RISC-V CPUs

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Introduction

RISC-V is an open and free standard or RISC ISA. It exists in 32,64 and still in draft stage, 128 bits wide versions. There are lot of free or non free implementations. You can find software, FPGA, or ASIC implementations on most codeforges like Github, Gitlab, Gitea, etc... Several research institute around the world also opened their own versions.

The RISC-V processor ISA

The RISC-V Processor is a RISC based architecture, with less instructions than most other RISC processors. For example, there is no GT (>) or GE (>=) hardware instructions, only LT (<) or LE (<=) instruction, but the RISC-V assembly language support GT and GE, and switch it to LT/LE by switching also registers to keep the test as needed. The same way, several instructions are supported in RISC-V assembly language that doesn't actually exists as code machine instructions.

One of the main goals of the specifications is to have modularity with high granularity. So bitwise operators, multiplications/divisions, vector instructions for example are on different optional extensions, that can be implemented or not by RISC-V processor makers, reducing the total number of transistors depending on processor usage. Each extension have its own letter added in the name of the processor.

The specifications are all available on the risc-v.org site: https://riscv.org/technical/specifications/

Tools

Assembler, compiler, linker

  • GNU tools contains both RISC-V 32 and 64bits compiler (GCC), assembler (gnu-binutils) and debugger (GDB), they are available for native or cross-compiling on most GNU/Linux distribution.
  • Bronzebeard is a standalone assembler for developing bare metal RISC-V programs. It has been used mainly on Longan Nano a RV32 board generally sold with an LCD screen. There are few demo on this board. The initialisation of the screen on this platform has an important footprint that can't allow to go in 128B or 256B competitions.
  • Arduino has several RISC-V implementations.

Most other languages are also available in RISC-V instruction set. Some Linux distribution have been ported and ISO are available (Debian, Ubuntu for example). Haiku OS also work on RISC-V platform, and there are plenty of embedded OS working on it (Arduino, FreeRTOS, HarmonyOS, LiteOS, RT-Thread, RustOS, Zephyr...).

Testing it

If you don't have one of those cheap boards, you can still test RISC-V assembly on an emulator. Qemu allow to execute RISC-V application or systems on any kind of architectures. You can use the same tricks than used on x86 to reduce the binary size of the ELF binary format made with GNU tools.

The main author of Qemu also made TinyEmu, a RISC−V only tiny emulator, it already has WASM ports, so you can play with it on the web.