commit 8d126278ece16e200c1f08f15b5f30c2ada79049 Author: riscvcxh1 Date: Tue Aug 25 19:39:13 2026 +0200 Initial readme diff --git a/README.MD b/README.MD new file mode 100644 index 0000000..5c2706d --- /dev/null +++ b/README.MD @@ -0,0 +1,400 @@ +# RISC-V CPU Series — XH-1 + +Welcome to the initial documentation for the **XH-series**, a family of custom processors based on the **RISC-V instruction set architecture (ISA)**. + +The XH-series is a planned family of **four processors**, beginning with **XH-1**, a custom **128-core RISC-V CPU** currently in the research and development phase. + +The XH-series is being developed as an exploration into large-scale processor architecture, multicore design, parallel execution, and the practical implementation of a high-core-count RISC-V system. XH-1 serves as the foundation of the series and will establish many of the architectural concepts, design methodologies, and engineering principles that may influence the processors that follow. + +The remaining processors — **XH-2, XH-3, and XH-4** — will be developed after sufficient research and experience has been gained from XH-1. + +--- + +## What is the XH-Series? + +The **XH-series** is a family of custom RISC-V processors designed around the idea of building increasingly capable and sophisticated multicore CPUs. + +The first processor in the series, **XH-1**, is designed around a **128-core architecture**. Rather than being a conventional single-core processor or a small multicore system, XH-1 is intended to investigate the challenges involved in coordinating a large number of CPU cores within a single processor design. + +The series will explore areas such as: + +* High-core-count CPU architecture +* RISC-V ISA implementation +* Parallel execution +* Multicore organization +* Inter-core communication +* Memory hierarchy design +* Cache architecture +* Core-to-core synchronization +* Hardware scheduling and coordination +* Interrupt handling +* Shared and distributed resources +* On-chip communication +* Verification of large multicore systems +* FPGA and/or hardware implementation +* Performance scaling +* Future CPU architectural extensions + +The exact implementation of these components is still being researched and will be documented as the XH-1 architecture matures. + +--- + +# XH-1 + +**XH-1 is the first processor in the XH-series and the foundation for all future development.** + +At its core, XH-1 is a **custom 128-core RISC-V CPU**. + +The processor is intended to demonstrate and investigate how a large number of RISC-V cores can operate together as a single coherent processing system. + +Instead of focusing exclusively on increasing the complexity of an individual CPU core, XH-1 places significant emphasis on **scaling the number of cores** and understanding the architectural problems that emerge when many processing units must work together. + +### Core Count + +**XH-1: 128 CPU cores** + +The 128 cores are intended to operate as part of a unified processor architecture, with the exact organization of the cores, memory system, interconnect, caches, and other shared resources being defined during the research phase. + +The core count is one of the defining characteristics of XH-1 and provides the basis for much of the project's architectural research. + +--- + +# Why 128 Cores? + +Modern computing workloads increasingly benefit from parallel execution. While increasing the performance of a single CPU core remains important, there are fundamental architectural and practical limits to continuously increasing single-thread performance. + +A high-core-count design approaches the problem from another direction: instead of relying entirely on one extremely powerful execution engine, the processor provides a large number of independent processing units capable of executing work concurrently. + +With **128 cores**, XH-1 is intended to explore questions such as: + +* How should 128 cores be organized? +* How should the cores communicate? +* How should memory be shared between them? +* How can contention for shared resources be minimized? +* How should interrupts be distributed? +* How should synchronization be handled? +* How does performance scale as additional cores are enabled? +* What types of workloads benefit most from the architecture? +* What bottlenecks emerge when moving from a small multicore system to a much larger one? + +These questions are central to the research behind XH-1. + +--- + +# RISC-V + +The XH-series uses **RISC-V** as its underlying instruction-set architecture. + +RISC-V provides an open and extensible ISA foundation upon which the XH-series can build its own processor implementations. + +This allows the project to concentrate on the architecture and implementation of the CPU itself while retaining compatibility with the broader RISC-V ecosystem where appropriate. + +The exact RISC-V ISA configuration used by XH-1 will be documented as the design is finalized. This includes the supported base ISA, extensions, privilege levels, and any implementation-specific features. + +If the XH-series introduces custom instructions or extensions, they will be documented separately and clearly distinguished from standard RISC-V functionality. + +--- + +# Multicore Architecture + +One of the primary research areas of XH-1 is the organization of its **128 processing cores**. + +A processor with this many cores introduces substantially different design considerations compared with a conventional 2-, 4-, or 8-core CPU. + +The architecture must account for communication and resource sharing between a large number of independent execution units. + +Areas of particular interest include: + +### Core Organization + +The physical and logical organization of the 128 cores will determine how efficiently they can communicate and access shared resources. + +The final architecture may organize cores into groups, clusters, tiles, or another structure depending on the results of the research. + +### Interconnect + +A high-core-count processor requires an efficient method for moving information between cores and shared components. + +The interconnect is therefore a critical part of XH-1. + +The design will investigate how requests, responses, interrupts, synchronization operations, and memory transactions can move through the processor without creating unnecessary bottlenecks. + +### Memory System + +With 128 cores potentially executing simultaneously, memory bandwidth becomes a significant architectural consideration. + +The XH-1 memory system will therefore be designed with scalability in mind. + +Documentation will cover areas such as: + +* Memory hierarchy +* Caches +* Cache coherency +* Shared memory +* Memory controllers +* Memory bandwidth +* Access latency +* Atomic operations +* Synchronization +* Memory ordering + +The exact implementation will be documented once these components have been finalized. + +--- + +# Parallelism + +The primary advantage of a 128-core processor is the amount of potential parallelism it provides. + +In an ideal workload, many independent operations could execute simultaneously across different cores. In practice, however, achieving useful scaling requires careful consideration of dependencies, synchronization, memory access, scheduling, and communication overhead. + +XH-1 will therefore investigate not only the number of available cores, but also how effectively those cores can be utilized. + +Performance measurements will eventually be used to evaluate questions such as: + +* How efficiently does the system scale with additional cores? +* What workloads benefit from large-scale parallelism? +* Where do synchronization bottlenecks occur? +* How much overhead is introduced by inter-core communication? +* How does memory contention affect scaling? +* At what point does adding additional parallel execution provide diminishing returns? + +--- + +# Design Goals + +The exact goals of XH-1 may evolve throughout the research phase, but the initial objectives include: + +### 1. Build a 128-Core RISC-V Processor + +The primary objective is to develop a functional processor containing **128 RISC-V CPU cores**. + +### 2. Explore Scalable Multicore Architecture + +XH-1 is intended to investigate architectures that remain practical as the number of cores increases. + +### 3. Establish the XH Architecture + +The first processor will establish the architectural foundation from which the remaining XH processors can evolve. + +### 4. Develop a Verifiable Design + +A large multicore processor introduces a significant verification challenge. XH-1 will therefore place substantial emphasis on simulation, testing, and verification. + +### 5. Identify Architectural Bottlenecks + +The project will be used to identify limitations in interconnects, memory systems, synchronization mechanisms, and other parts of the architecture. + +### 6. Create a Foundation for Future CPUs + +The lessons learned from XH-1 will inform the design of XH-2, XH-3, and XH-4. + +--- + +# Research Areas + +Development of XH-1 may involve research across several areas of computer architecture. + +## CPU Core Design + +Each of the 128 cores requires a processor implementation capable of executing the selected RISC-V instruction set. + +Research may include: + +* Datapath design +* Register files +* ALUs +* Control logic +* Instruction decoding +* Branch handling +* Pipeline design +* Exceptions +* Interrupts +* Atomic operations +* Privileged execution + +## Cache and Memory Architecture + +The memory hierarchy is especially important in a 128-core processor. + +Research will consider: + +* L1 caches +* Higher-level caches +* Cache coherency +* Shared memory +* Memory ordering +* Memory bandwidth +* Latency +* Contention +* Atomic memory operations + +## Inter-Core Communication + +The cores must be able to communicate and coordinate efficiently. + +This includes research into: + +* Core-to-core messaging +* Shared memory +* Synchronization primitives +* Interrupt delivery +* Interconnect topology +* Request routing +* Arbitration +* Congestion + +## Verification + +Verification becomes increasingly difficult as system complexity increases. + +XH-1 will therefore require testing at multiple levels: + +1. Individual instruction verification +2. Individual core verification +3. Multicore subsystem verification +4. Memory-system verification +5. Interconnect verification +6. Full 128-core system verification + +The verification methodology will be documented alongside the processor implementation. + +--- + +# XH-Series Roadmap + +The initial XH-series consists of four planned processors: + +| Processor | Core Count | Status | Role | +| --------- | ---------: | ------------------------- | ------------------------------------- | +| **XH-1** | **128** | 🔬 Research & Development | Foundation of the XH-series | +| **XH-2** | TBD | ⏳ Planned | Future-generation processor | +| **XH-3** | TBD | ⏳ Planned | Future-generation processor | +| **XH-4** | TBD | ⏳ Planned | Final processor of the initial series | + +The specifications of XH-2 through XH-4 have intentionally not been fixed at this stage. + +The architecture of each future processor will be influenced by the findings from the previous generation. This means that the XH-series roadmap is evolutionary rather than being a rigid set of predetermined specifications. + +--- + +# Current Development Status + +> **XH-1 is currently in the research and development phase.** + +The architecture is subject to change. + +At this stage, documentation may describe proposed designs, experimental concepts, or research directions rather than finalized hardware. + +To avoid confusion, documentation will distinguish between different stages of implementation: + +* **Proposed** — under consideration +* **Research** — currently being investigated +* **Experimental** — being tested +* **Implemented** — present in the design +* **Verified** — tested and confirmed +* **Deprecated** — no longer part of the intended design +* **Planned** — intended for future development + +A feature should not be considered part of the final XH-1 specification unless it has been explicitly marked as such. + +--- + +# Documentation + +This repository is the central documentation repository for the XH-series. + +Documentation will cover both the **individual CPU cores** and the **system as a whole**. + +As development progresses, the repository may contain documentation covering: + +```text +XH-Series +│ +├── Architecture +│ ├── ISA +│ ├── Privilege Model +│ ├── Registers +│ └── Memory Model +│ +├── XH-1 +│ ├── CPU Core +│ ├── 128-Core Organization +│ ├── Pipeline +│ ├── Cache System +│ ├── Memory System +│ ├── Interconnect +│ ├── Interrupt System +│ └── Verification +│ +├── XH-2 +│ +├── XH-3 +│ +└── XH-4 +``` + +The structure is expected to evolve as the project becomes more mature. + +--- + +# Project Philosophy + +XH-1 is not intended to be merely a collection of CPU cores. + +The primary challenge is building a **coherent processor around 128 cores**. + +A design containing many cores can theoretically provide substantial parallel processing capability, but core count alone does not determine performance. The architecture surrounding those cores — particularly the memory system, interconnect, synchronization mechanisms, and resource management — plays a major role in determining how effectively the cores can be utilized. + +For this reason, XH-1 treats the processor as a complete system rather than focusing exclusively on the individual CPU core. + +The project will prioritize: + +* Scalability +* Modularity +* Verifiability +* Clear architectural boundaries +* Reproducibility +* Documentation +* Measurable performance +* Iterative development + +--- + +# Future Development + +Once XH-1 has reached a sufficiently mature state, the project will move toward the next generation of the XH-series. + +The transition to XH-2 will be informed by the practical experience gained from building the 128-core XH-1 system. + +This may include improvements to: + +* CPU core performance +* Core organization +* Cache hierarchy +* Memory bandwidth +* Interconnect performance +* Synchronization +* Power efficiency +* Verification +* Software support +* Scalability + +The same process will continue through XH-3 and XH-4. + +The intention is for each processor to represent a meaningful step forward rather than simply being a minor revision of the previous design. + +--- + +# Summary + +**XH-1 is the first processor in the XH-series: a custom 128-core RISC-V CPU currently in research and development.** + +The project focuses on the design and implementation of a large-scale multicore processor, with particular attention given to scalability, parallel execution, memory architecture, inter-core communication, and verification. + +XH-1 will establish the foundation for the three processors that follow. + +The long-term goal of the XH-series is to explore how a custom RISC-V architecture can evolve across multiple generations while addressing the increasingly complex challenges associated with high-performance, high-core-count processor design. + +**XH-1 is where the XH-series begins — 128 cores, one architecture, and a foundation for everything that follows.** +