Latest revision as of 20:43, 15 April 2025

Cortex-X1 (codename Hera) is a performance-enhanced version of the Cortex-A78 (Hercules), a low-power high-performance ARM microarchitecture designed by Arm for the mobile market. The Cortex-X1 was designed by Arm's Austin, Texas team. This microarchitecture is designed as a synthesizable IP core and is licensed to other semiconductor companies to be implemented in their own chips.

The Cortex-X1, which implements the ARMv8.2 ISA, is a higher performance core that is designed to be combined with the Cortex-A78 in a DynamIQ big.LITTLE combination in order to provide even higher single-thread performance. This core, along with the Cortex-A78, are often combined with a number of low(er) power cores (e.g. Cortex-A55) in order to achieve better energy/performance.

Cortex-X[edit]

ARM • Cortex

Process Technology[edit]

Although the Cortex-X1 may be fabricated on various process nodes, it has been primarily designed for the 10 nm, 7 nm,

and 5 nm process nodes with performance, power and area numbers mainly targeting the 5-nanometer node.

Architecture[edit]

Key changes from Cortex-A78[edit]

See also: Cortex-A78 § Key changes from Cortex-A77

The Cortex-X1 is a custom performance-enhanced variant of the Cortex-A78, therefore it

inherits most of the changes that were done to the Cortex-A78 from the Cortex-A77.

• Higher performance (See § Performance claims)
  • Arm self-reported around 30% performance over the Cortex-A77
    (compared to +20% with the Cortex-A78)
  • 2.0x (machine learning) performance
• Silicon area
  • 15% more silicon area (on N5)
• Front-end
  • 1.25x wider decode (5-way, up from 4-way)
  • 1.33x wider decoded cache bandwidth
    (8 MOPs/cycle, up from 6 MOPs/cycle)
• Memory subsystem
  • Only 64 KiB L1I cache option (from 32-64 KiB)
  • Only 64 KiB L1D cache option (from 32-64 KiB)
  • Up to 1 MiB L2 cache option (from 512 KiB)
  • Up to 8 MiB L3 cache option (from 4 MiB)

Comparison[edit]

"Prime" core

Performance claims[edit]

• Compared to the Cortex-A77, the Cortex-X1 is said to be 30% faster in peak performance on SPEC CPU2006.

The improvement comes from both architectural improvements and frequency improvement with the help

of process improvement moving from the 7 nm to the 5 nm node.

• Arm says that, at ISO-process and frequency, the Cortex-X1 achieves 22% higher integer performance (SPEC CPU2006)

over the Cortex-A78 and 30% higher integer performance over the Cortex-A77. Likewise, due to the doubling

of the number of NEON units, the Cortex-X1 can achieve twice the ML performance as both the A77 and A78.

Overview[edit]

• The Cortex-X1 is a high-performance synthesizable core designed by Arm. It is delivered as Register

Transfer Level (RTL) description in Verilog and is designed to be integrated into customer's SoCs.

• This core supports the ARMv8.2 extension as well as a number of other partial extensions.

This is the first from Arm's Cortex-X custom program. The X1 is a performance-enhanced

version of the A78, it therefore uses the A78 as the starting point for its modifications.

• The Cortex-X1 is built on top of the Cortex-A78, but enhances it in order to extract additional performance,

albeit at a slight reduction in power efficiency and area. To that end, whereas the Hercules was said to provide

a 20% sustain performance uplift over the Cortex-A77, the Cortex-X1 offers up to 30% peak performance.

• In other words, whereas the Cortex-A78 is designed for high sustained performance at high performance-efficiency,

the Cortex-X1 is designed to supplement it with higher peak performance while relaxing the power and area constraints.

• The Cortex-X1 is a fatter version of the Cortex-A78, relying on bigger buffers and a large out-of-order window

in order to extract further performance. To that end, the X1 features a 5-way decode, twice as many NEON units,

and larger overall buffers in order to allow for a bigger out-of-order window with more in-flight operations.

• The Cortex-X1 enlarges the pipeline while still retaining the higher frequency which was introduced in the Cortex-A77.
• The Cortex-X1 is intended to be combined with a number of Cortex-A78 cores in DynamIQ Shared Unit (DSU)

cluster along with possibly with other lower-power cores such as the Cortex-A55 to more efficiently support

a wide range of workloads at various performance and power levels beyond what's possible with any one core.

DSU Cluster[edit]

• The Cortex-X1 provides additional peak performance beyond what the Cortex-A78 can offer.

Therefore the X1 is designed to be combined with a number of Cortex-A78 cores in DynamIQ

Shared Unit (DSU) cluster in order to provide a balance in both power and performance.

• Compared to a quad-core Cortex-A77 cluster on 7 nm, a quad-core Cortex-A78 cluster provides

+20% sustained performance improvement while reducing the silicon area by about 15%.

• When replacing one of those big Cortex-A78 cores with a single Cortex-X1 core, the cluster

can now provide a peak single-thread performance of up to 30% versus the Cortex-A77

at the cost of 15% additional silicon area (or neural area-wise from N7 to N5).

References[edit]

1. ↑ Arm's New Cortex-A78 and Cortex-X1 Microarchitectures: An Efficiency and Performance Divergence.
2. ↑ Schor, David (2020-05-26). Arm Cortex-X1: The First From The Cortex-X Custom Program.
3. ↑ (2023-05-29) Arm Cortex-X4, A720, and A520: 2024 smartphone CPUs deep dive.