# 1. Introduction¶

## 1.1. Overview¶

You are someone looking for a real-time operating system. This document

• presents the basic features of RTEMS, so that you can decide if it is worth to look at,
• gives you a quick start to install all the tools necessary to work with RTEMS, and
• helps you to build an example application on top of RTEMS.

## 1.2. Features¶

The Real-Time Executive for Multiprocessor Systems (RTEMS) is a multi-threaded, single address-space, real-time operating system with no kernel-space/user-space separation. It is capable to operate in an SMP configuration providing a state of the art feature set.

RTEMS provides the following basic feature set:

• APIs

• Programming languages

• C/C++/OpenMP (RTEMS Source Builder, RSB)
• Ada (RSB, --with-ada)
• Erlang
• Fortran (RSB, --with-fortran)
• Python and MicroPython
• Parallel languages

• Mutexes with and without locking protocols
• Counting semaphores
• Binary semaphores
• Condition variables
• Events
• Message queues
• Barriers
• Futex (used by OpenMP barriers)
• Epoch Based Reclamation (libbsd)
• Locking protocols

• Transitive Priority Inheritance
• OMIP (SMP feature)
• Priority Ceiling
• MrsP (SMP feature)
• Scalable timer and timeout support

• Lock-free timestamps (FreeBSD timecounters)

• Responsive interrupt management

• C11/C++11 TLS [3]

• Fixed-priority
• Job-level fixed-priority (EDF)
• Constant Bandwidth Server (experimental)
• Clustered scheduling (SMP feature)

• Job-level fixed-priority scheduler (EDF) with support for one-to-one and one-to-all thread to processor affinities (default SMP scheduler)
• Fixed-priority scheduler
• Proof-of-concept strong APA scheduler
• Focus on link-time application-specific configuration

• Linker-set based initialization (similar to global C++ constructors)

• Operating system uses fine-grained locking (SMP feature)

• Dynamic memory allocators

• First-fit (default)
• Universal Memory Allocator (UMA , libbsd)
• File systems

• Device drivers

• Termios (serial interfaces)
• I2C (Linux user-space API compatible)
• SPI (Linux user-space API compatible)
• Network stacks (legacy, libbsd, lwIP)
• USB stack (libbsd)
• SD/MMC card stack (libbsd)
• Framebuffer (Linux user-space API compatible, Qt)
• Application runs in kernel-space and can access hardware directly
• libbsd

• Port of FreeBSD user-space and kernel-space components to RTEMS
• Support to stay in synchronization with FreeBSD

## 1.3. Ecosystem¶

The RTEMS Ecosystem is the collection of tools, packages, code, documentation and online content provided by the RTEMS Project. The ecosystem provides a way to develop, maintain, and use RTEMS. It’s parts interact with the user, the host environment, and each other to make RTEMS accessible, useable and predicable.

The ecosystem is for users, developers and maintainers and it is an ongoing effort that needs your help and support. The RTEMS project is always improving the way it delivers the kernel to you and your feedback is important so please join the mailing lists and contribute back comments, success stories, bugs and patches.

What the RTEMS project describes here to develop, maintain and use RTEMS does not dictate what you need to use in your project. You can and should select the work-flow that best suites the demands of your project and what you are delivering.

### 1.3.1. Rational¶

RTEMS is complex and the focus of the RTEMS Ecosystem is to simplify the complexity for users by providing a stable documented way to build, configure and run RTEMS. RTEMS is more than a kernel running real-time applications on target hardware, it is part of a project’s and therefore team’s workflow and every project and team is different.

RTEMS’s ecosystem does not mandate a way to work. It is a series of parts, components, and items that are used to create a suitable development environment to work with. The processes explained in this manual are the same things an RTEMS maintainer does to maintain the kernel or an experienced user does to build their production system. It is important to keep this in mind when working through this manual. We encourage users to explore what can be done and to discover ways to make it fit their needs. The ecosystem provided by the RTEMS Project will not install in a single click of a mouse because we want users to learn the parts they will come to depend on as their project’s development matures.

The RTEMS Ecosystem provides a standard interface that is the same on all supported host systems. Standardizing how a user interacts with RTEMS is important and making that experience portable is also important. As a result the ecosystem is documented at the command line level and we leave GUI and IDE integration for users and integrators.

Standardizing the parts and how to use them lets users create processes and procedures that are stable over releases. The RTEMS Ecosystem generates data that can be used to audit the build process so their configuration can be documented.

The ecosystem is based around the source code used in the various parts, components and items of the RTEMS development environment. A user can create an archive of the complete build process including all the source code for long term storage. This is important for projects with a long life cycle.

### 1.3.2. Open Source¶

RTEMS is an open source operating system and an open source project and this extends to the ecosystem. We encourage users to integrate the processes to build tools, the kernel and any third-party libraries into their project’s configuration management processes.

All the parts that make up the ecosystem are open source. The ecosystem uses a package’s source code to create an executable on a host so when an example RTEMS executable is created and run for the first time the user will have built every tool as well as the executable from source. The RTEMS Project believes the freedom this gives a user is as important as the freedom of having access to the source code for a package.

### 1.3.3. Deployment¶

The RTEMS Project provides the ecosystem as source code that users can download to create personalised development environments. The RTEMS Project does not provide packaging and deployment for a specific host environment, target architecture or BSP. The RTEMS Project encourages users and organizations to fill this role for the community. The RTEMS Source Builder provides some aid to build and deploy tool binaries.

## 1.4. Real-time Application Systems¶

Real-time application systems are a special class of computer applications. They have a complex set of characteristics that distinguish them from other software problems. Generally, they must adhere to more rigorous requirements. The correctness of the system depends not only on the results of computations, but also on the time at which the results are produced. The most important and complex characteristic of real-time application systems is that they must receive and respond to a set of external stimuli within rigid and critical time constraints referred to as deadlines. Systems can be buried by an avalanche of interdependent, asynchronous or cyclical event streams.

Deadlines can be further characterized as either hard or soft based upon the value of the results when produced after the deadline has passed. A deadline is hard if the results have no value after the deadline has passed, or a catastrophic event results from their intended use if not completed on time. In contrast, results produced after a soft deadline may still have some value.

Another distinguishing requirement of real-time application systems is the ability to coordinate or manage a large number of concurrent activities. Since software is a synchronous entity, this presents special problems. One instruction follows another in a repeating synchronous cycle. Even though mechanisms have been developed to allow for the processing of external asynchronous events, the software design efforts required to process and manage these events and tasks are growing more complicated.

The design process is complicated further by spreading this activity over a set of processors instead of a single processor. The challenges associated with designing and building real-time application systems become very complex when multiple processors are involved. New requirements such as interprocessor communication channels and global resources that must be shared between competing processors are introduced. The ramifications of multiple processors complicate each and every characteristic of a real-time system.

## 1.5. Real-time Executive¶

Fortunately, real-time operating systems, or real-time executives, serve as a cornerstone on which to build the application system. A real-time multitasking executive allows an application to be cast into a set of logical, autonomous processes or tasks which become quite manageable. Each task is internally synchronous, but different tasks execute independently, resulting in an asynchronous processing stream. Tasks can be dynamically paused for many reasons resulting in a different task being allowed to execute for a period of time. The executive also provides an interface to other system components such as interrupt handlers and device drivers. System components may request the executive to allocate and coordinate resources, and to wait for and trigger synchronizing conditions. The executive system calls effectively extend the CPU instruction set to support efficient multitasking. By causing tasks to travel through well-defined state transitions, system calls permit an application to demand-switch between tasks in response to real-time events.

By properly grouping stimuli responses into separate tasks a system can now asynchronously switch between independent streams of execution. This allows the system to directly respond to external stimuli as they occur, as well as meet critical performance specifications that are typically measured by guaranteed response time and transaction throughput. The multiprocessor extensions of RTEMS provide the features necessary to manage the extra requirements introduced by a system distributed across several processors. It removes the physical barriers of processor boundaries from the world of the system designer, enabling more critical aspects of the system to receive the required attention. Such a system, based on an efficient real-time, multiprocessor executive, is a more realistic model of the outside world or environment for which it is designed. As a result, the system will always be more logical, efficient, and reliable.

By using the directives provided by RTEMS, the real-time applications developer is freed from the problem of controlling and synchronizing multiple tasks and processors. In addition, one need not develop, test, debug, and document routines to manage memory, pass messages, or provide mutual exclusion. The developer is then able to concentrate solely on the application. By using standard software components, the time and cost required to develop sophisticated real-time applications are significantly reduced.

 [1] The goal is to use the BSD 2-Clause license for new code or code those copyright holder agreed to a license change, see #3053 for the details.
 [2] See #2832.
 [3] Thread-local storage requires some support by the tool chain and the RTEMS architecture support, e.g. context-switch code. It is supported at least on ARM, PowerPC, RISC-V, SPARC and m68k. Check the RTEMS CPU Architecture Supplement if it is supported.