.. _introducing_zephyr:

Introduction
############

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

The Zephyr kernel supports multiple architectures, including:

 - ARCv2 (EM and HS) and ARCv3 (HS6X)
 - ARMv6-M, ARMv7-M, and ARMv8-M (Cortex-M)
 - ARMv7-A and ARMv8-A (Cortex-A, 32- and 64-bit)
 - ARMv7-R, ARMv8-R (Cortex-R, 32- and 64-bit)
 - Intel x86 (32- and 64-bit)
 - MIPS (MIPS32 Release 1 specification)
 - NIOS II Gen 2
 - RISC-V (32- and 64-bit)
 - SPARC V8
 - Tensilica Xtensa

The full list of supported boards based on these architectures can be found :ref:`here <boards>`.

Licensing
*********

Zephyr is permissively licensed using 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/main/LICENSE

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


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

Zephyr offers a large and ever growing number of features including:

**Extensive suite of Kernel services**
   Zephyr offers a number of familiar services for development:

   * *Multi-threading Services* for cooperative, priority-based,
     non-preemptive, and preemptive threads with optional round robin
     time-slicing. Includes POSIX pthreads compatible API support.

   * *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 overarching, application or
     policy-defined, System Power Management and fine-grained, driver-defined,
     Device Power Management.

**Multiple Scheduling Algorithms**
   Zephyr provides a comprehensive set of thread scheduling choices:

   * Cooperative and Preemptive Scheduling
   * Earliest Deadline First (EDF)
   * Meta IRQ scheduling implementing "interrupt bottom half" or "tasklet"
     behavior
   * Timeslicing: Enables time slicing between preemptible threads of equal
     priority
   * Multiple queuing strategies:

     * Simple linked-list ready queue
     * Red/black tree ready queue
     * Traditional multi-queue ready queue

.. _zephyr_intro_configurability:

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

**Cross Architecture**
   Supports a wide variety of :ref:`supported boards<boards>` with different CPU
   architectures and developer tools. Contributions have added support
   for an increasing number of SoCs, platforms, and drivers.

**Memory Protection**
   Implements configurable architecture-specific stack-overflow protection,
   kernel object and device driver permission tracking, and thread isolation
   with thread-level memory protection on x86, ARC, and ARM architectures,
   userspace, and memory domains.

   For platforms without MMU/MPU and memory constrained devices, supports
   combining 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.

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

**Optimized Device Driver Model**
   Provides a consistent device model for configuring the drivers that are part
   of the platform/system and a consistent model for initializing all the
   drivers configured into the system and allows the reuse of drivers across
   platforms that have common devices/IP blocks.

**Devicetree Support**
   Use of :ref:`devicetree <dt-guide>` to describe hardware.
   Information from devicetree is used to create the application image.

**Native Networking Stack supporting multiple protocols**
   Networking support is fully featured and optimized, including LwM2M and BSD
   sockets compatible support.  OpenThread support (on Nordic chipsets) is also
   provided - a mesh network designed to securely and reliably connect hundreds
   of products around the home.

**Bluetooth Low Energy 5.0 support**
   Bluetooth 5.0 compliant (ESR10) and Bluetooth Low Energy Controller support
   (LE Link Layer). Includes Bluetooth Mesh and a Bluetooth qualification-ready
   Bluetooth controller.

   * Generic Access Profile (GAP) with all possible LE roles
   * Generic Attribute Profile (GATT)
   * Pairing support, including the Secure Connections feature from Bluetooth
     4.2
   * Clean HCI driver abstraction
   * Raw HCI interface to run Zephyr as a Controller instead of a full Host
     stack
   * Verified with multiple popular controllers
   * Highly configurable

   Mesh Support:

   * Relay, Friend Node, Low-Power Node (LPN) and GATT Proxy features
   * Both Provisioning bearers supported (PB-ADV & PB-GATT)
   * Highly configurable, fitting in devices with at least 16k RAM

**Native Linux, macOS, and Windows Development**
   A command-line CMake build environment runs on popular developer OS
   systems. A native port (:ref:`native_sim <native_sim>`) lets you build and run Zephyr as a native
   application on Linux, aiding development and testing.

**Virtual File System Interface with ext2, FatFs, and LittleFS Support**
   ext2, LittleFS and FatFS support; FCB (Flash Circular Buffer) for memory constrained
   applications.

**Powerful multi-backend logging Framework**
   Support for log filtering, object dumping, panic mode, multiple backends
   (memory, networking, filesystem, console, ...) and integration with the shell
   subsystem.

**User friendly and full-featured Shell interface**
   A multi-instance shell subsystem with user-friendly features such as
   autocompletion, wildcards, coloring, metakeys (arrows, backspace, ctrl+u,
   etc.) and history. Support for static commands and dynamic sub-commands.

**Settings on non-volatile storage**
   The settings subsystem gives modules a way to store persistent per-device
   configuration and runtime state. Settings items are stored as key-value pair
   strings.

**Non-volatile storage (NVS)**
  NVS allows storage of binary blobs, strings, integers, longs, and any
  combination of these.

**Native port**
  :ref:`Native sim <native_sim>` allows running Zephyr as a Linux application with support
  for various subsystems and networking.


.. include:: ../../README.rst
   :start-after: start_include_here


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

See :ref:`glossary`