marketed as sixth-generation Core.^[15][16][17]

Intel officially declared end of life and discontinued Skylake LGA 1151 CPUs (except i3-6100, i5-6500, and Xeon E3 v5) on March 4, 2019.^[18]

Skylake's development, as with previous processors such asBanias,Dothan,Conroe,Sandy Bridge, andIvy Bridge, was primarily undertaken by Intel Israel at its engineering research center inHaifa, Israel.^[19] The final design was largely an evolution ofHaswell, with minor improvements to performance and several power-saving features being added.^[20] A major priority of Skylake's design was to design a microarchitecture for envelopes as low as 4.5W to embed withintablet computers andnotebooks in addition to higher-powerdesktop computers andservers.^[21]

In September 2014, Intel announced the Skylake microarchitecture at theIntel Developer Forum inSan 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 desktoptestbed system, running the latest version of3DMark, while the second computer was a fully functional laptop, playing4K video.^[22]

An initial batch of Skylake CPU models (i5-6600K and i7-6700K) was announced for immediate availability during theGamescom on August 5, 2015,^[23] unusually soon after the release of its predecessor, Broadwell, which had suffered from launch delays.^[24] 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;^[25] yet, the 14 nm production was back on track and in full production as of Q3 2014.^[26] 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.^[27][28] As a result, the Broadwell architecture had an unusually short run.^[27]

Officially Intel supportedoverclocking 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 asASRock (which prominently promoted it under the name Sky OC) allowed the base clock to be modified in this manner.^[29][30]

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.^[31]

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 CPUmicrocode update that removes the function.^[32][33][34] In April 2016, ASRock started selling motherboards that allow overclocking of unsupported CPUs using an external clock generator.^[35][36]

In January 2016, Microsoft announced that it would end support ofWindows 7 andWindows 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 (until July 31, 2019; August 2019 critical update requires at leastWindows 10), and Windows 10 would be the onlyMicrosoft Windows platform officially supported on Skylake and on later Intel CPU microarchitectures beginning with Skylake's successorKaby 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.^[37][38]

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.^[39][40] 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.^[41][42] In addition, an enthusiast-createdmodification was released that disabled theWindows Update check and allowed Windows 8.1 and earlier to continue to be updated on this and later platforms.^[43]

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.^[44]

Skylake is fully supported onOpenBSD 6.2 and later, includingaccelerated graphics.^[45]

ForWindows 11, only the high-end Skylake-X processors are officially listed as compatible.^[46] All other Skylake processors are not officially supported due to security concerns.^[47] However, it is still possible to manually upgrade using an ISO image (as Windows 10 users on those processors will not be offered to upgrade to Windows 11 via Windows Update), or perform a clean installation as long as the system hasTrusted Platform Module (TPM) 2.0 enabled,^[48] 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.^[49][50]

Like its predecessor,Broadwell, Skylake is available in five variants, identified by thesuffixes S (SKL-S), X (SKL-X), H (SKL-H), U (SKL-U), and Y (SKL-Y). SKL-S and SKL-X containoverclockable K and X variants withunlocked multipliers.^[51] The H, U and Y variants are manufactured inball grid array (BGA) packaging, while the S and X variants are manufactured inland grid array (LGA) packaging using a new socket,LGA 1151 (LGA 2066 for Skylake X).^[52] Skylake is used in conjunction withIntel 100 Series chipsets, also known asSunrise Point.^[53]

The major changes between the Haswell and Skylake architectures include the removal of thefully integrated voltage regulator (FIVR) introduced with Haswell.^[54] On the variants that will use a discretePlatform Controller Hub (PCH),Direct Media Interface (DMI) 2.0 is replaced byDMI 3.0, which allows speeds of up to 8 GT/s.

Skylake's U and Y variants support oneDIMM slot per channel, while H and S variants support two DIMM slots per channel.^[52] Skylake's launch and sales lifespan occur at the same time as the ongoingSDRAM market transition, withDDR3 SDRAM memory gradually being replaced byDDR4 SDRAM. Rather than working exclusively with DDR4, the Skylake microarchitecture remainsbackward 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, calledUniDIMM, was also announced.^[55]

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 includeThunderbolt 3.0,Serial ATA Express,Iris Pro graphics withDirect3D feature level 12_1 with up to 128 MB of L4eDRAM cache on certain SKUs.^[56] The Skylake line of processors retiresVGA support,^[57] while supporting up to three monitors connected via HDMI 1.4, DisplayPort 1.2 or Embedded DisplayPort (eDP) interfaces.^[58] HDMI 2.0 (4K@60 Hz) is only supported on motherboards equipped with Intel's Alpine Ridge Thunderbolt controller.^[59]

The Skylake instruction set changes includeIntel MPX (Memory Protection Extensions) andIntel SGX (Software Guard Extensions). Future Xeon variants will also haveAdvanced Vector Extensions 3.2 (AVX-512F).^[3][4]

Skylake-based laptops were predicted to use wireless technology calledRezence 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.^[60][61]

The integrated GPU of Skylake's S variant supports on WindowsDirectX 12 Feature Level 12_1,OpenGL 4.6 with latest Windows 10 driver update^[62] (OpenGL 4.5 on Linux^[63]) andOpenCL 3.0 standards. TheQuick Sync video engine now includes support forVP9 (GPU accelerated decode only),VP8 andHEVC (hardware accelerated 8-bit encode/decode and GPU accelerated 10-bit decode), and supports for resolutions up to 4096 × 2048.^[64][65][66]

Intel also released unlocked (capable of overclocking) mobile Skylake CPUs.^[67]

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.

Started from Skylake, Intel had removed IDE mode (of SATA controller) and EHCI controller from its client platform chipsets.

Short loops with a specific combination of instruction use may cause unpredictable system behavior on CPUs with hyperthreading. Amicrocode update was issued to fix the issue.^[68]

Skylake is vulnerable toSpectre attacks.^[69]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 thespinlockPAUSE 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.^[70] Intel documents the increased latency as a feature that improves power efficiency.^[71]

• Improved front-end, deeper out-of-order buffers, improvedexecution units, more execution units (third vector integerALU(VALU)) for five ALUs in total, more load/storebandwidth, improvedhyper-threading (wider retirement), speedup of AES-GCM and AES-CBC by 17% and 33% accordingly.^[72][73]
• Up to four cores as the default mainstream configuration^[74] and up to 18 cores for X-series
• AVX-512: F, CD, VL, BW, and DQ for Xeon Scalable and W variants, but not Xeon E3^[3]
• Intel Memory Protection Extensions (MPX)
• IntelSoftware Guard Extensions (SGX)
• IntelTransactional Synchronization Extensions (Disabled in 2021^[75])
• Intel Speed Shift^[76]
• Largerre-order buffer (224 entries, up from 192)
• L2 cache was changed from 8-way to 4-way set associative^[77]
• 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 forInstruction Pointer (IP) address filtering.^[78]
• 64 to 128 MB L4eDRAM cache on certain SKUs
• Add C10C-states on certain SKUs (mobile)^[79]

• Skylake's integratedGen9 GPU supports Direct3D 12 at thefeature level 12_1^[6][80][81]
• Vulkan 1.3 support (1.4 on Linux by Mesa 25.0 and intel ANV driver^[82])^[83]
• Full fixed functionHEVC Main/8bit encoding(only4:2:0^[84]) and decoding acceleration (Level 5.1).
• Hybrid/PartialHEVC Main10/10bit decoding acceleration.
• PartialVP9 decoding acceleration.^[85]
• JPEG encoding acceleration for resolutions up to 16,000×16,000 pixels.
• Offloading some media decoding functionality to HEVC/H.265 micro controller (HuC)^[86]
• Frame Buffer Compression (FBC)
• Up to 72 Execution Units (from 48)^[87]
• AddMultiplane Overlay (MPO)^[87]
• 16-bit float support^[87]

• LGA 1151 socket for mainstream desktop processors andLGA 2066 socket for enthusiast gaming/workstation X-series processors
• 100-series chipset (Sunrise Point)^[88] and 200-series chipset (Union Point)
• X-series uses X299-series chipset
• DMI 3.0 (FromDMI 2.0)
• Support for bothDDR3L SDRAM andDDR4 SDRAM in mainstream variants, using customUniDIMM SO-DIMM form factor^[74][89][90] with up to 64 GB ofRAM onLGA 1151 variants. UsualDDR3 memory is also supported by certain motherboard vendors even though Intel does not officially support it.^[91][92]
• Support for 16PCI Express 3.0 lanes from CPU, 20PCI Express 3.0 lanes from PCH (LGA 1151), 44PCI Express 3.0 lanes for Skylake-X
• Support forThunderbolt 3 (Alpine Ridge)^[93]
• AddNVMePCIe 3.0 x4 support through chipset

• Thermal design power (TDP) up to 95 W (LGA 1151); up to 165 W (LGA 2066)^[94]
• 14 nm manufacturing process^[95]

Skylake processors are produced in seven main families: Y, U, H, S, X, W, and SP. Multiple configurations are available within each family:^[52]

Common features of the mainstream desktop Skylake CPUs:

• DMI 3.0 andPCIe 3.0 interfaces
• Dual-channel memory support in the following configurations: DDR3L-1600 1.35 V (32 GB maximum) or DDR4-2133 1.2 V (64 GB maximum). DDR3 is unofficially supported through some motherboard vendors^[96][97][98]
• 16PCIe 3.0 lanes
• The Core-branded processors support the AVX2 instruction set. The Celeron and Pentium-branded ones support only SSE4.1/4.2
• 350 MHz base graphics clock rate

Common features of the high-performance Skylake-X CPUs:

• In addition to the AVX2 instruction set, they also support theAVX-512 instructions
• No built-in iGPU (integrated graphics processor)
• Turbo Boost Max Technology 3.0 for up to two/four threads workloads for CPUs that have eight cores and more (7820X, 7900X, 7920X, 7940X, 7960X, 7980XE, and all ninth generation chips)^[100]
• A different cache hierarchy (when compared to client Skylake CPUs or previous architectures)

• Marketed as a Xeon
• Uses the C621 chipset
• Xeon W-3175X was the only Xeon with a multiplier officially unlocked foroverclocking until the introduction ofSapphire Rapids-WS Xeon CPUs in 2023.^[106]

For mobile workstation processors, seeServer processors

• All models support:MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,AVX,AVX2,AVX-512,FMA3,MPX, Enhanced IntelSpeedStep Technology (EIST),Intel 64, XD bit (anNX bit implementation),Intel VT-x,Intel VT-d,Turbo Boost (excluding W-2102 and W-2104),Hyper-threading (excluding W-2102 and W-2104),AES-NI,Intel TSX-NI, Smart Cache.
• PCI Express lanes: 48
• Supports up to eight DIMMs of DDR4 memory, maximum 512 GB.

E3 series server chips all consist of System Bus 9 GT/s, maximum memory bandwidth of 34.1GB/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.

• Xeon Platinum supports up to eight sockets. Xeon Gold supports up to four sockets. Xeon Silver and Bronze support up to two sockets.
  • −M: 1536 GB RAM per socket instead of 768 GB RAM fornon−M SKUs
  • −F: integratedOmniPath fabric
  • −T: High thermal-case and extended reliability
• Support for up to 12DIMMs ofDDR4 memory per CPU socket.
• Xeon Platinum, Gold 61XX, and Gold 5122 have twoAVX-512 FMA units per core. Xeon Gold 51XX (except 5122), Silver, and Bronze have a singleAVX-512 FMA unit per core.

• Xeon Bronze 31XX has no HT or Turbo Boost support.
• Xeon Bronze 31XX supports DDR4-2133 MHz RAM. Xeon Silver 41XX supports DDR4-2400 MHz RAM.
• Xeon Bronze 31XX and Xeon Silver 41XX support two UPI links at 9.6 GT/s.

• Xeon Gold 51XX and F SKUs has two UPIs at 10.4 GT/s. Xeon Gold 61XX has three UPIs at 10.4 GT/s.
• Xeon Gold 51XX support DDR4-2400 MHz RAM (except 5122). Xeon Gold 5122 and 61XX support DDR4-2666 MHz RAM.

• 
  Intel Skylake Xeon gold processor
• 
  Intel Skylake Xeon gold processor, delidded
• 
  Die shot

• Xeon Platinum non-F SKUs have three UPIs at 10.4 GT/s. Xeon Platinum F-SKUs have two UPIs at 10.4 GT/s.
• Xeon Platinum supports DDR4-2666 MHz RAM.

• List of Intel CPU microarchitectures

• Ung, Gordon (August 18, 2015)."Intel Skylake CPU details revealed: These faster, more power-efficient chips can even drive three 4K monitors". PC world.
• Shields, Anne (September 19, 2015)."Intel's Skylake a New Lifeline for the PC Market?". Yahoo Finance.
• "Graphics Driver Support Update for 10th Generation and Older Intel Processor Graphics". Intel Support. December 30, 2022.

• Intel x86 microprocessors
• Intel microarchitectures
• Transactional memory
• X86 microarchitectures
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