RISC-V RV32I[MAFC] emulator

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rv32emu is an emulator for the 32 bit RISC-V processor model (RV32), faithfully implementing the RISC-V instruction set architecture (ISA). It serves as an exercise in modeling a modern RISC-based processor, demonstrating the device's operations without the complexities of a hardware implementation. The code is designed to be accessible and expandable, making it an ideal educational tool and starting point for customization. It is primarily written in C99, with a focus on efficiency and readability.

Features:

• Fast interpreter for executing the RV32 ISA
• Comprehensive support for RV32I and M, A, F, C extensions
• Memory-efficient design
• Built-in ELF loader
• Implementation of commonly used newlib system calls
• Experimental SDL-based display/event/audio system calls for running video games
• Support for remote GDB debugging
• Experimental JIT compiler for performance boost while maintaining a small footprint

Build and Verify

rv32emu relies on certain third-party packages for full functionality and access to all its features. To ensure proper operation, the target system should have the SDL2 library and SDL2_Mixer library installed.

• macOS: brew install sdl2 sdl2_mixer
• Ubuntu Linux / Debian: sudo apt install libsdl2-dev libsdl2-mixer-dev

Build the emulator.

$ make

Run sample RV32I[M] programs:

$ make check

Run Doom, the classical video game, via rv32emu:

$ make doom

The build script will then download data file for Doom automatically. When Doom is loaded and run, an SDL2-based window ought to appear.

If RV32F support is enabled (turned on by default), Quake demo program can be launched via:

$ make quake

The usage and limitations of Doom and Quake demo are listed in docs/demo.md.

Docker image

The image containing all the necessary tools for development and testing can be executed by docker run -it sysprog21/rv32emu:latest. It works for both x86-64 and aarch64 (Apple's M1 chip) machines.

Customization

rv32emu is configurable, and you can override the below variable(s) to fit your expectations:

• ENABLE_EXT_M: Standard Extension for Integer Multiplication and Division
• ENABLE_EXT_A: Standard Extension for Atomic Instructions
• ENABLE_EXT_F: Standard Extension for Single-Precision Floating Point Instructions
• ENABLE_EXT_C: Standard Extension for Compressed Instructions (RV32C.D excluded)
• ENABLE_Zicsr: Control and Status Register (CSR)
• ENABLE_Zifencei: Instruction-Fetch Fence
• ENABLE_GDBSTUB : GDB remote debugging support
• ENABLE_SDL : Experimental Display and Event System Calls
• ENABLE_JIT : Experimental JIT compiler

e.g., run make ENABLE_EXT_F=0 for the build without floating-point support.

Alternatively, configure the above items in advance by executing make config and specifying them in a configuration file. Subsequently, run make according to the provided configurations. For example, employ the following commands:

$ make config ENABLE_SDL=0
$ make

RISCOF

RISCOF (RISC-V Compatibility Framework) is a Python based framework that facilitates testing of a RISC-V target against a golden reference model.

The RISC-V Architectural Tests, also known as riscv-arch-test, provide a fundamental set of tests that can be used to verify that the behavior of the RISC-V model aligns with RISC-V standards while executing specific applications. These tests are not meant to replace thorough design verification.

Reference signatures are generated by the formal RISC-V model RISC-V SAIL in Executable and Linkable Format (ELF) files. ELF files contain multiple testing instructions, data, and signatures, such as cadd-01.elf. The specific data locations that the testing model (this emulator) must write to during the test are referred to as test signatures. These test signatures are written upon completion of the test and are then compared to the reference signature. Successful tests are indicated by matching signatures.

To install RISCOF:

$ python3 -m pip install git+https://github.com/riscv/riscof

RISC-V GNU Compiler Toolchain should be prepared in advance. You can obtain prebuilt GNU toolchain for riscv32-elf from the Automated Nightly Release. Then, run the following command:

$ make arch-test

For macOS users, installing sdiff might be required:

$ brew install diffutils

To run the tests for specific extension, set the environmental variable RISCV_DEVICE to one of I, M, A, F, C, Zifencei, privilege.

$ make arch-test RISCV_DEVICE=I

Current progress of this emulator in riscv-arch-test (RV32):

• Passed Tests
  • I: Base Integer Instruction Set
  • M: Standard Extension for Integer Multiplication and Division
  • A: Standard Extension for Atomic Instructions
  • F: Standard Extension for Single-Precision Floating-Point
  • C: Standard Extension for Compressed Instruction
  • Zifencei: Instruction-Fetch Fence
  • privilege: RISCV Privileged Specification

Detail in riscv-arch-test:

• RISCOF document
• riscv-arch-test repository
• RISC-V Architectural Testing Framework
• RISC-V Architecture Test Format Specification

Benchmarks

The benchmarks are classified into three categories based on their characteristics:

Category	Benchmark	Description
Computing intensive	puzzle	A sliding puzzle where numbered square tiles are arranged randomly with one tile missing, designed for solving the N-puzzle problem.
	Pi	Calculates the millionth digit of π.
	miniz	Compresses and decompresses 8 MiB of data.
	primes	Finds the largest prime number below 33333333.
	sha512	Computes the SHA-512 hash of 64 MiB of data.
I/O intensive	Richards	An OS task scheduler simulation benchmark for comparing system implementations.
	Dhrystone	Evaluates string operations, involves frequent memory I/O, and generates the performance metric.
Computing and I/O Hybrid	Mandelbrot	A benchmark based on the Mandelbrot set, which uses fixed-point arithmetic and involves numerous integer operations.
	AES	Includes 23 encryption and decryption algorithms adhering to the Advanced Encryption Standard.
	Nqueens	A puzzle benchmark where n queens are placed on an n × n chessboard without attacking each other, using deep recursion for execution.
	qsort	Sorts an array with 50 million items.

These benchmarks performed by rv32emu (interpreter-only mode) and Spike v1.1.0. Ran on Intel Core i7-11700 CPU running at 2.5 GHz and an Ampere eMAG 8180 microprocessor equipped with 32 Arm64 cores, capable of speeds up to 3.3 GHz. Both systems ran Ubuntu Linux 22.04.1 LTS. We utilized gcc version 12.3, configured as riscv32-unknown-elf-gcc.

The figures below illustrate the normalized execution time of rv32emu and Spike, where the shorter indicates better performance.

x86-64

Arm64

Continuous Benchmarking

Continuous benchmarking is integrated into GitHub Actions, allowing the committer and reviewer to examine the comment on benchmark comparisons between the pull request commit(s) and the latest commit on the master branch within the conversation. This comment is generated by the benchmark CI and provides an opportunity for discussion before merging.

The results of the benchmark will be rendered on a GitHub page. Check benchmark-action/github-action-benchmark for the reference of benchmark CI workflow.

There are several files that have the potential to significantly impact the performance of rv32emu, including:

• src/decode.c
• src/rv32_template.c
• src/emulate.c

As a result, any modifications made to these files will trigger the benchmark CI.

GDB Remote Debugging

rv32emu is permitted to operate as gdbstub in an experimental manner since it supports a limited number of GDB Remote Serial Protocol (GDBRSP). To enable this feature, you need to build the emulator and set ENABLE_GDBSTUB=1 when running the make command. After that, you might execute it using the command below.

$ build/rv32emu -g <binary>

The <binary> should be the ELF file in RISC-V 32 bit format. Additionally, it is advised that you compile programs with the -g option in order to produce debug information in your ELF files.

You can run riscv-gdb if the emulator starts up correctly without an error. It takes two GDB commands to connect to the emulator after giving GDB the supported architecture of the emulator and any debugging symbols it may have.

$ riscv32-unknown-elf-gdb
(gdb) file <binary>
(gdb) target remote :1234

Congratulate yourself if riscv-gdb does not produce an error message. Now that the GDB command line is available, you can communicate with rv32emu.

Dump registers as JSON

If the -d [filename] option is provided, the emulator will output registers in JSON format. This feature can be utilized for tests involving the emulator, such as compiler tests.

You can also combine this option with -q to directly use the output. For example, if you want to read the register x10 (a0), then run the following command:

$ build/rv32emu -d - -q out.elf | jq .x10

Usage Statistics

RISC-V Instructions/Registers

This is a static analysis tool for assessing the usage of RV32 instructions/registers in a given target program. Build this tool by running the following command:

$ make tool

After building, you can launch the tool using the following command:

$ build/rv_histogram [-ar] [target_program_path]

The tool includes two optional options:

• -a: output the analysis in ascending order(default is descending order)
• -r: output usage of registers(default is usage of instructions)

Example Instructions Histogram

Example Registers Histogram

Basic Block

To install lolviz, use the following command:

$ pip install lolviz

For macOS users, it might be necessary to install additional dependencies:

$ brew install graphviz

Build the profiling data by executing rv32emu. This can be done as follows:

$ build/rv32emu -p build/[test_program].elf

To analyze the profiling data, use the rv_profiler tool with the desired options:

$ tools/rv_profiler [--start-address|--stop-address|--graph-ir] [test_program]

WebAssembly Translation

Build and run

rv32emu relies on Emscripten to be compiled to WebAssembly. Thus, the target system should have the Emscripten version 3.1.51 installed.

Moreover, rv32emu leverages the tail call optimization(TCO) strategy and we have tested the WebAssembly execution in Chrome with at least MAJOR 112 and Firefox with at least MAJOR 121 since they supports tail call feature. Thus, please check and update your browsers if necessary or install the suitable browsers before going further.

Source your Emscripten SDK environment before make. For macOS and Linux user:

$ source ~/emsdk/emsdk_env.sh

Change the Emscripten SDK environment path if necessary.

At this point, you can build and start a web server service to serve WebAssembly by running:

$ make CC=emcc start-web

You would see the server's IP:PORT in your terminal. Copy and paste it to the browsers and you just access the index page of rv32emu.

Index page

You would see a dropdown menu which you can use to select the ELF executable. Select one and click the Run button to run it.

Alternatively, you may want to view a hosted rv32emu demo page since building takes some time.

Contributing

See CONTRIBUTING.md for contribution guidelines.

License

rv32emu is available under a permissive MIT-style license. Use of this source code is governed by a MIT license that can be found in the LICENSE file.

External sources

In rv32emu repository, there are some prebuilt ELF files for testing purpose.

• aes.elf : See tests/aes.c
• captcha.elf : See tests/captcha.c
• cc.elf : See tests/cc
• chacha20.elf : See tests/chacha20
• coremark.elf : See eembc/coremark [RV32M]
• dhrystone.elf : See rv8-bench
• donut.elf : See donut.c
• doom.elf : See sysprog21/doom_riscv [RV32M]
• fcalc.elf : See fcalc.c
• hamilton.elf : See hamilton.c
• ieee754.elf : See tests/ieee754.c [RV32F]
• jit-bf.elf : See ezaki-k/xkon_beta
• lena.elf: See tests/lena.c
• line.elf : See tests/line.c [RV32M]
• maj2random.elf : See tests/maj2random.c [RV32F]
• mandelbrot.elf : See tests/mandelbrot.c
• nqueens.elf : See tests/nqueens.c
• nyancat.elf : See tests/nyancat.c
• pi.elf : See tests/pi.c [RV32M]
• qrcode.elf : See tests/qrcode.c
• quake.elf : See sysprog21/quake-embedded [RV32F]
• readelf.elf : See tests/readelf
• richards.elf : See tests/richards.c
• rvsim.elf : See tests/rvsim.c
• scimark2.elf : See tests/scimark2 [RV32MF]
• smolnes.elf : See tests/smolnes [RV32M]
• spirograph.elf : See tests/spirograph.c
• stream.elf : See tests/stream [RV32MF]

Reference

• Writing a simple RISC-V emulator in plain C
• Writing a RISC-V Emulator in Rust
• Bare metal C on my RISC-V toy CPU
• Juraj's RISC-V note
• libriscv: RISC-V userspace emulator library
• GUI-VP: RISC-V based Virtual Prototype (VP) for graphical application development
• LupV: an education-friendly RISC-V based system emulator
• mini-rv32ima / video: Writing a Really Tiny RISC-V Emulator
• RVVM