biRISC-V - 32-bit dual issue RISC-V CPU

Github: http://github.com/ultraembedded/biriscv

Features

• 32-bit RISC-V ISA CPU core.
• Superscalar (dual-issue) in-order 6 or 7 stage pipeline.
• Support RISC-V’s integer (I), multiplication and division (M), and CSR instructions (Z) extensions (RV32IMZicsr).
• Branch prediction (bimodel/gshare) with configurable depth branch target buffer (BTB) and return address stack (RAS).
• 64-bit instruction fetch, 32-bit data access.
• 2 x integer ALU (arithmetic, shifters and branch units).
• 1 x load store unit, 1 x out-of-pipeline divider.
• Issue and complete up to 2 independent instructions per cycle.
• Supports user, supervisor and machine mode privilege levels.
• Basic MMU support - capable of booting Linux with atomics (RV-A) SW emulation.
• Implements base ISA spec v2.1 and privileged ISA spec v1.11.
• Verified using Google's RISCV-DV random instruction sequences using cosimulation against C++ ISA model.
• Support for instruction / data cache, AXI bus interfaces or tightly coupled memories.
• Configurable number of pipeline stages, result forwarding options, and branch prediction resources.
• Synthesizable Verilog 2001, Verilator and FPGA friendly.
• Coremark: 4.1 CoreMark/MHz
• Dhrystone: 1.9 DMIPS/MHz ('legal compile options' / 337 instructions per iteration)

A sequence showing execution of 2 instructions per cycle;

Documentation

• Configuration
• Booting Linux
• Integration
• Custom Features

Similar Cores

• SiFive E76
  • RV32IMAFC
  • Dual issue in-order 8 stage pipeline
  • 4 ALU units (2 early, 2 late)
  • ✖️ Commercial closed source core/$$
• WD SweRV RISC-V Core EH1
  • RV32IMC
  • Dual issue in-order 9 stage pipeline
  • 4 ALU units (2 early, 2 late)
  • ✖️ System Verilog + auto signal hookup
  • ✖️ No data cache option
  • ✖️ Not able to boot Linux

Project Aims

• Boot Linux all the way to a functional userspace environment. ✔️
• Achieve competitive performance for this class of in-order machine (i.e. aim for 80% of WD SweRV CoreMark score). ✔️
• Reasonable PPA / FPGA resource friendly. ✔️
• Fit easily onto cheap hobbyist FPGAs (e.g. Xilinx Artix 7) without using all LUT resources and synthesize > 50MHz. ✔️
• Support various cache and TCM options. ✔️
• Be constructed using readable, maintainable and documented IEEE 1364-2001 Verilog. ✔️
• Simulate in open-source tools such as Verilator and Icarus Verilog. ✔️
• In later releases, add support for atomic extensions.

Booting the stock Linux 5.0.0-rc8 kernel built for RV32IMA to userspace on a Digilent Arty Artix 7 with biRISC-V (with atomic instructions emulated in the bootloader);

Prior Work

Based on my previous work;

• Github: http://github.com/ultraembedded/riscv

Getting Started

Cloning

To clone this project and its dependencies;

git clone --recursive https://github.com/ultraembedded/biriscv.git

Running Helloworld

To run a simple test image on the core RTL using Icarus Verilog;

# Install Icarus Verilog (Debian / Ubuntu / Linux Mint)
sudo apt-get install iverilog

# [or] Install Icarus Verilog (Redhat / Centos)
#sudo yum install iverilog

# Run a simple test image (test.elf)
cd tb/tb_core_icarus
make

The expected output is;

Starting bench
VCD info: dumpfile waveform.vcd opened for output.

Test:
1. Initialised data
2. Multiply
3. Divide
4. Shift left
5. Shift right
6. Shift right arithmetic
7. Signed comparision
8. Word access
9. Byte access
10. Comparision

Configuration