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When the writeback stage is present the retired instruction counter
(minstret) and the retired compressed instruction counter could see an
off by one error when an instruction was in the writeback stage when
reading the counters. With this fix the ID stage observes the
incremented value of the counters when an instruction that would
increment them is in writeback.
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Build Status

Ibex RISC-V Core

Ibex is a production-quality open source 32-bit RISC-V CPU core written in SystemVerilog. The CPU core is heavily parametrizable and well suited for embedded control applications. Ibex is being extensively verified and has seen multiple tape-outs. Ibex supports the Integer (I) or Embedded (E), Integer Multiplication and Division (M), Compressed (C), and B (Bit Manipulation) extensions.

The block diagram below shows the small parametrization with a 2-stage pipeline.

Ibex was initially developed as part of the PULP platform under the name "Zero-riscy", and has been contributed to lowRISC who maintains it and develops it further. It is under active development.

Configuration

Ibex offers several configuration parameters to meet the needs of various application scenarios. The options include different choices for the architecture of the multiplier unit, as well as a range of performance and security features. The table below indicates performance, area and verification status for a few selected configurations. These are configurations on which lowRISC is focusing for performance evaluation and design verification (see supported configs).

Config "micro" "small" "maxperf" "maxperf-pmp-bmfull"
Features RV32EC RV32IMC, 3 cycle mult RV32IMC, 1 cycle mult, Branch target ALU, Writeback stage RV32IMCB, 1 cycle mult, Branch target ALU, Writeback stage, 16 PMP regions
Performance (CoreMark/MHz) 0.904 2.47 3.13 3.05
Area - Yosys (kGE) 17.44 26.06 35.64 58.74
Area - Commercial (estimated kGE) ~16 ~24 ~33 ~54
Verification status Red Green Amber Amber

Notes:

  • Performance numbers are based on CoreMark running on the Ibex Simple System platform. Note that different ISAs (use of B and C extensions) give the best results for different configurations. See the Benchmarks README for more information. The "maxperf-pmp-bmfull" configuration sets a SpecBranch parameter in ibex_core.sv; this helps timing but has a small negative performance impact.
  • Yosys synthesis area numbers are based on the Ibex basic synthesis flow using the latch-based register file.
  • Commercial synthesis area numbers are a rough estimate of what might be achievable with a commercial synthesis flow and technology library.
  • For comparison, the original "Zero-riscy" core yields an area of 23.14kGE using our Yosys synthesis flow.
  • Verification status is a rough guide to the overall maturity of a particular configuration. Green indicates that verification is close to complete. Amber indicates that some verification has been performed, but the configuration is still experimental. Red indicates a configuration with minimal/no verification. Users must make their own assessment of verification readiness for any tapeout.
  • v0.92 of the RISC-V Bit Manipulation Extension is supported. This is not ratified and there may be changes for the v1.0 ratified version. See Standards Compliance in the Ibex documentation for more information.

Documentation

The Ibex user manual can be read online at ReadTheDocs. It is also contained in the doc folder of this repository.

Contributing

We highly appreciate community contributions. To ease our work of reviewing your contributions, please:

  • Create your own branch to commit your changes and then open a Pull Request.
  • Split large contributions into smaller commits addressing individual changes or bug fixes. Do not mix unrelated changes into the same commit!
  • Write meaningful commit messages. For more information, please check out the contribution guide.
  • If asked to modify your changes, do fixup your commits and rebase your branch to maintain a clean history.

When contributing SystemVerilog source code, please try to be consistent and adhere to our Verilog coding style guide.

When contributing C or C++ source code, please try to adhere to the OpenTitan C++ coding style guide. All C and C++ code should be formatted with clang-format before committing. Either run clang-format -i filename.cc or git clang-format on added files.

To get started, please check out the "Good First Issue" list.

Issues and Troubleshooting

If you find any problems or issues with Ibex or the documentation, please check out the issue tracker and create a new issue if your problem is not yet tracked.

Questions?

Do not hesitate to contact us, e.g., on our public Ibex channel on Zulip!

License

Unless otherwise noted, everything in this repository is covered by the Apache License, Version 2.0 (see LICENSE for full text).

Credits

Many people have contributed to Ibex through the years. Please have a look at the credits file and the commit history for more information.