XMM Architecture (eXpress Memory Module) offers an innovative open hardware specification designed for decentralized, high-bandwidth compute-attached memory. This cutting-edge architecture aims to resolve the limitations posed by current memory technologies in modern computing environments, particularly as CPU core counts continue to rise.

Motivation

The increasing number of CPU cores leads to the "Memory Wall," where traditional parallel dual-channel DDR interfaces struggle to deliver adequate memory bandwidth, complicating motherboard design with extensive signal routing. High-Bandwidth Memory (HBM) solutions are often too costly and lack the modularity that is essential for consumers in building PCs. The XMM Architecture proposes a paradigm shift by relocating the complex Memory Controller (IMC) directly onto a discrete RAM board. By leveraging a serial interface such as PCIe/CXL.mem, XMM provides:

• Massive bandwidth
• High modularity
• Advanced cooling capabilities
• True "Unified Memory" for desktop setups

Key Concepts

• Card Edge Form Factor: The memory module is no longer a compact stick but a dedicated board (LP-CEM) resembling a low-profile GPU.
• Serial Host Link: Modules are interconnected using PCIe 6.0/7.0 physical layers with CXL.mem protocols, simplifying motherboard design by removing thousands of parallel traces.
• On-Board Controller: Each module autonomously manages timing, refresh cycles, and error correction code (ECC) through a wide internal bus, supporting 256-bit or 512-bit configurations.
• Democratized Bandwidth: Bridging the performance gap between DDR5 and HBM3, XMM modules can achieve 200–500 GB/s per slot using standard planar DRAM dies.

Project Structure

• SPECIFICATION.md: The core technical whitepaper detailing the architectural specification.
• FAQ.md: Answers to frequently asked questions and technical discussions regarding the architecture.
• CONTRIBUTING.md: Guidelines for submitting Requests for Comments (RFC) or contributions to the specification.
• CODE_OF_CONDUCT.md: Community guidelines to foster a respectful and constructive environment.
• .github/ISSUE_TEMPLATE/: Templates for proposing changes and discussions.

Getting Involved

As a draft specification, the XMM Architecture welcomes contributions from hardware engineers, memory controller designers, and PC enthusiasts. Interested contributors can:

1. Review the Specification: Begin with SPECIFICATION.md to comprehend the goals and design constraints.
2. Submit an RFC: Ideas for connector pinouts, cooling specifications, or protocol enhancements can be shared by opening an Issue using the RFC Template.

This specification is developed under the Creative Commons Attribution 4.0 International (CC-BY-4.0) license, credited to Benjamin Leimer from Vienna, Austria.