Micro-Controller Operating Systems (MicroC/OS, stylized as μC/OS, or Micrium OS) is a real-time operating system (RTOS) designed by Jean J. Labrosse in 1991. It is a priority-based preemptive real-time kernel for microprocessors, written mostly in the programming language C. It is intended for use in embedded systems. MicroC/OS allows defining several functions in C, each of which can execute as an independent thread or task. Each task runs at a different priority, and runs as if it owns the central processing unit (CPU). Lower priority tasks can be preempted by higher priority tasks at any time. Higher priority tasks use operating system (OS) services (such as a delay or event) to allow lower priority tasks to execute. OS services are provided for managing tasks and memory, communicating between tasks, and timing.
History The MicroC/OS kernel was published originally in a three-part article in Embedded Systems Programming magazine and the book μC/OS The Real-Time Kernel by Labrosse. He intended at first to simply describe the internals of a portable OS he had developed for his own use, but later developed it as a commercial product in his own company Micrium, Inc. in versions II and III. In 2016 Micrium, Inc. was acquired by Silicon Laboratories and it was subsequently released as open-source under the Apache license. Silicon Labs continues to maintain an open-source product named Micrium OS for use on their own silicon and a group of former Micrium, Inc. employees (including Labrosse) provides consultancy and support for both μC/OS and Cesium RTOS, a proprietary fork made just after the open-source release.
μC/OS-II Based on the source code written for μC/OS, and introduced as a commercial product in 1998, μC/OS-II is a portable, ROM-able, scalable, preemptive, real-time, deterministic, multitasking kernel for microprocessors, and digital signal processors (DSPs). It manages up to 64 tasks. Its size can be scaled (between 5 and 24 Kbytes) to only contain the features needed for a given use. Most of μC/OS-II is written in highly portable ANSI C, with target microprocessor-specific code written in assembly language. Use of the latter is minimized to ease porting to other processors.
Uses in embedded systems μC/OS-II was designed for embedded uses. If the producer has the proper toolchain (i.e., C compiler, assembler, and linker-locator), μC/OS-II can be embedded as part of a product. μC/OS-II is used in many embedded systems, including:
Avionics Medical equipment and devices Data communications equipment White goods (appliances) Mobile phones, personal digital assistants (PDAs), MIDs Industrial controls Consumer electronics Automotive
Task states μC/OS-II is a multitasking operating system. Each task is an infinite loop and can be in any one of the following five states (see figure below additionally)
Dormant Ready Running Waiting (for an event) Interrupted (interrupt service routine (ISR)) Further, it can manage up to 64 tasks. However, it is recommended that eight of these tasks be reserved for μC/OS-II, leaving an application up to 56 tasks.
Kernels The kernel is the name given to the program that does most of the housekeeping tasks for the operating system. The boot loader hands control over to the kernel, which initializes the various devices to a known state and makes the computer ready for general operations. The kernel is responsible for managing tasks (i.e., for managing the CPU's time) and communicating between tasks. The fundamental service provided by the kernel is context switching. The scheduler is the part of the kernel responsible for determining which task runs next. Most real-time kernels are priority based. In a priority-based kernel, control of the CPU is always given to the highest priority task ready to run. Two types of priority-based kernels exist: non-preemptive and preemptive. Nonpreemptive kernels require that each task do something to explicitly give up control of the CPU. A preemptive kernel is used when system responsiveness is more important. Thus, μC/OS-II and most commercial real-time kernels are preemptive. The highest priority task ready to run is always given control of the CPU.
Assigning tasks Tasks with the highest rate of execution are given the highest priority using rate-monotonic scheduling. This scheduling algorithm is used in real-time operating systems (RTOS) with a static-priority scheduling class.
Managing tasks In computing, a task is a unit of execution. In some operating systems, a task is synonymous with a process, in others with a thread. In batch processing computer systems, a task is a unit of execution within a job. The system user of μC/OS-II is able to control the tasks by using the following features:
Task feature Task creation Task stack & stack checking Task deletion Change a task's priority Suspend and resume a task Get information about a task
Managing memory To avoid fragmentation, μC/OS-II allows applications to obtain fixed-sized memory blocks from a partition made of a contiguous memory area. All memory blocks are the same size, and the partition contains an integral number of blocks. Allocation and deallocation of these memory blocks is done in constant time and is a deterministic system.
Managing time μC/OS-II requires that a periodic time source be provided to keep track of time delays and timeouts. A tick should occur between 10 and 1000 times per second, or Hertz. The faster the tick rate, the more overhead μC/OS-II imposes on the system. The frequency of the clock tick depends on the desired tick resolution of an application. Tick sources can be obtained by dedicating a hardware timer, or by generating an interrupt from an alternating current (AC) power line (50 or 60 Hz) signal. This periodic time source is termed a clock tick. After a clock tick is determined, tasks can be:
Delaying a task Resume a delayed task
Communicating between tasks Intertask or interprocess communication in μC/OS-II occurs via: semaphores, message mailbox, message queues, tasks, and interrupt service routines (ISRs). They can interact with each other when a task or an ISR signals a task through a kernel object called an event control block (ECB). The signal is considered to be an event.
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