BOOM: Berkeley Out-of-Order Machine

The Berkeley Out-of-Order RISC-V Processor

The Berkeley Out-of-Order Machine (BOOM) is a synthesizable and parameterizable open source RV64G RISC-V core written in the Chisel hardware construction language. While BOOM is primarily ASIC optimized, it is also usable on FPGAs. We support the FireSim flow to run BOOM at 90+ MHz on FPGAs on Amazon EC2 F1. Created at the University of California, Berkeley in the Berkeley Architecture Research group, its focus is to create a high performance, synthesizable, and parameterizable core for architecture research.

Feature BOOM
Floating Point (IEEE 754-2008)
Atomic Memory Op Support
Viritual Memory
Boots Linux
Privileged Arch v1.11
External Debug

Google group: (!forum/riscv-boom)

For documentation and publications on BOOM visit the BOOM website (


This repository is NOT A SELF-RUNNING repository. To instantiate a BOOM core, please use the boom-template SoC generator found in the git repository (

Note: you MUST build the riscv-tools as described to build the correct version. A copy of riscv-tools you have built yourself previously may be out of date! Likewise, the master branch of risv-tools may be running ahead and may also not work!


You must set the $RISCV environment variable to where you would like the RISC-V toolchain to be installed. You must also add $RISCV/bin to your $PATH.

The instructions below will walk you through installing the RISC-V toolchain. If you run into problems, go to the README in riscv-tools for additional information.

Code Organization

The Chisel source code is found in src/main/scala:

  • bpu - branch predictor unit
  • exu - execute/core unit
  • ifu - instruction fetch unit
  • lsu - load/store/memory unit
  • common - configs, bundle, and tile definitions
  • system - Non-core system-level infrastructure
  • util - utilities


To build a BOOM Verilator emulator and its corresponding RISC-V toolchain, and run BOOM through a couple of simple tests:

   $ git clone
   $ cd boom-template
   $ ./scripts/
   $ ./scripts/ 
   $ cd verisim
   $ make CONFIG=BoomConfig
   $ make run CONFIG=BoomConfig

Note: the above script builds a specific commit of the risv-tools that BOOM is compatible with. Building your own riscv-tools copy may produce an incompatible version (there is too much development churn in risv-tools currently!). The will also build a RV64G toolchain -- the default riscv-tool build scripts produce an incompatible RV64GC toolchain.

There are many BOOM configurations to choose from (and modify!). In fact, the CONFIG variable defaults to BoomConfig, so it is not necessary to pass a CONFIG option.

Installing the RISC-V Toolchain

First, set the $RISCV environment variable (to where you want the toolchain to be installed). You will also need to add $RISCV/bin to your $PATH.

   $ git clone
   $ cd boom-template
   $ ./scripts/ 

That's it. But read on for some more information about what's going on behind the scenes.

By default, riscv-tools/ builds a RV64GC compiler. Therefore, We need to change that as BOOM does not support the RVC extension.

   $ export RISCV=/path/to/install/riscv/toolchain
   $ export PATH="${PATH}:$RISCV/bin"
   $ git clone
   $ cd boom-template/rocket-chip
   $ git submodule update --init
   $ cd riscv-tools
   $ git submodule update --init --recursive
   $ cp
   $ vim

Modify the riscv-gnu-toolchain and riscv-isa-sim entries to specify rv64imafd as the ISA we want to build:

build_project riscv-isa-sim --prefix=$RISCV --with-fesvr=$RISCV --with-isa=rv64imafd
build_project riscv-gnu-toolchain --prefix=$RISCV --with-arch=rv64imafd

Now we can build the riscv-tools within (boom-template/rocket-chip/riscv-tools):

   $ ./

For more detailed information on the toolchain, visit the riscv-tools repository.

Using the gem5 O3 Pipeline Viewer with BOOM

The O3 Pipeline Viewer is an out-of-order pipeline viewer included in the gem5 suite. BOOM is capable of generating traces compatible with the pipeline viewer, which is useful for understanding what causes pipeline stalls and flushes.

To generate gem5 compatible traces, first set O3PIPEVIEW_PRINTF in boom/src/main/scala/consts.scala to true:

val O3PIPEVIEW_PRINTF = true  // dump trace for O3PipeView from gem5

Rebuild and rerun BOOM. You should find the traces (*.out) in emulator/output/. To generate the visualization run:

   $ boom/util/ -f <TRACE_FILE> > cleaned_trace.out
   $ path_to_gem5/util/ --color --store_completions -o pipeview.out cleaned_trace.out

You can view the visualization by running:

   $ less -r pipeview.out

For more details (and to download, visit the gem5 wiki.

More Info

Check out the BOOM website @!


The RISC-V privileged ISA, platform, and Debug specs are still in flux. BOOM will do its best to stay up-to-date with it!

BOOM is a work-in-progress and remains in active development.


Help! BOOM isn't working.

First verify the software is not an issue. Run spike first:

# Verify it works on spike.
spike --isa=rv64imafd my_program

# Then we can run on BOOM.
./emulator-freechips.rocketchip.system-SmallBoomConfig my_program 

Also verify the riscv-tools you built is the one pointed to within the boom-template/rocket-chip/riscv-tools repository. Otherwise a version mismatch can easily occur!

Master branch is broken! How do I get a working BOOM?

The boom-template super-repo should always be pointing to a working boom/rocket-chip/riscv-tools combination. The master branch of riscv-boom may run ahead though. Ideally, master should never be broken, but it may be somewhat unstable as development continues. For more stability, I recommend using one of the tagged releases.