Spear

Spear is a RISC-V emulator that tries to be correct as possible, without focusing on high-perfomance, similair to the official spike emulator, but with more features and more devices to support.

Table of Contents

• Table of Contents
  • Status
  • Usage
  • Plugins
  • Compliance

Status

Note that spear is in very early development phase and it will take a long time until you can run real programs on it. Currently, most of the things mentioned in this README are plans that I would like to implement, however they are far from being implemented.

Right now spear only supports the following extensions and features:

Extensions

Thanks to the dynamic and pluggable design of spear, it's very easy to enable/disable extensions which allows to support many extesions, even ones that are not rattified yet, without much hassle.

Support for all rattified extensions is planned, and important other extensions, like the V extension will also come to play with cool new features.

Extension roadmap:

• Base
  • RV32I v2.1
  • RV64I v2.1
  • RV32E (low priority)
  • RV128I (low priority)
• Extensions
  • RV32/64G
    • M v2.0
    • A v2.1
    • F v2.2
    • D v2.2
    • Zicsr v2.0 (note that many CSRs do not work correctly or might not even be implemented)
    • Zifencei v2.0 (ICACHE emulation is not yet implemented, so fence.i is basically a NOP)
  • Q v2.2
  • C v2.0
  • All other extensions (low priority)

Privilege modes

Spear supports all ratified privilege modes (M, S, and U), however, there are probably many bugs left to fix.

Usage

Spear currently requires at least the latest beta version (1.53.0-beta.3) of the Rust compiler.

Install the latest spear version from main branch by running the following command.

cargo install --locked --git https://github.com/Stupremee/spear

Now you can run spear! 🎉

You can take a look at the example. It contains the hello.S assembly file and the link.lds which is used to compile the hello.S file. To compile the and run hello.S file, run the following commands (on NixOS the prefix is riscv32-none-elf, but it might differ on your platform):

riscv32-none-elf-gcc -Wl,-Texample/link.lds -nostdlib -ffreestanding -mabi=ilp32 -march=rv32i example/hello.S -o example/hello

# And the run the resulting binary using spear
spear example/hello

You should see the state of all registers at the end of the program. As you can see t1 is 0xFF, which is correct since we executed li t1, 255 in the example binary.

If you want to see what the emulator is doing under the hood, set the SPEAR_LOG environment variable to debug or even trace. The trace log level will show every instruction that is executed, thus the output can be huge for larger programs.

Plugins

Spear allows one to write plugins and inject them into the emulator, allowing custom devices to be written in any language that can be compiled to WASM. Infact, every standard device that is supported in spear (e.g. the UART driver, RTIC, etc) are written using WASM plugins.

Compliance

Spear has support for running the riscv-tests (already implemented and used for testing) or riscof (not yet implemented) test suites to prove the correctnes of the emulator and it's execution.