115 lines
4.3 KiB
ReStructuredText
115 lines
4.3 KiB
ReStructuredText
.. _introducing_zephyr:
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Introducing Zephyr
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##################
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The Zephyr kernel is a small-footprint kernel designed for use on
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resource-constrained systems: from simple embedded environmental
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sensors and LED wearables to sophisticated smart watches and IoT
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wireless gateways.
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It is designed to be supported by multiple architectures, including
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ARM Cortex-M, Intel x86, and ARC. The full list of supported boards
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can be found :ref:`here <board>`.
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Licensing
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*********
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The Zephyr project associated with the kernel makes it available
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to users and developers under the Apache License, version 2.0.
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Distinguishing Features
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***********************
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The Zephyr kernel offers a number of features that distinguish it from other
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small-footprint OSes:
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#. **Single address-space**. Combines application-specific code
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with a custom kernel to create a monolithic image that gets loaded
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and executed on a system's hardware. Both the application code and
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kernel code execute in a single shared address space.
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#. **Highly configurable**. Allows an application to incorporate *only*
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the capabilities it needs as it needs them, and to specify their
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quantity and size.
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#. **Compile-time resource definition**. Allows system resources
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to be defined at compile-time, which reduces code size and
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increases performance.
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#. **Minimal error checking**. Provides minimal run-time error checking
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to reduce code size and increase performance. An optional error-checking
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infrastructure is provided to assist in debugging during application
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development.
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#. **Extensive suite of services**. Offers a number of familiar services
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for development:
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* *Multi-threading Services* for priority-based, non-preemptive and
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preemptive threads with optional round robin time-slicing.
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* *Interrupt Services* for compile-time registration of interrupt handlers.
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* *Memory Allocation Services* for dynamic allocation and freeing of
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fixed-size or variable-size memory blocks.
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* *Inter-thread Synchronization Services* for binary semaphores,
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counting semaphores, and mutex semaphores.
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* *Inter-thread Data Passing Services* for basic message queues, enhanced
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message queues, and byte streams.
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* *Power Management Services* such as tickless idle and an advanced idling
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infrastructure.
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Fundamental Terms and Concepts
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******************************
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This section outlines the basic terms used by the Zephyr kernel ecosystem.
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:dfn:`kernel`
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The set of Zephyr-supplied files that implement the Zephyr kernel,
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including its core services, device drivers, network stack, and so on.
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:dfn:`application`
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The set of user-supplied files that the Zephyr build system uses
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to build an application image for a specified board configuration.
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It can contain application-specific code, kernel configuration settings,
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and at least one Makefile.
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The application's kernel configuration settings direct the build system
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to create a custom kernel that makes efficient use of the board's resources.
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An application can sometimes be built for more than one type of board
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configuration (including boards with different CPU architectures),
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if it does not require any board-specific capabilities.
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:dfn:`application image`
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A binary file that is loaded and executed by the board for which
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it was built.
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Each application image contains both the application's code and the
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Zephyr kernel code needed to support it. They are compiled as a single,
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fully-linked binary.
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Once an application image is loaded onto a board, the image takes control
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of the system, initializes it, and runs as the system's sole application.
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Both application code and kernel code execute as privileged code
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within a single shared address space.
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:dfn:`board`
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A target system with a defined set of devices and capabilities,
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which can load and execute an application image. It may be an actual
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hardware system or a simulated system running under QEMU.
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The Zephyr kernel supports a :ref:`variety of boards <board>`.
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:dfn:`board configuration`
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A set of kernel configuration options that specify how the devices
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present on a board are used by the kernel.
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The Zephyr build system defines one or more board configurations
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for each board it supports. The kernel configuration settings that are
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specified by the build system can be over-ridden by the application,
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if desired.
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