.. _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)
 - Renesas RX
 - 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>`.

In the context of the Zephyr OS, a :term:`subsystem` refers to a logically distinct
part of the operating system that handles specific functionality or provides
certain services. Subsystems can include components such as networking,
file systems, device driver classes, power management, and communication protocols,
among others. Each subsystem is designed to be modular and can be configured,
customized, and extended to meet the requirements of different embedded
applications.

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 (:zephyr:board:`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**
  :zephyr:board:`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`
