FreeRTOS-support for RV32I Core Project

1. Project Overview

CARP Core is the flagship design effort of the Computer Architecture Research Project (CARP). It extends the original OTTER RV32I five-stage pipelined processor into a feature-complete, open-source RISC-V core capable of running modern embedded workloads, ported to ASIC flows, and validated against standard reference models.

Key Objectives:

  • Extend OTTER’s baseline RV32I to RV32I_Zicsr with CLINT.

  • Target both FPGA and ASIC implementations using open-source tools.

  • Port and run FreeRTOS on the core.

  • Verify functionality against the RISC-V Spike simulator.

2. Target ISA and Features

Current baseline

  • RV32I, 5-stage pipeline (IF, ID, EX, MEM, WB).

  • Basic hazard handling (stall + forwarding).

  • No branch or jump support yet (to be extended).

Planned extensions

  • Zicsr: Control/status registers for privileged mode and interrupts.

  • Zicntr: Base counters/timers (cycle, instret).

3. Microarchitecture Roadmap

  1. Pipeline Enhancements

    • Add branch/jump handling.

    • Forwarding + hazard detection unit.

    • Instruction/data memory interfaces with handshake support.

  2. CSR & Privilege Support

    • Implement Zicsr instructions and machine-mode CSRs.

    • Hook up counters (Zicntr).

    • Add interrupt/trap handler flow.

  3. Any other extensions

4. Verification Strategy

  • Unit tests: Self-checking SystemVerilog/C++ testbenches for ALU, multiplier, CSR unit, etc.

  • ISA tests: Run RISC-V compliance suite for Zicsr extensions.

  • Reference model: Compare against Spike using differential testing.

  • Simulation: Use Verilator for cycle-driven C++ simulation.

  • Waveform debug: VCD/FST dumps for GTKWave.

5. Toolchain & Software Support

  • Toolchain: Build custom GCC / binutils targeting rv32i_zicsr_zicntr.

  • RTOS: Port FreeRTOS kernel to CARP Core (machine mode, UART console, timers).

  • Test Programs: Bare-metal apps, benchmarks (CoreMark), RTOS demo tasks.

6. ASIC Flow & Implementation

  • Use OpenLane 2 / OpenROAD on Sky130 PDK.

  • Explore standard cell synthesis, place & route, and GDSII export.

  • Analyze PPA (Power, Performance, Area) vs FPGA baseline.

  • Deliver a tapeout-ready netlist for academic publication.

7. Impact

  • Students gain hands-on experience with ISA design, pipelining, RTOS, and ASIC flows.

  • Open-source contribution of a teaching-quality yet industrially relevant RISC-V core.

  • Builds a reusable foundation for future CARP projects (accelerators, coprocessors, autonomic SoCs).