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Skylake µarch

General Info

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Skylake (SKL) is Intel's microarchitecture using 14 nm process for mainstream desktops, servers, and mobiles. Skylake became the successor to the short-lived Broadwell which experienced severe delays.

For desktop and mobile, Skylake is branded as 6th Generation Intel Core processors. For server class processors, Intel branded it as Xeon E3 v5, Xeon E5 v5, and Xeon E7 v5.

Codenames

Core	Abbrev	Target
Skylake Y	SKL-Y	2-in-1s detachable, tablets, and computer sticks
Skylake U	SKL-U	Light notebooks, portable All-in-Ones (AiOs), Minis, and conference room
Skylake H	SKL-H	Ultimate mobile performance, mobile workstations
Skylake S	SKL-S	Desktop performance to value, AiOs, and minis

Technology

Main article: Broadwell § Technology

Skylake uses the same 14 nm process used for the Broadwell microarchitecture.

Architecture

Overall Skylake builds upon Intel's previous microarchitecture, Broadwell, but includes a more beefed up front end, more optimized execution engine, and numerous number of smaller enhancements including

Key changes from Broadwell

• Front End
  • Larger legacy pipeline delivery (5 µOPs, up from 4)
  • Larger IDQ delivery (6 µOPs, up from 4)
  • 2.28x larger allocation queue (64/thread, up from 28/thread)
  • Improved branch prediction unit
• Execution Engine
  • Larger re-order buffer (224 entries, up from 192)
  • Larger scheduler (97 entries, up from 60)
    • Larger Integer Register File (180 entries, up from 160)
  • Larger store buffer (56 entries, up from 42)
• Memory
  • L2$ was changed from 8-way to 4-way set associative
• TLBs
  • ITLB
    • 4KB page translations was changed from 4-way to 8-way associative
  • STLB
    • 4KB+2M page translations was changed from 6-way to 12-way associative

Graphics

• Skylake retires VGA support, multi-monitor support for up to 3 displays via HDMI 1.4, DP 1.2, and eDP 1.3 interfaces.
• Direct X 12
• OpenCL 2.0
• OpenGL 4.4
• Up to 24 EUs (same as Haswell, 48 EUs on Iris Pro Graphics)

New instructions

Main article: See §Added_instructions for the complete list

Skylake introduced a number of new instructions:

• SGX - Software Guard Extensions
• MPX -Memory Protection Extensions
• AVX-512 - Advanced Vector Extensions 512

Block Diagram

Memory Hierarchy

Other than a few organizational changes (e.g. L2$ went from 8-way to 4-way set associative), the overall memory structure is identical to Broadwell/Haswell.

• Cache
  • L1I Cache:
    • 32 KB 8-way set associative
      • 64 B line size
      • Write-back policy
      • shared by the two threads, per core
  • L1D Cache:
    • 32 KB 8-way set associative
    • 64 B line size
    • shared by the two threads, per core
    • 4 cycles for fastest load-to-use
    • 64 Bytes/cycle load bandwidth
    • 32 Bytes/cycle store bandwidth
    • Write-back policy
  • L2 Cache:
    • unified, 256 KB 4-way set associative
    • 12 cycles for fastest load-to-use
    • 64B/cycle bandwidth to L1$
    • Write-back policy
  • L3 Cache:
    • Up to 2 MB
    • Per core
    • Up to 16-way set associative
    • Write-back policy
  • L4 Cache:
    • 128 MB
    • Per package
    • Only on the Iris Pro GPUs

Skylake TLB consists of dedicated level one TLB for instruction cache and another one for data cache. Additionally there is a unified second level TLB.

• TLBs:
  • ITLB
    • 4KB page translations:
      • 128 entries; 8-way set associative
      • dynamic partition; divided between the two threads
    • 2MB/4MB page translations:
      • 8 entries; fully associative
      • Duplicated for each thread
  • DTLB
    • 4KB page translations:
      • 64 entries; 4-way set associative
      • fixed partition; divided between the two threads
    • 2MB/4MB page translations:
      • 32 entries; 4-way set associative
      • fixed partition
    • 1G page translations:
      • 4 entries; 4-way set associative
      • fixed partition
  • STLB
    • 4KB+2M page translations:
      • 1536 entries; 12-way set associative
      • fixed partition
    • 1G page translations:
      • 16 entries; 4-way set associative
      • fixed partition