View source for Micro-Controller Operating Systems

{{Short description|Real-time operating system}}
{{Infobox OS
| name = MicroC/OS (μC/OS)
| logo = 
| caption = 
| developer = Micrium, Inc.,<br/>Silicon Labs
| family = 
| working state = Current
| source model = [[Open-source software|Open-source]] as of 2020
| released = {{Start date and age|1991}}
| latest release version = OS-III
| latest release date = {{Start date and age|2016}}
| repo = {{URL|https://github.com/weston-embedded/uC-OS3}}
| marketing target = [[Embedded device]]s
| programmed in = [[ANSI C]]
| language = English
| supported platforms = [[ARM Cortex-M#Cortex-M3|ARM Cortex-M3]], [[ARM Cortex-M#Cortex-M4|-M4F]], [[ARM7#ARM7TDMI|ARM7TDMI]]; [[Atmel AVR]]; [[eSi-RISC]], and many others
| kernel type = [[Real-time operating system|Real-time]] [[microkernel]]
| ui = μC/[[Graphical user interface|GUI]] 
| license = [[Apache License|Apache]] as of 2020; former [[Commercial software|Commercial]], [[freeware]] education use
| website = {{URL|https://weston-embedded.com/micrium/overview}}
}}
{{Infobox OS
| name = Micrium OS
| logo = 
| caption = 
| developer = Silicon Labs
| family = 
| working state = Current
| source model = [[Open-source software|Open-source]]
| released = {{Start date and age|2020}}
| latest release version = Part of Gecko Platform 4.2.0.0,<ref>{{cite web|url=https://www.silabs.com/documents/public/release-notes/gecko-platform-release-notes-4.2.0.0.pdf|title=Gecko Platform 4.2.0.0 GA|access-date=2023-01-04|date=2022-12-14}}</ref> part of Gecko SDK 4.2.0.0<ref>{{cite web|url=https://github.com/SiliconLabs/gecko_sdk/releases|title=gecko_sdk Releases on github.com|website=[[GitHub]] |access-date=2023-01-04}}</ref>
| latest release date = {{Start date and age|2022|12|14}}
| repo = {{URL|https://github.com/SiliconLabs/gecko_sdk/tree/gsdk_4.2/platform/micrium_os}}
| marketing target = [[Embedded device]]s
| programmed in = [[ANSI C]]
| language = English
| supported platforms = exclusively Silicon Labs silicon
| kernel type = [[Real-time operating system|Real-time]] [[microkernel]]
| license = [[Apache License|Apache]]
| website = {{URL|https://www.silabs.com/developers/micrium-os}}
}}
{{Infobox OS
| name = Cesium RTOS
| logo = 
| caption = 
| developer = Weston Embedded Solutions
| family = 
| working state = Current
| source model = [[Commercial software|Commercial]]
| released = {{Start date and age|2020|06|23}} (forked from uC/OS-III V3.08.00)<ref name=cesium_changelog>{{cite web|url=https://weston-embedded.com/cesium-release-notes/cs-os3-release-notes|title=Cs/OS3 Release Notes|publisher= Weston Embedded Solutions}}</ref>
| latest release version = Cs/OS3 3.09.05<ref name=cesium_changelog />
| latest release date = {{Start date and age|2025|04|22}}<ref name=cesium_changelog />
| marketing target = [[Embedded device]]s
| programmed in = [[ANSI C]]
| language = English
| supported platforms = 50+ unclear whether there is a 1-to-1 overlap with {{nowrap|μC/OS}}
| kernel type = [[Real-time operating system|Real-time]] [[microkernel]]
| license = [[Commercial software|Commercial]]
| website = {{URL|weston-embedded.com/products/cesium}}
}}
'''Micro-Controller Operating Systems''' ('''MicroC/OS''', stylized as '''{{nowrap|μC/OS}}''', or '''Micrium OS''') is a [[real-time operating system]] (RTOS) designed by Jean J. Labrosse in 1991. It is a priority-based [[Preemption (computing)|preemptive]] [[Real-time computing|real-time]] kernel for [[microprocessor]]s, written mostly in the programming language [[C (programming language)|C]]. It is intended for use in [[embedded system]]s.

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.<ref>{{cite web |url=https://people.ece.cornell.edu/land/courses/ece5760/NiosII_muCOS/ |title=NiosII GCC with MicroC/OS |author=<!--Unstated--> |date=June 2006 |website=School of Electrical and Computer Engineering |publisher=Cornell University |access-date=25 April 2017}}</ref>

==History==
The MicroC/OS kernel was published originally in a three-part article in Embedded Systems Programming magazine and the book ''{{nowrap|μC/OS}} The Real-Time Kernel'' by Labrosse.<ref>{{cite book |last=Labrosse |first=Jean J. |date=15 June 2002 |title=μC/OS The Real-Time Kernel |edition=2nd |publisher=CRC Press |isbn=978-1578201037}}</ref> He intended at first to simply describe the internals of a [[Software portability|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<ref>{{cite web|url=https://weston-embedded.com/about-micrium|title=What is Micrium?|access-date=2023-01-04|publisher=Weston Embedded Solutions}}</ref> 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<ref>{{cite web|url=https://www.silabs.com/developers/micrium|title=Micrium Software and Documentation|access-date=2023-01-04}}</ref> and a group of former Micrium, Inc. employees (including Labrosse) provides consultancy and support for both {{nowrap|μC/OS}} and Cesium RTOS, a proprietary fork made just after the open-source release.<ref>{{cite web|url=https://weston-embedded.com/why-cesium|title=Why Cesium RTOS?|access-date=2023-01-04|publisher= Weston Embedded Solutions}}</ref>

==μC/OS-II==
Based on the source code written for {{nowrap|μC/OS}}, and introduced as a commercial product in 1998, {{nowrap|μC/OS-II}} is a [[Software portability|portable]], ROM-able, [[scalable]], preemptive, real-time, deterministic, multitasking [[Kernel (operating system)|kernel]] for [[microprocessor]]s, and [[digital signal processor]]s (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 {{nowrap|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 ===
{{nowrap|μC/OS-II}} was designed for embedded uses. If the producer has the proper [[toolchain]] (i.e., C compiler, assembler, and linker-locator{{clarify|date=May 2024}}), {{nowrap|μC/OS-II}} can be embedded as part of a product.

{{nowrap|μC/OS-II}} is used in many embedded systems, including:
* [[Avionics]] 
* [[Medical equipment]] and devices
* [[Data communications equipment]]
* White goods ([[Home appliance|appliances]])
* [[Mobile phone]]s, [[personal digital assistant]]s (PDAs), MIDs
* Industrial controls
* [[Consumer electronics]]
* [[Automotive]]

===Task states===
{{nowrap|μC/OS-II}} is a [[Computer multitasking|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 handler|interrupt service routine]] (ISR))
Further, it can manage up to 64 tasks. However, it is recommended that eight of these tasks be reserved for {{nowrap|μC/OS-II}}, leaving an application up to 56 tasks.<ref>{{cite book|last=Labrosse|first=Jean J.|title=MicroC/OS-II: The Real Time Kernel|page=77|edition=2nd}}</ref>

===Kernels===
The [[Kernel (operating system)|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.{{CN|date=February 2026}} The kernel is responsible for managing tasks (i.e., for managing the CPU's time) and communicating between tasks.<ref>{{cite book|last=Labrosse|first=Jean J.|title=MicroC/OS-II: The Real Time Kernel|page=39|edition=2nd}}</ref> The fundamental service provided by the kernel is [[context switch]]ing.

The [[scheduler]] is the part of the kernel responsible for determining which task runs next.<ref name="LabrosseP40">{{cite book|last=Labrosse|first=Jean J.|title=MicroC/OS-II: The Real Time Kernel|page=40|edition=2nd}}</ref> 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: [[Computer multitasking#Cooperative multitasking|non-preemptive]] and [[Preemption (computing)|preemptive]]. Nonpreemptive kernels require that each task do something to explicitly give up control of the CPU.<ref name="LabrosseP40" /> A preemptive kernel is used when system responsiveness is more important. Thus, {{nowrap|μC/OS-II}} and most commercial real-time kernels are preemptive.<ref>{{cite book|last=Labrosse|first=Jean J.|title=MicroC/OS-II: The Real Time Kernel|page=42|edition=2nd}}</ref> 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]].<ref>{{cite journal|last1=Liu|first1=Chung Lang|last2=Layland|first2=James W.|title=Scheduling algorithms for multiprogramming in a hard real-time environment|journal=Journal of the ACM |volume=20|issue=1|pages=46–61|doi=10.1145/321738.321743|year=1973|citeseerx=10.1.1.36.8216|s2cid=59896693 }}</ref> This scheduling algorithm is used in real-time operating systems (RTOS) with a [[static-priority scheduling class]].<ref>{{cite web|last1=Bovet |first1=Daniel |title=Understanding The Linux Kernel |url=http://oreilly.com/catalog/linuxkernel/chapter/ch10.html#85347 |url-status=dead |archive-url=https://web.archive.org/web/20140921000832/http://oreilly.com/catalog/linuxkernel/chapter/ch10.html |archive-date=2014-09-21 }}</ref>

===Managing tasks===
In [[computing]], a task is a unit of [[execution]]. In some [[operating systems]], a task is synonymous with a [[Process (computing)|process]], in others with a [[Thread (computing)|thread]]. In [[batch processing]] computer systems, a task is a unit of execution within a [[Job stream|job]].
The system user of {{nowrap|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<ref>{{cite book|last=Labrosse|first=Jean J.|title=MicroC/OS-II: The Real Time Kernel|pages=45–49|edition=2nd}}</ref>

===Managing memory===
To avoid [[Fragmentation (computing)|fragmentation]], {{nowrap|μC/OS-II}} allows applications to obtain fixed-sized memory blocks from a [[Memory management (operating systems)#Partitioned allocation|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]].<ref>{{cite book|last=Labrosse|first=Jean J.|title=MicroC/OS-II: The Real Time Kernel|pages=273–285|edition=2nd}}</ref>

===Managing time===
{{nowrap|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 (computing)|overhead]] {{nowrap|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&nbsp;Hz) signal. This periodic time source is termed a clock tick.<ref>{{cite book|last=Labrosse|first=Jean J.|title=MicroC/OS-II: The Real Time Kernel|pages=145–152|edition=2nd}}</ref>

After a ''clock tick'' is determined, tasks can be: 
*Delaying a task 
*Resume a delayed task

===Communicating between tasks===
Intertask or interprocess communication in {{nowrap|μC/OS-II}} occurs via: [[Semaphore (programming)|semaphores]], message mailbox, message queues, tasks, and [[Interrupt handler|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.

==μC/OS-III==
{{nowrap|μC/OS-III}} is the acronym for Micro-Controller Operating Systems Version 3, introduced in 2009 and adding functionality to the {{nowrap|μC/OS-II}} RTOS.

{{nowrap|μC/OS-III}} offers all of the features and functions of {{nowrap|μC/OS-II}}. The biggest difference is the number of supported tasks. {{nowrap|μC/OS-II}} allows only 1 task at each of 255 priority levels, for a maximum of 255 tasks. {{nowrap|μC/OS-III}} allows any number of application tasks, priority levels, and tasks per level, limited only by processor access to memory.<ref>{{cite web |url=http://micrium.com/rtos/ucosiii/rtos-comparison/ |title=μC/OS-II and μC/OS-III Features Comparison |website=Micrium}}</ref><ref>{{cite web |url=http://micrium.com/rtos/ucosiii/overview/ |title=μC/OS-III overview |website=Micrium}}</ref>

{{nowrap|μC/OS-II}} and {{nowrap|μC/OS-III}} are currently maintained by Micrium, Inc., a subsidiary of Silicon Labs, and can be licensed per product or per product line.

===Uses in embedded systems===
The uses are the same as for {{nowrap|μC/OS-II}}

===Task states===
{{nowrap|μC/OS-III}} is a [[Computer multitasking|multitasking]] operating system. Each task is an infinite loop and can be in any one of five states (dormant, ready, running, interrupted, or pending). {{nowrap|μC/OS-III}} supports an unlimited number of task priorities but configuring {{nowrap|μC/OS-III}} to have between 32 and 256 task priorities typically suits most embedded systems well.<ref>{{Cite web | url=https://media.digikey.com/PDF/Data%20Sheets/Micrium%20PDFs/UC_OS-III_RTOS.pdf#:~:text=Micrium%E2%80%99s%20%CE%BCC%2FOS-III%20supports%20ARM7%2F9%2C%20Cortex-MX%2C%20Nios-II%2C%20PowerPC%2C%20Coldfire%2C,are%20available%20for%20download%20from%20the%20Micrium%20website | title=μC/OS-III™ Real-Time Operating System | website=media.digikey.com}}</ref>

===Round robin scheduling===
When two or more tasks have the same priority, the kernel allows one task to run for a predetermined amount of time, named a ''quantum'', and then selects another task. This process is termed [[round robin scheduling]] or time slicing. The kernel gives control to the next task in line if:
*The current task has no work to do during its time slice, or 
*The current task completes before the end of its time slice, or 
*The time slice ends.

===Kernels===
The kernel functionality for {{nowrap|μC/OS-III}} is the same as for {{nowrap|μC/OS-II}}.

===Managing tasks===
Task management also functions the same as for {{nowrap|μC/OS-II}}. However, {{nowrap|μC/OS-III}} supports multitasking and allows an application to have any number of tasks. The maximum number of tasks is limited by only the amount of computer memory (both code and data space) available to the processor.

A task can be implemented viarunning to scheduled completion, in which the task deletes itself when it is finished, or more typically as an infinite loop, waiting for events to occur and processing those events.

===Managing memory===
Memory management is performed in the same way as in {{nowrap|μC/OS-II}}.

===Managing time===
{{nowrap|μC/OS-III}} offers the same time managing features as {{nowrap|μC/OS-II}}. It also provides services to applications so that tasks can suspend their execution for user-defined time delays. Delays are specified by a number of either clock ticks, or hours, minutes, seconds, and [[millisecond]]s.

===Communicating between tasks===
Sometimes, a task or ISR must communicate information to another task, because it is ''unsafe'' for two tasks to access the same specific data or hardware resource at once. This can be resolved via an information transfer, termed inter-task communication. Information can be communicated between tasks in two ways: through global data, or by sending messages.

When using global variables, each task or ISR must ensure that it has exclusive access to variables. If an ISR is involved, the only way to ensure exclusive access to common variables is to disable [[interrupt]]s. If two tasks share data, each can gain exclusive access to variables by either disabling interrupts, locking the scheduler, using a [[Semaphore (programming)|semaphore]], or preferably, using a [[mutual exclusion]] semaphore. Messages can be sent to either an intermediate object called a [[message queue]], or directly to a task, since in {{nowrap|μC/OS-III}}, each task has its own built-in message queue. Use an external message queue if multiple tasks are to wait for messages. Send a message directly to a task if only one task will process the data received. While a task waits for a message to arrive, it uses no CPU time.

==Ports==
A port involves three aspects: CPU, OS, and board specific (BSP) code. {{nowrap|μC/OS-II}} and {{nowrap|μC/OS-III}} have ports for most popular processors and boards in the market and are suitable for use in [[safety critical]] embedded systems such as aviation, medical systems, and nuclear installations. A {{nowrap|μC/OS-III}} port involves writing or changing the contents of three kernel specific files: <code>OS_CPU.H</code>, <code>OS_CPU_A.ASM</code>, and <code>OS_CPU_C.C</code>. Finally create or change a board support package (BSP) for the evaluation board or target board being used. A {{nowrap|μC/OS-III}} port is similar to a {{nowrap|μC/OS-II}} port. There are significantly more ports than listed here, and ports are subject to continuous development. Both {{nowrap|μC/OS-II}} and {{nowrap|μC/OS-III}} are supported by popular [[Transport Layer Security|SSL/TLS]] libraries such as [[wolfSSL]], which ensure security across all connections.

==Licensing change==
After acquisition by Silicon Labs, Micrium in 2020 changed to [[open-source model]] licensing in February 2020. This includes uC/OS III, all prior versions, all components: USB, [[file system]], GUI, TCP/IP, etc.

==Documentation and support==
Support is available via a typical support forum, and several comprehensive books, of which some are tailored to a given microcontroller architecture and development platform, as free PDFs, or as low-cost purchase in hard-cover. Paid support is available from Weston Embedded Solutions.

==References==
{{Reflist}}

==Sources==
*[http://www.mil-embedded.com/news/db/?13968 Protocol Support for {{nowrap|μC/OS-II}} from Fusion Embedded]
*Micrium-uCOS-III-UsersManual 1st Edition
*[http://micrium.com/download/%C2%B5cos-iii-the-real-time-kernel-for-the-renesas-rx62n/ uC/OS-III: The Real-Time Kernel for the Renesas RX62N]

==External links==
*{{Official website|https://web.archive.org/web/20231206170818/https://www.silabs.com/developers/micrium}}
*{{GitHub|SiliconLabs}}
*[https://people.ece.cornell.edu/land/courses/ece5760/NiosII_muCOS/uC_Functions.html Summary of Commonly Used uC/OS-II Functions and Data Structures] 
*[https://people.ece.cornell.edu/land/courses/ece5760/NiosII_muCOS/ NiosII GCC with MicroC/OS]
*[http://www.farnell.com/datasheets/1950186.pdf {{nowrap|μC/OS-II}} Reference Manual]
*[http://ftp1.digi.com/support/documentation/0220047_e.pdf How to Get a {{nowrap|μC/OS-II}} Application Running]
* [[Wikiversity:Operating Systems/Kernel Models#Monolithic Kernel]]

{{Real-time operating systems}}
{{Microkernel}}

{{DEFAULTSORT:Microc Os-II}}
[[Category:Real-time operating systems]]
[[Category:Embedded operating systems]]
[[Category:ARM operating systems]]
[[Category:Microkernel-based operating systems]]
[[Category:Microkernels]]