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Zen 2 µarch
General Info
Arch Type	CPU
Designer	AMD
Manufacturer	TSMC
Introduction	2019
Process	7 nm
Succession
Zen+ Zen 3

Zen 2 is a planned microarchitecture being developed by AMD as a successor to Zen+. Zen 2 is expected to be succeeded by Zen 3.

History

Zen 2 is set to succeed Zen in the future, sometimes around 2019. In February of 2017 Lisa Su, AMD's CEO announced their future roadmap to include Zen 2 and later Zen 3. On Investor's Day May 2017 Jim Anderson, AMD Senior Vice President, confirmed that Zen 2 is set to utilize 7 nm process.

Codenames

Preliminary Data! Information presented in this article deal with future products, data, features, and specifications that have yet to be finalized, announced, or released. Information may be incomplete and can change by final release.

Core	C/T	Target
Rome	Up to 64/128	High-end server multiprocessors
Castle Peak	?/?	workstation & enthusiasts market processors
Matisse	?/?	Mainstream to high-end desktops & enthusiasts market processors
Picasso	?/?	Mainstream desktop & mobile processors with GPU

Process technology

Zen 2 is fabricated on TSMC's 7 nm process.

Compiler support

Compiler	Arch-Specific	Arch-Favorable
GCC	-march=znver2	-mtune=znver2

• Note: Initial support in GCC 9.

Architecture

Zen 2 inherits most of the design from Zen+ but improves the instruction stream bandwidth and floating-point throughput performance.

Key changes from Zen+

• 7 nm process (from 12 nm)
  • I/O die still utilizes 14 nm
• Core
  • Front-end
    • Improved branch prediction unit
      • Improved prefetcher
    • Improved µOP cache tags
    • Improved µOP cache
      • Larger µOP cache (?? enters, up from 2048)
    • Increased dispatch bandwidth
  • Back-end
    • Increased retire bandwidth (??-wide, up from 8-wide)
    • FPU
      • 2x wider datapath (256-bit, up from 128-bit)
      • 2x wider EUs (256-bit FMAs, up from 128-bit FMAs)
      • 2x wider LSU (2x256-bit L/S, up from 128-bit)
• Security
  • In-silicon Spectre enhancements
  • Increase number of keys/VMs supported
• I/O
  • PCIe 4.0 (from 3.0)
  • Infinity Fabric 2
    • 2.3x transfer rate per link (25 GT/s, up from ~10.6 GT/s)

This list is incomplete; you can help by expanding it.

New instructions

Zen 2 introduced a number of new x86 instructions:

• CLWB - Force cache line write-back without flush
• RDPID - Read Processor ID
• WBNOINVD - Force cache line write-back without invalidation

Block Diagram

Individual Core

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Memory Hierarchy

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Core

Zen 2 largely builds on Zen. Most of the fine details have not been revealed by AMD yet.

Front End

In order to feed the backend, which has been widened to support 256-bit operation, the front-end throughput was improved. AMD reported that the branch prediction unit has been reworked. This includes improvements to the prefetcher and various undisclosed optimizations to the instruction cache. The µOP cache was also tweaked including changes to the µOP cache tags and the µOP cache itself which has been enlarged to improve the instruction stream throughput.

Execution Engine

AMD stated that both the dispatch bandwidth and the retire bandwidth has been increased.

Floating Point

The floating-point unit underwent major modifications in Zen 2. In Zen, AVX2 256 bit single and double precision vector floating-point data types were supported through the use of two 128 bit micro-ops per instruction. Likewise, the floating-point load and store operations were 128 bits wide. In Zen 2, the datapath and the execution units were widened to 256 bits, doubling the vector throughput of the core.

With two 256-bit FMAs, Zen 2 is capable of 16 FLOPs/cycle.

Bibliography

• AMD 'Tech Day', February 22, 2017
• AMD 2017 Financial Analyst Day, May 16, 2017
• AMD GCC 9 znver2 enablement patch
• AMD 'Next Horizon', November 6, 2018

See Also

• Intel Ice Lake