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Outline for some new documentation. Maybe we should be doing this wit LaTeX and using pandoc (LaTeX->mediawiki converter)?


Getting Started

What is M5?

M5 is a modular discrete event driven computer system simulator platform. That means that:

  1. M5's components can be rearranged, parameterized, extended or replaced easily to suite your needs.
  2. It simulates the passing of time as a series of discrete events.
  3. It's intended use is to simulate one or more computer systems in various ways.
  4. It's more than just a simulator, it's a simulator platform that lets you use as many of its premade components as you want to build up your own simulation system.

M5 is written primarily in C++ and python and most components are provided under a BSD style license. It can simulate a complete system with devices and an operating system in full system mode (FS mode), or user space only programs where system services are provided directly by the simulator in syscall emulation mode (SE mode). There are varying levels of support for executing Alpha, ARM, MIPS, Power, SPARC, and 64 bit x86 binaries on CPU models including two simple single CPI models, an out of order model, and an in order pipelined model. A memory system can be flexibly built out of caches and interconnects. Recently the Ruby simulator has been integrated with M5 to provide even better, more flexible memory system modeling.

There are many components and features not mentioned here, but from just this partial list it should be obvious that M5 is a sophisticated and capable simulation platform. Even with all M5 can do today, active development continues through the support of individuals and some companies, and new features are added and existing features improved on a regular basis. For the most up to date information you can check out the project's website at

Getting a copy

M5's source code is managed using the mercurial revision control system. There are several repositories you may be interested in:

  1. m5 – The main repository is where active development takes place.
  2. m5-stable – A repository which lags behind “m5” repository but has basically the same contents. It’s usually better to use “m5” instead of “m5-stable”
  3. encumbered – A repository for extensions to M5 that are under a different, more restrictive license. Currently this only includes support for SimpleScalar's EIO trace format.
  4. linux-patches – A repository for patches to the linux kernel that modify it so it can be simulated more efficiently. These patches are optional, but it's a good idea to use them if possible to cut down on simulation run time.

To check out a copy, first, make sure you have mercurial installed on your system and that you can run the hg command. Then use hg clone to create your own local copy using the URL where XXX is replaced by the name of the repository your interested in. For example, to check out the main repository, you'd use the command:

hg clone

You can find out more about mercurial and its commands using its built in help by running:

hg help


M5 uses the scons build system which is based on python. To build the simulator binary, run scons from the top of the source directory with a target of the form build/<config>/<binary> where <config> is replaced with one of the predefined set of build parameters and <binary> is replaced with one of the possible m5 binary names. The predefined set of parameters determine build wide configuration settings that affect the behavior, composition, and capabilities of the binary being built. These include whether the simulator will run in FS or SE mode, if Ruby support is included, which ISA will be supported, which CPU models to build, and what coherence protocol Ruby should use. Examples are ARM_FS, X86_SE, and ALPHA_SE_MOESI_CMP_token. All of the available options can be found in the build_opts directory, and it should be fairly easy to see what each is for. We'll talk about the build system in more detail later. Valid binary names are m5.debug, m5.opt,, and These binaries all have different properties suggested by their extension. m5.debug has optimization turned off to make debugging easier in tools like gdb, m5.opt has optimizations turned on but debug output and asserts left in, removes those debugging tools, and is built to use with gprof. Normally you'll want to use m5.opt. To build the simulator in syscall emulation mode with Alpha support, optimizations turned on, and debugging left in, you would run:

scons build/ALPHA_SE/m5.opt

In your source tree, you'd then find a new build/ALPHA_FS/ directory with the requested m5.opt in it. For the rest of this chapter we'll assume this is the binary you're using.


Now that you've built M5, it's time to try running it. An M5 command line is composed of four parts, the binary itself, options for M5, a configuration script to run, and then finally options for the configuration script. Several example configuration scripts are provided in the “configs/example” directory and are generally pretty powerful. You are encouraged to make your own scripts, but these are a good starting point. The example script we'll use in SE mode is called and sets up a basic SE mode simulation for us. We'll tell it to run the hello world binary provided in the M5 source tree.

build/ALPHA_SE/m5.opt configs/example/ -c tests/test-progs/hello/bin/alpha/linux/hello

This builds up a simulated system, tells it to run the binary found at the location specified, and kicks off the simulation. As the binary runs, it's output is sent to the console by default and looks like this:

M5 Simulator System

Copyright (c) 2001-2008
The Regents of The University of Michigan
All Rights Reserved

M5 compiled Feb 12 2011 20:43:55
M5 revision e00ef55a2c49 7933 default tip
M5 started Feb 12 2011 20:45:47
M5 executing on fajita
command line: build/ALPHA_SE/m5.opt configs/example/ -c tests/test-progs/hello/bin/alpha/linux/hello
Global frequency set at 1000000000000 ticks per second
0: system.remote_gdb.listener: listening for remote gdb #0 on port 7000

info: Entering event queue @ 0. Starting simulation...
info: Increasing stack size by one page.
Hello world!
hack: be nice to actually delete the event here
Exiting @ tick 3240000 because target called exit()

You can see a lot of output from the simulator itself, but the line “Hello world!” came from the simulated program. In this example we didn't provide any options to M5 itself. If we had, they would have gone on the command line between m5.opt and If you'd like to see what command line options are supported, you can pass the --help option to either M5 or the configuration script. The two groups of options are different, so make sure you keep track of whether they go before or after the configuration script.

Asking for help

M5 has two main mailing lists where you can ask for help or advice. m5-dev is for developers who are working on the main version of M5. This is the version that's distributed from the website and most likely what you'll base your own work off of. m5-users is a larger mailing list and is for people working on their own projects which are not, at least initially, going to be distributed as part of the official version of M5. Most of the time m5-users is the right mailing list to use. Most of the people on m5-dev are also on m5-users including all the main developers, and in addition many other members of the M5 community will see your post. That helps you because they might be able to answer your question, and it also helps them because they'll be able to see the answers people send you. To find more information about the mailing lists, to sign up, or to look through archived posts visit:

Source Code

Tour of the tree

These are the files and directories at the top of the tree:

AUTHORS LICENSE README RELEASE_NOTES SConstruct build_opts configs ext src system tests util

AUTHORS, LICENSE, README are files with general information about the simulator. AUTHORS is a list of people who have historically contributed to M5. LICENSE has the license terms that applies to M5 as a whole, unless overridden by a more specific license. README has some very basic information introducing M5 and explaining how to get started.

The SConstruct file is part of the build system, as is the build_opts directory. build_opts holds files that define default settings for build different build configurations. These include X86_FS and MIPS_SE, for instance.

The configs directory is for simulation configuration scripts which are written in python. These are described in more detail later. The files in this directory help make writing configurations easier by providing some basic prepackaged functionality, and include a few examples which can be used directly or as a starting point for your own scripts.

The ext directory is for things M5 depends on but which aren’t actually part of M5. Specifically these are for dependencies that are harder to find, not likely to be available, or where a particular version is needed.

The src directory is where most of M5 is located. This is where all of the C++ and python source that contributes to the M5 binary is kept, excluding components in the ext directory.

The system directory is for the source for low level software like firmware or bootloaders for use in simulated systems. Currently this includes Alpha’s PAL and console code, and a simple bootloader for ARM.

The tests directory stores files related to M5’s regression tests. These include the scripts that build up the configurations used in the tests and reference outputs. Simple hello world binaries are also stored here, but other binaries need to be downloaded separately.

Finally, in the util directory are utility scripts, programs and useful files which are not part of the M5 binary but are generally useful when working on M5.

Style rules

Generated files - where do they end up

.m5 config files

jobfile to run multiple jobs

Build System

Build targets

Configuration variables

What are they

=What do they do

How do you get at their values

Running regressions

Adding files to the build



The python side

Param types (table?)


Special attribute names

Rules for importing - how to get what

Pro tips - avoiding cycles, always descend from root, etc.

The C++ side

create functions

Stages of initialization

Header files to include

Configuration Scripts

SimObject hierarchy

Explanation of infrastructure scripts in configs

How to use and

How to use other top level scripts (what are they?)

General memory system

Ports system

Ports in general

Various port types



Atomic/Timing/Functional accesses

Classic memory system



Hooking them up





Anything else?


Ruby (no idea what to say here)


Event queue

EventManager objects (I don’t know a lot about these)

Event objects

Time sync


I/O device base classes




PCI devices

DMA devices

UARTs and serial terminals

Explanation of platforms and systems, how they’re related, and what they’re each for

Execution Basics



Microcode support




Architectural State


Register types - float, int, misc

Indexing - register spaces stuff


Address Translation

TLB management

Delayed translation

Page table walkers (defer to ISA discussion?)

Differences between SE and FS mode


Simple CPU

Flow of execution

Sharing of code between atomic and timing


Anatomy of the pipeline




Load/store handling



InOrder (what to say here?)


Traditional handling with platform object

New, currently X86 only system

Interrupt sources/sinks/wires/pins

Interrupt messages

Interrupt managing objects in the CPU

ISA parser



decode tree

let blocks

microcode assembler




rom object

Lots more stuff

Multiple ISA support

Switching header files

Specialized classes and functions

Matrix of supported ISA/mode/CPU combinations

Alpha Implementation

ARM Implementation

Supported features and modes

Thumb support

Special PC management

MIPS Implementation

Power Implementation

SPARC Implemenation

Supported features and modes

X86 Implementation

Microcode ISA

Macroop specialization

Platform objects

Physical memory space allocation

Microcode registers and other hidden implementation details

Supported features and modes

Pseudo Instructions

List of pseudo instructions and what they do

How to get at and use them from a binary, script, etc.

SE mode

Loading binaries

System call instruction handling

Implementation of various system calls


Statistics (don’t know what to say here)

Utility Code

Bitfield functions

BitUnion classes



panic, warn, etc., when to use what

random number generation

reference counting pointers


Trace flags

State trace/Tracer objects

Remote GDB support



Development Tools/Contributing

Mercurial queues

Review board

Mercurial repo browser

External Dependencies

Required versions

Things in ext