.. _introducing_zephyr:

Introducing Zephyr
##################

The Zephyr OS is based on a small-footprint kernel designed for use on
resource-constrained systems: from simple embedded environmental sensors and LED
wearables to sophisticated smart watches and IoT wireless gateways.

The Zephyr kernel supports multiple architectures, including ARM Cortex-M, Intel
x86, ARC, NIOS II, Tensilica Xtensa and RISC V. The full list of supported
boards can be found :ref:`here <boards>`.

Licensing
*********

Zephyr uses the `Apache 2.0 license`_ (as found in the LICENSE file in the
project's `GitHub repo`_).  There are some
imported or reused components of the Zephyr project that use other licensing,
as described in :ref:`Zephyr_Licensing`.

.. _Apache 2.0 license:
   https://github.com/zephyrproject-rtos/zephyr/blob/master/LICENSE

.. _GitHub repo: https://github.com/zephyrproject-rtos/zephyr


Distinguishing Features
***********************

The Zephyr kernel offers a number of features that distinguish it from other
small-footprint OSes:

#. **Single address-space**. Combines application-specific code
   with a custom kernel to create a monolithic image that gets loaded
   and executed on a system's hardware. Both the application code and
   kernel code execute in a single shared address space.

#. **Highly configurable**. Allows an application to incorporate *only*
   the capabilities it needs as it needs them, and to specify their
   quantity and size.

#. **Compile-time resource definition**. Allows system resources
   to be defined at compile-time, which reduces code size and
   increases performance.

#. **Minimal error checking**. Provides minimal run-time error checking
   to reduce code size and increase performance. An optional error-checking
   infrastructure is provided to assist in debugging during application
   development.

#. **Extensive suite of services**. Offers a number of familiar services
   for development:

   * *Multi-threading Services* for priority-based, non-preemptive and
     preemptive threads with optional round robin time-slicing.

   * *Interrupt Services* for compile-time registration of interrupt handlers.

   * *Memory Allocation Services* for dynamic allocation and freeing of
     fixed-size or variable-size memory blocks.

   * *Inter-thread Synchronization Services* for binary semaphores,
     counting semaphores, and mutex semaphores.

   * *Inter-thread Data Passing Services* for basic message queues, enhanced
     message queues, and byte streams.

   * *Power Management Services* such as tickless idle and an advanced idling
     infrastructure.

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Fundamental Terms and Concepts
******************************

See :ref:`glossary`