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README.md

About The Sodor Processor Collection

Note: This repo has been updated to be used with the Chipyard SoC Generator. For the old self-contained version of Sodor (which is no longer maintained), see https://github.com/ucb-bar/riscv-sodor/tree/sodor-old.

Diagrams: Sodor Github wiki

More documentation: Librecores Sodor wiki

Downstream development: Librecores Sodor

This repo has been put together to demonstrate a number of simple RISC-V integer pipelines written in Chisel:

  • 1-stage (essentially an ISA simulator)
  • 2-stage (demonstrates pipelining in Chisel)
  • 3-stage (uses sequential memory; supports both Harvard and Princeton versions)
  • 5-stage (can toggle between fully bypassed or fully interlocked)
  • "bus"-based micro-coded implementation

All of the cores implement the RISC-V 32b integer base user-level ISA (RV32I) version 2.0. None of the cores support virtual memory, and thus only implement the Machine-level (M-mode) of the Privileged ISA v1.10 .

All processors talk to a simple scratchpad memory (asynchronous, single-cycle), with no backing outer memory (the 3-stage is the exception - its scratchpad is synchronous). Programs are loaded in via JTAG or TSI, scratchpads 3-port memories (instruction, data, debug).

This repository is set up to use the Verilog file generated by Chisel3 which is fed to Verilator along with a test harness in C++ to generate and run the Sodor emulators.

This repo works great as an undergraduate lab (and has been used by Berkeley's CS152 class for 3 semesters and counting). See doc/ for an example, as well as for some processor diagrams. Be careful though - admittedly some of those documents may become dated as things like the Privileged ISA evolve.

Getting the repo and Building the processor emulators

This repo is NOT a self-running repository. Please follow the instruction in https://chipyard.readthedocs.io/en/latest/ to set up Chipyard and simulate Sodor cores.

FAQ

What is the goal of these cores?

First and foremost, to provide a set of easy to understand cores that users can easily modify and play with. Sodor is useful both as a quick introduction to the RISC-V ISA and to the hardware construction language Chisel3.

Are there any diagrams of these cores?

Diagrams of some of the processors can be found either in the Sodor Github wiki, in doc/, or in doc/lab1.pdf. A more comprehensive write-up on the micro-code implementation can be found at the CS152 website.

How do I generate Verilog code for use on a FPGA?

Chisel3 outputs verilog by default which can be generated by

cd emulator/rv32_1stage
make generated-src/Top.v

I want to help! Where do I go?

You can participate in the Sodor conversation on gitter. Downstream development is also taking place at Librecores. Major milestones will be pulled back here. Check it out! We also accept pull requests here!

TODO

Here is an informal list of things that would be nice to get done. Feel free to contribute!

  • Reduce the port count on the scratchpad memory by having the HTIF port share one of the cpu ports.
  • Provide a Verilog test harness, and put the 3-stage on a FPGA.
  • Add support for the ma_addr, ma_fetch ISA tests. This requires detecting misaligned address exceptions.
  • Greatly cleanup the common/csr.scala file, to make it clearer and more understandable.
  • Refactor the stall, kill, fencei, and exception logic of the 5-stage to be more understandable.
  • Update the u-code to properly handle illegal instructions (rv32mi-p-illegal) and to properly handle exceptions generated by the CSR file (rv32mi-p-csr).