[HOTFIX] A patch was applied to fix the RubyPrefetcher with MESI_Three_Level. Prior to this fix a segfault occurred.


[HOTFIX] A fix was applied to stop incorrect clock frequences being reported due to rounding errors.


Welcome to our first “official” gem5 release! gem5 v19.0.0.0 was a “test” release, but this one has release notes, so it must be official!

Thank you to everyone that made this release possible! This has been a very productive release with over 70 issues closed, over 500 commits, and 31 unique contributors. Below are some of the highlights, though I‘m sure I’ve missed some important changes.

New features

  • gem5-resources repository
    • This new repository will store all of the sources (e.g., code) used to create testing and research resources. This includes disk images, testing binaries, kernel binaries, etc.
    • Binaries created with the sources are hosted on
    • Details on the new page for resources:
  • Memory SimObjects can now be initialized using an image file using the image_file parameter.
  • [USER-FACING CHANGE] The m5 utility has been revamped with a new build system based on scons, tests, and updated and more consistent feature support.
    • To build, now use scons build/<arch>/out/m5, not make.
    • Documentation coming soon.
  • Robust support for marshalling data from a function call inside the simulation to a function within gem5 using a predefined set of rules.
    • Developers can specify an ABI for guest<->simulator calls and then “just call functions”.
    • Unifies pseudo-inst, syscall, and other support.
    • Code within gem5 has been updated. However, users which added new pseudo-ops may have to update their code.
  • [PYTHON API CHANGE] Workload configuration pulled out into its own object, simplifying the System object and making workload configuration more modular and flexible.
  • Sv39 paging has been added to the RISC-V ISA, bringing gem5 close to running Linux on RISC-V.
    • (Some) Baremetal OSes are now supported.
  • Improvements to DRAM model:
    • Added support for verifying available command bandwidth.
    • Added support for multi-cycle commands.
    • Added new timing parameters.
    • Added ability to interleave bursts.
    • Added LPDDR5 configurations.
  • [Developer change] We are beginning to document gem5 APIs.
    • Currently, only SimObjects and the APIs they depend on have been documented.
    • We are using doxygen to mark “stable APIs” and will use manual code review to make sure the APIs stay stable.
    • More information will be coming during gem5-20.1 development.

Removed features

  • Support for the ALPHA ISA has been dropped.
    • All ALPHA ISA code has been removed
    • Old “rcS” scripts for ALPHA have been removed

New supported platforms

  • Compiling and running gem5 with Python 3 is now fully supported.
    • Lots of code changes required for this.
    • There may still be some python code that‘s not up to date. Please open a Jira ticket if you find any code that doesn’t work with python3.
  • gem5 now supports Ubuntu 20.04.
  • Compiling gem5 with GCC 8 and 9 is now supported.
  • Compiling with clang up to version 9 is now supported.

Testing improvements

  • Scons-based tests have been migrated to the testlib framework.
    • Tests can now be run with tests/, except for the unittests.
    • Please consult for more information on how these may be run.
  • We are continuing to work on CI tests. Most of the plumbing is there for Google Cloud Build integration. See the Jira issue for details.

Other API changes

  • [API CHANGE] Ruby's prefetcher renamed to RubyPrefetcher.
    • Any SLICC protocols with prefetchers need to be updated.
    • Some config scripts for Ruby protocols with prefetchers may need to be updated.
  • [API CHANGE] SE mode improvements.
    • Better support for the mmap and related syscalls.
    • A new virtual memory area API for tracking SE mode allocations.
    • When implementing syscalls, the way that guest memory is allocated changes. All code in gem5 is updated, but if there are any external syscalls, they may need be updated.
  • [COMMAND LINE CHANGE] The --disk-image argument to is now optional.
    • However, the disk image names are no longer implied.
    • The script still implicitly searches M5_PATH, but the name of the disk image must be specified.
  • [API CHANGE] SLICC queueMemory is now enqueue.
    • All protocol configs must be updated with another message buffer in the memory controllers (directories).
    • All protocol SLICC files must replace queueMemoryRead and queueMemoryWrite with enqueue to another “special” message buffer named memQueue.
    • This allows finite buffering between the cache controllers and DRAMCtrl.
  • [API CHANGE] Added Prefetcher namespace
    • All prefetchers' names have changed from *Prefetcher to Prefetcher::*
    • If you have any prefetchers that are not in the gem5 mainline, your code will likely need to be updated.

Other changes

  • Implemented ARMv8.3-CompNum, SIMD complex number extension.
  • Support for Arm Trusted Firmware + u-boot with the new VExpress_GEM5_Foundation platform
  • Removed author list from source files.
    • This was originally so future people would know who to contact.
    • However, it was difficult to maintain and quickly out of date.
    • Copyright is unchanged.
  • Improvements to gem5's power model.
  • MESI_Three_Level Ruby protocol bugfixes.
  • Ruby functional reads now work in more cases.
  • Indirect branch stats work correctly now.