The Zephyr kernel is 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.
It is designed to be supported by multiple architectures, including ARM Cortex-M, Intel x86, and ARC. The full list of supported boards can be found here.
The Zephyr project associated with the kernel makes it available to users and developers under the Apache License, version 2.0.
The Zephyr kernel offers a number of features that distinguish it from other small-footprint OSes:
- Single address-space OS. 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.
- Resources defined at compile-time. Requires all system resources be defined at compilation 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
- Multi-threading Services for both priority-based, non-preemptive fibers and priority-based, preemptive tasks with optional round robin time-slicing.
- Interrupt Services for both compile-time and run-time registration of interrupt handlers.
- 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.
- Memory Allocation Services for dynamic allocation and freeing of fixed-size or variable-size memory blocks.
- Power Management Services such as tickless idle and an advanced idling infrastructure.
Fundamental Terms and Concepts
This section outlines the basic terms used by the Zephyr kernel ecosystem.
- The set of Zephyr-supplied files that implement the Zephyr kernel, including its core services, device drivers, network stack, and so on.
The set of user-supplied files that the Zephyr build system uses to build an application image for a specified board configuration. It can contain application-specific code, kernel configuration settings, kernel object definitions, and at least one Makefile.
The application’s kernel configuration settings direct the build system to create a custom kernel that makes efficient use of the board’s resources.
An application can sometimes be built for more than one type of board configuration (including boards with different CPU architectures), if it does not require any board-specific capabilities.
- application image
A binary file that is loaded and executed by the board for which it was built.
Each application image contains both the application’s code and the Zephyr kernel code needed to support it. They are compiled as a single, fully-linked binary.
Once an application image is loaded onto a board, the image takes control of the system, initializes it, and runs as the system’s sole application. Both application code and kernel code execute as privileged code within a single shared address space.
A target system with a defined set of devices and capabilities, which can load and execute an application image. It may be an actual hardware system or a simulated system running under QEMU.
The Zephyr kernel supports a variety of boards.
- board configuration
A set of kernel configuration options that specify how the devices present on a board are used by the kernel.
The Zephyr build system defines one or more board configurations for each board it supports. The kernel configuration settings that are specified by the build system can be over-ridden by the application, if desired.