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Yes, We Are Open for Business

If you have any questions, send us an e-mail to or tweet at us @echronosrtos.

If there is something in the project that you think is worth improving, please create a github issue.

Of course, we are also keen on your changes and contributions if you have any - here is a primer.


The eChronos RTOS is a real-time operating system (RTOS) originally developed by NICTA and Breakaway Consulting Pty. Ltd.

It is intended for tightly resource-constrained devices without memory protection. To this end, the RTOS code base is designed to be highly modular and configurable on multiple levels, so that only the minimal amount of code necessary is ever compiled into a given system image.

Available implementations currently target ARM Cortex-M4, and (as of writing, only QEMU-emulated) PowerPC e500.

To obtain the source code, use the following commands:

git clone
cd echronos


See the Prerequisites section for instructions on downloading and installing all prerequisites. The following assumes they have been set up.

Build and run an example system for the RTOS variant Rigel on QEMU-emulated ARMv7-M:

prj/app/ build machine-qemu-simple.example.rigel-system

# Run the generated system in qemu (press `ctrl-c` to close QEMU after it is finished)
qemu-system-arm -M simple-armv7m -S -s -nographic -semihosting -kernel out/machine-qemu-simple/example/rigel-system/system

# To connect and view debug output run gdb in another shell prompt
# Note: The Rigel example will end by deliberately triggering a fatal error
arm-none-eabi-gdb -ex "target remote :1234" -ex "b fatal" out/machine-qemu-simple/example/rigel-system/system
(gdb) c
task a: start delay
irq tick
irq tick
irq tick
irq tick
task a: complete delay

Breakpoint 1, fatal (error_id=error_id@entry=1 '\001')
(gdb) quit

Build and run an example system for the RTOS variant Kochab on QEMU-emulated PowerPC e500:

prj/app/ build machine-qemu-ppce500.example.kochab-system

# Run the generated system in qemu (press `ctrl-a` then 'x' to close QEMU after you are finished)
qemu-system-ppc -M ppce500 -S -s -nographic -kernel out/machine-qemu-ppce500/example/kochab-system/system

# To connect and view debug output run gdb in another shell prompt
# Note: The Kochab example will end with task b cycling forever between "blocked" and "unblocked"

powerpc-linux-gdb -ex "target remote :1234" -ex "b debug_print" out/machine-qemu-ppce500/example/kochab-system/system
(gdb) c
Breakpoint 1, debug_print (msg=0x33b4 "tick")
(gdb) c
Breakpoint 1, debug_print (msg=0x33e8 "task b blocking")
(gdb) c
Breakpoint 1, debug_print (msg=0x33b4 "tick")
(gdb) c
Breakpoint 1, debug_print (msg=0x33f8 "task b unblocked")
(gdb) quit


The following tools are supplied with this repository in the tools directory:

  • Python 3 for the Python scripts in the repository
  • The arm-none-eabi GNU toolchain and arm-none-eabi-gdb for building and debugging the RTOS for ARMv7-M

To obtain and build qemu-system-arm with target simple-armv7m for testing the machine-qemu-simple systems, run:

git clone
./configure --target-list=arm-softmmu --disable-cocoa --disable-curses --disable-vnc --disable-tools --without-pixman --disable-console --disable-default-devices --disable-slirp --disable-curl --disable-guest-base --disable-guest-agent --disable-blobs --audio-drv-list= --audio-card-list= --disable-usb --disable-ide --disable-pie --enable-debug --disable-sdl --disable-fdt --disable-spice --disable-xen --disable-werror
export PATH=`pwd`/arm-softmmu:$PATH

To obtain the powerpc-linux-gnu GNU toolchain for building the RTOS for PowerPC e500 on Ubuntu Linux systems, run:

sudo apt-get install gcc-powerpc-linux-gnu

To obtain the qemu-system-ppc emulator for running the machine-qemu-ppce500 systems on Ubuntu Linux systems, run:

sudo apt-get install qemu-system-ppc

To obtain, build, and install powerpc-linux-gdb for debugging PowerPC e500 systems, run:

tar xaf gdb-7.12.tar.xz
cd gdb-7.12
./configure --target=powerpc-linux --prefix=/usr/
make -s
sudo make -s install

To obtain pandoc and wkhtmltopdf needed for building user documentation on Ubuntu Linux systems:

sudo apt-get install pandoc
sudo apt-get install wkhtmltopdf


Basic RTOS concepts and usage are documented in this README.

More detailed documentation for the tool can be found inside itself. More detailed documentation for the prj tool can be found in prj/manual/prj-user-manual.

Pregenerated RTOS API manuals can be found on the eChronos GitHub wiki. They can also be generated with the command build docs.

Software model

The software model and structure of the RTOS is governed by two stages of customization.

In the first stage, features, in the form of components, are customized for and composed into variants of the RTOS such that each variant has a specific feature set. This stage is supported by the tool.

In the second stage, the RTOS variant is customized to the specific properties and requirements of a specific application. Although this customization is limited to the functionality provided by the given variant, it controls details such as the number of tasks required by the application. This stage is supported by the prj tool.

The two stages can optionally be separated by deploying a product release to an application project. The application project is then only exposed to the second stage and the variant and functionality of the RTOS they require.

The following sections cover these concepts in more detail.

Variants and Components

The RTOS comes in a number of different variants, each offering a specific set of features for a particular platform.

For example:

  • The RTOS variant Rigel supports tasks, co-operative round-robin scheduling, mutexes, signals, and interrupt events which can trigger the sending of signals. It is available for QEMU-emulated ARMv7-M.

  • The RTOS variant Kochab supports tasks, preemptive priority scheduling, mutexes with priority inheritance, semaphores, signals, and interrupt events which can cause task preemption and trigger the sending of signals. It is available for the ARMv7-M STM32F4-Discovery board, and QEMU-emulated PowerPC e500.

Features are implemented as individual components in individual C files. Unlike typical C-based software, the RTOS does not compile components individually and later link them into a single binary. Instead, the RTOS tool merges the component files of a variant into a single C file called the RTOS module. The feature set of each RTOS variant is specified within the tool itself. This allows for better compile-time optimizations.

The tool also supports building product releases of the RTOS. A product release bundles a set of variants and target platforms tailored for a certain application product that builds on the RTOS. Thus, the application product sees only what it needs without being needlessly exposed to all features and platforms supported by the RTOS. The variants and platforms contained in a release are defined by

Systems, Modules and Packages

An RTOS system encompasses the entirety of the OS and an application on top of it. It consists in particular of:

  • the OS in the form of a variant of the RTOS with a feature set suitable for the application (e.g., which form of scheduling is supported)
  • a system configuration that tailors the variant to the specific application instance (e.g., how many task or mutexes the application requires)
  • the application code itself

Systems are built via the prj tool which implements the RTOS build system. At the build system level, systems are composed of modules (such as the RTOS module), so modules provide the unit of composition and reusability.

In its simplest form, a module is a C file and that is usually all that applications need to know about modules. However, modules can consist of the following elements:

  • Entity definition file named or <module_name>.py, specifying the module contents and customization options by providing a module Python object.
  • C and header file named <module_name>.c/h, providing the public interface and its implementation of the module in C
  • Assembly file named <module_name>.s, providing the module functionality as assembly code
  • Linker script named <module_name>.ld, specifying linker commands for linking the system (not just the module)
  • XML Configuration schema (as a standalone file or integrated into the entity definition script or source code files), specifying the configuration parameters supported by the module
  • Builder module script <module_name>.py, defining a system_build() function to be executed in order for the system to be built. The presence of this function distinguishes the module as a Builder module.

A system is statically defined and configured in its system configuration in a .prx file. This is an XML file that lists the modules that make up a system and provides configuration parameters for each of the modules. The .prx file includes a static declaration of all the RTOS resources used by the system, including all tasks, mutexes, semaphores, interrupt handlers, etc. The prj tool reads .prx files and composes, compiles, and links all the code to produce a system binary.

A complete system typically consists of

  • an RTOS module
  • modules dictating the build process for the target platform
  • one or more modules containing platform-specific assembly code needed by the RTOS variant to implement low-level OS functionality
  • one or more modules containing user-provided code that implement the application functionality

For example, the Kochab RTOS example system for QEMU-emulated PowerPC e500 (defined in packages/machine-qemu-ppce500/example/kochab-system.prx) contains the following modules:

  • ppce500.rtos-kochab, the Kochab variant of the RTOS for ppce500.
  • and ppce500.default-linker, which define building and linking for ppce500.
  • ppce500.interrupts-util and ppce500.vectable, which provide assembly-level RTOS code for ppce500.
  • ppce500.debug and generic.debug, which define stubs for debug printing.
  • machine-qemu-ppce500.example.kochab-test, the user-provided test program for Kochab QEMU-emulated PowerPC e500.

On the file system, modules are grouped into packages, allowing modules to be organised based on common characteristics (such as platform or intended usage). For example, the PowerPC e500 RTOS variant modules are grouped together with the build, default-linker, interrupts-util and vectable modules in the ppce500 package. As another example, platform-agnostic RTOS example code such as the kochab-mutex-test and timer-test modules are grouped together in the rtos-example package.

Tool support

As described above, provides the means to generate all the different RTOS variants and prj provides the means to combine an RTOS module with other modules to produce a system binary.

There are two main steps in building an RTOS-based system from the RTOS repository.

Step 1: Generate the RTOS variants.

 ./ build packages

This generates all the RTOS variants. For each variant specified in it finds the appropriate components and combines them into the RTOS variant. The resulting variant can be found in packages/<platform>/rtos-<variant>.

Please look at the documentation inside for more information on the tool.

Step 2: Build a system.

 prj/app/ build <system name>

This finds the appropriate .prx file, combines the required modules and generates a system binary. prj can be further configured using the top level project.prj file, which specifies tasks that are automatically performed when prj is run as well as the locations to look for modules and .prx configuration files.

As a convenience prj can be configured to automatically regenerate RTOS modules whenever it is run. This is done by including the following line in the project.prj file:

 <startup-script>./ build partials --allow-unknown-filetypes</startup-script>

Please see prj/manuals/prj-user-manual for more information on the prj tool.

Repository contents

The components directory contains the RTOS component source and documentation.

The packages directory provides various platform-specific RTOS modules and other source modules. RTOS modules generated by are output to locations within the packages directory structure.

The tool provides an interface for:

  • RTOS module and product release generation ( build),
  • task management ( task), and
  • testing ( test).

Much of's underlying implementation resides in the pylib directory, and its self-test suite ( test x) is implemented in

The prj directory contains everything related to the prj tool, which provides an interface for configuring and building systems as compositions of source modules.

The release directory is created by, and contains all releasable artifacts such as product releases.

The external_tools and tools directories contain external tools committed to the repository in order for the repository to be self-contained in its ability to provide everything that is needed for development. See external_tools/LICENSE.txt and tools/LICENSE.txt for license information regarding the contents of these directories.

The pm directory contains project management related meta-data. This meta-data is crucial for documenting that our internal development processes are correctly followed, as well as providing a record for external audit. The feature documentation for development tasks resides in pm/tasks, and their reviews can be found under pm/reviews.

The docs directory contains various release documentation-related content, including templates for auto-generated manuals and top-level README files for product releases.

The rtos directory contains a model of the RTOS schedulers implemented in Python for testing purposes.

Common Development Tasks

Developing a RTOS variant

To generate the code for all available RTOS variants:

./ build packages

The resulting RTOS code is placed into packages/<platform>/rtos-<variant>/.

The RTOS variants themselves are specified as lists of components within itself. Adding a new RTOS variant means adding the appropriate list entries to, and adding new component code in the components directory structure if necessary.

Please see the existing component code under components for examples on how to develop RTOS components.

Building user documentation

Manuals for all RTOS variants are built using:

./ build docs

The manuals are output to packages/<platform>/rtos-<variant>/documentation.* in HTML, Markdown and PDF.

Like the RTOS component code, the user manual content is componentized so that only the user documentation for components present in a particular RTOS variant appears in the user manual for that variant.

See components/*/ for examples of componentized user documentation.

Building the prj tool

To build just the prj tool binary for stand-alone use:

# The `prj` binary is output to `prj_build_x86_64-unknown-linux-gnu/prj`
./ build prj

Configuring and building a system

To build a system, use the prj build sub-tool of prj, supplying the system name as argument. The name of the system is the basename of its .prx file, minus its .prx extension, appended to a dot-separated string indicating the sub-package location in which it can be found.

For example, to build the system whose .prx file is located at packages/machine-qemu-ppce500/example/kochab-timer-demo.prx, from the top level of the repository:

prj/app/ build machine-qemu-ppce500.example.kochab-timer-demo

Alternatively, assuming the prj tool binary is on the PATH:

prj build machine-qemu-ppce500.example.kochab-timer-demo

Building a product release

To build all product releases, run:

./ build prj
./ build docs
./ build partials
./ build release

The releases are defined in the top-level script and are generated by into release/*.tar.gz.

Each product release contains:

  • a LICENSE file
  • a file containing a brief introduction to the release
  • a build_info file containing the commit hash of the RTOS repository from which the release was built
  • a share directory, containing the package directory structure containing all the released packages
  • a pre-built prj binary for the host platform targeted by the release

Note that any user manuals available for each RTOS variant in the release can be found at share/packages/<platform>/rtos-<variant>/documentation.* in HTML, Markdown and PDF.

Using a product release

After unpacking the product release, create a project.prj file for your software project in order to be able to use the prj tool that comes packaged with the release.

A minimal project.prj contains, at the very least, a search-path entry to give prj a hint as to where to find modules. Here is a example that points to its relative location from the root of the release:

<?xml version="1.0" encoding="UTF-8" ?>

After this, the prj binary packaged with the release can be used in the same way as in the RTOS repository. Additional application code can be placed in other directories as long as they are included as search paths in the project.prj file.

For example, to build the Kochab timer demo system provided with the PowerPC e500 release of the RTOS for Linux, run:

x86_64-unknown-linux-gnu/bin/prj build machine-qemu-ppce500.example.kochab-timer-demo

Note that the PATH environment variable needs to be set up manually so that it includes the tools invoked by prj.


The eChronos real-time operating system




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