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Revision as of 12:48, 7 October 2012 by Nilayvaish (talk | contribs) (Replaced m5 with gem5)
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gem5 uses the Mercurial (a.k.a. hg) revision control system. The repository is directly accessible at This website provides browser access to history and changesets and the ability to "clone" (create your own copy of) the repository on your local machine.

We host two source repositories:

  • gem5 --- The development repository. Code changes are always committed to this repository first. This code should work, because developers are supposed to run a limited set of regression tests before committing to this repository. However, this code has generally not been tested against the full regression suite, and developers occasionally mess up and commit things that don't work as well as they expected.
  • gem5-stable --- The (relatively) stable repository. The head of this repository is the latest version that has passed a full set of regression tests. This repository is updated periodically from the development repository. Our goal is to update it from the development repository on a monthly basis, although that has slipped in the past.

Q: How do I know which repository to use?

A: Whichever one strikes your fancy can be tried first. If you like bleeding edge - go for dev. If you like knowing gem5-stable has passed all of our regression tests, try that one. If whatever you try doesn't work for you, try the other one, as there may have been bugs fixed/introduced between stable and dev. If neither works for you, go ahead and ask the mailing list. Read Reporting Problems before sending your question to the mailing list.

What is it?

Mercurial (hg) is a distributed version control system. Every copy of a mercurial repository is complete and fully functional. Any operation (examining history, committing changes, etc) can be done without communicating with a central repository. (In fact, from mercurial's point of view, there is no central repository; the repositories are "central" only by user convention.) Instead of having to manually diff/patch your changes when a new version is released simply executing a command will "pull" all changes in the repository into your local repository launching a merge tool whenever necessary. Additionally, patches can be sent to us for inclusion in the repository with a single command.

Where can I get more information about it?

The main Mercurial website is here. Joel Spolsky wrote a nice tutorial about mercurial. Other good sources of information about Mercurial is the hg book and the HG Cheat Sheets. We promise that it is worth taking 30 minutes to read some of the documentation to get you started. You may even decide that you really like hg and want to use it in your own projects. We do!

How do I use it?

The first thing you need to do is install hg (from either package for your OS/distribution or from source). Debian and Ubuntu have packages named mercurial that you can install. MacPorts also has a package. The next thing you should do is setup a $HOME/.hgrc. An example .hgrc is provided below. It enables various extensions that come with Mercurial and allows you to mail patches to the M5 mailing list if you so choose.

# Set the username you will commit code with
username=Your Name <your@email.address> 
ssh = ssh -C

# Always use git diffs since they contain permission changes and rename info
qrefresh = --git
email = --git
diff = --git

# These are various extensions we find useful

# Mercurial Queues -- allows managing of changes as a series of patches =

# PatchBomb -- send a series of changesets as e-mailed patches
hgext.patchbomb = 
# External Diff tool (e.g. kdiff3, meld, vimdiff, etc)
hgext.extdiff =

# Fetch allows for a pull/update operation to be done with one command and automatically commits a merge changeset
hgext.fetch = 

# Path to the style file for the M5 repository 
# This file enforces our coding style requirements
style = /path/to/your/m5/util/

method = smtp
from = Your Name <your@email.address>

host =

Basic Commands

Here are some basic commands for Mercurial, however this is not exhaustive and you should read the Mercurial documentation.

Cloning creates a complete and fully functional copy of a repository. To get started you should execute:

hg clone

Once you've got a clone of the repository, here are a few basic commands you can use:

  • hg status shows what files have been modified in your repository
  • hg diff shows a diff of the modified files
  • hg fetch fetches any new updates from the repository you cloned (in this case, gem5-stable), and if necessary merges them with any changes you have made since the last update. Note that fetch is a convenient extension that combines several other primitive hg operations that you can also do separately: hg pull, hg update, and if needed hg merge and hg commit. See the mercurial documentation for more details.

Making Your Own Changes

For guidance on how to effectively extend M5 within the Mercurial framework, see our section on Adding Functionality.