Zen (microarchitecture)

Zen

Zen's introductory slide, showing major changes
Produced	2016
Designed by	AMD
Common manufacturer(s)	GlobalFoundries Samsung
Instruction set	AMD64 (x86-64)
Cores	2 (essential) 2–6 (mainstream) 8–16 (enthusiast) 8–32 (server)
Created	2012–2016
Transistors	14 nm (FinFET)
Socket(s)	Socket AM4
Predecessor	Excavator
Successor	Zen+
Application	Desktop, Laptop, Server, Workstation
Core name(s)	Raven Ridge (APUs) Summit Ridge (desktop CPUs) Zeppelin (server APUs/CPUs)

Zen was the codename for AMD's 2017 CPU fresh-design "Summit Ridge" microarchitecture, first used in their 14 nm AM4 socket series of CPUs starting in March 2017. Enthusiast Ryzen CPUs debuted on 2 March 2017, Zen-derived Opteron server processors are expected in 2Q 2017, and Zen-derived APUs are expected in 2H17.

The previously released "Bristol Ridge" APUs in Fall 2016, were the first CPU to embrace the AM4 socket, based on the fading Bulldozer architecture.

These APUs and the Ryzen chips embody SoC approaches. Intel provides a CPU and a complex chipset which provides interface functionality such as SATA; AMD with Ryzen incorporates much of that functionality onto the processor package itself. As a general rule, this is likely to somewhat simplify AM4 motherboard designs and SKUs.

Architecture summary

According to AMD, Zen's main focus will be on increasing per-core performance. A 2016 presentation from CERN engineer Liviu Valsan reported that the process would use SMT (simultaneous multithreading) microarchitecture. The change from a clustered multi-thread (CMT) design in the previous Bulldozer microarchitecture) to SMT is expected to offer higher per thread floating point performance at cost of lower core count/larger die size.

This is the first time in a very long time that we engineers have been given the total freedom to build a processor from scratch and do the best we can do. It is a multi-year project with a really large team. It's like a marathon effort with some sprints in the middle. The team is working very hard, but they can see the finish line. I guarantee that it will deliver a huge improvement in performance and power consumption over the previous generation.

— Suzanne Plummer, Zen team leader, on September 19th, 2015.

Reportedly, the Zen architecture will be built on a more efficient 14 nanometer process (probably GloFo 14 LPP), rather than the 32 nm and 28 nm processes of previous AMD FX CPUs and AMD APUs, respectively. The "Summit Ridge" Zen family will also feature a unified AM4 socket with its GPU-equipped "Bristol Ridge" APU counterparts, and feature DDR4 support and a 95 W TDP. Zen does not support DDR3; only 7th generation AMD APUs (also fitting the AMD AM4 unified socket) support DDR3 and DDR4. While newer roadmaps don't confirm the TDP for desktop products, they suggest a range for low-power mobile products with up to two Zen cores from 5 to 15 W and 15 to 35 W for performance-oriented mobile products with up to four Zen cores.

Zen will support the high-performance hardware random number generator instructions recently introduced by Intel's CPUs, RDSEED.

Each Zen core will have four integer units, two address generation units and four floating point units, and the decoder can decode four instructions per clock cycle. L1 cache size is 32 KiB and L2 cache size 512 KiB per core. Two of the floating point units are adders, two are multipliers.

Development

AMD began planning the Zen microarchitecture shortly after re-hiring Jim Keller in early 2012.

The team in charge of the Zen microarchitecture was led by Keller until he left in September 2015 after a 3-year tenure.

Zen was originally planned for 2017 following the ARM64-based K12 sister core, but on AMD's 2015 Financial Analyst Day it was revealed that K12 was delayed in favour of the Zen design, to allow it to enter the market within the 2016 timeframe, with the release of first Zen-based processors expected for October 2016.

In November 2015 a source inside AMD reported that Zen microprocessors had been tested and "met all expectations" with "no significant bottlenecks found".

In December 2015, it was rumored that Samsung may be contracted as a fabricator for AMD's 14 nm FinFET processors, including both Zen and their upcoming Polaris GPU architecture.

Advantages over predecessors

Zen's from-scratch design is notably different from its predecessors, with many different types of changes and enhancements being made across the board in hopes of making Zen more competitive with Intel's architectures, and the software most often built with Intel's processor features in mind. Processors built using Zen will utilize low-power 14 nm FinFET silicon. Prior to Zen, AMD's smallest process size was 28 nm, as utilized by their Kaveri APUs.

Performance

One of Zen's major goals was to focus on performance per-core, and it is rumored to have 40% higher instructions per clock over its predecessor, Excavator, which in comparison offered 4-15% improvement over previous architecture. Zen will also support simultaneous multithreading, a feature previously offered in some Intel processors in the form of HyperThreading. Previous Bulldozer-derived chips from AMD had single blocks of resources that had to be shared between cores, resulting in lower performance per core in multi-threaded applications.

Memory

APUs utilizing the Zen architecture will also support High Bandwidth Memory (HBM). Previous APUs from AMD had to rely on traditional shared DDR3 RAM for video memory.

Zen supports DDR4 memory (up to 8 channels). Previous CPUs and APUs from AMD only supported up to DDR3.

Power consumption

All processors using the Zen architecture will be built at the 14 nm node on FinFET silicon, potentially reducing power consumption and heat over their 28 nm and 32 nm non-FinFET predecessors.

Heat output

Also a byproduct of lower power usage and leakage, processors built on the 14 nm node with the Zen architecture are expected to produce less heat than their predecessors.

Socket compatibility

Due to Zen integrating previously external motherboard components in an SoC-like nature, Zen's AM4 socket will unify the future variants of older AM1, FM2+, and AM3+ sockets.

Cooling

Although the cooling solution is not a component of the architecture itself, AMD announced that their quieter and more powerful CPU cooler, named "Wraith", would be included with Zen chips. Further, it has been reported by a manufactuer that Socket AM2 and Socket AM3 compatible coolers will also work for the AM4 socketed Zen processor.

Enhanced Security and Virtualization Support

Zen added the support for AMD's Secure Memory Encryption (SME) and AMD's Secure Encrypted Virtualization (SEV). Secure Memory Encryption is real time memory encryption done per Page Table Entry. This is done utilizing the onboard "Security" Processor (ARM Cortex-A5) at boot time to encrypt each page, allowing any DDR-4 Memory (including Non Volatile varieties) to be encrypted. AMD SME also makes the contents of the memory more resistant to memory snooping and cold boot attacks.

SME can be used to mark individual pages of memory as encrypted through the page tables. A page of memory that is marked encrypted will be automatically decrypted when read from DRAM and will be automatically encrypted when written to DRAM. The SME feature is identified through a CPUID function and enabled through the SYSCFG MSR. Once enabled, page table entries will determine how the memory is accessed. If a page table entry has the memory encryption mask set, then that memory will be accessed as encrypted memory. The memory encryption mask (as well as other related information) is determined from settings returned through the same CPUID function that identifies the presence of the feature.

Products

Official names for products utilizing the Zen architecture have not yet been officially released, as of January 2016. Zen is expected to be utilized in future FX CPUs, future Opteron CPUs, and future APUs.

Zen will also be utilized for future Opteron server processors.

See also

• Steamroller (microarchitecture)