A real-time operating system (RTOS) is similar to a general-purpose operating system, however it is designed with special characteristics to support real-time applications. In such applications, the dependability that a certain task will finish before a particular deadline is just as important as obtaining the correct results. Besides meeting required deadlines, a RTOS must also be able to respond predictably to unpredictable outside events and process multiple events concurrently. Embedded applications programmers use commercial RTOSs as a means of reducing the cost and time involved in development of their system. A POSIX standard exists for portability of real-time applications between RTOSs.

Background

Control of nuclear power plants, control of laboratory experiments, medical monitoring, space navigation and guidance, anti-lock breaking, air traffic control, and burglar alarms are all classified under a special set of systems described as real-time. In such systems, correctness depends not only on retrieving accurate data but also on retrieving it within a specified time frame. By definition, failure is said to occur if the results are returned later then the allotted time.

Real-time systems are not necessarily fast (fast is a relative term), however there is a bounded latency on the time required to complete certain tasks. Depending on the system, the latency may be only a few microseconds. A real-world example of a familiar real-time system is a cashier line in a grocery store. There is some upper limit on the time that a customer is willing to wait in line. Normally, customers can get through in a matter of minutes, but if that time became instead hours, customers would probably go elsewhere.

In general, real-time systems must be able to respond predictably to unpredictable events, meet completion deadlines of particular tasks, and process multiple tasks at once. There are two flavors of real-time systems, depending on the degree of failure if the system does not meet a deadline. In hard real-time systems, no lateness is accepted. Catastrophic failure, possibly the loss of human life, is possible if deadlines are not met (e.g., an aircraft control system). On the other hand, in soft real-time systems there is usually a rising cost associated with lateness (e.g., a vending machine). Failure is not catastrophic in this case, but more of an inconvenience where the inconvenience increases over time.

Overview & Description

A real-time operating system (RTOS) may be a stripped-down version of an operating system that is common on desktop PCs, with a few special characteristics. Typically, a RTOS provides the same functionality as a general-purpose operating system (GPOS), but eliminates advanced features that slow operation and are seldom used. RTOSs are generally preemptible (a process that has control of the processor must yield to an arriving process with higher priority, allowing the higher priority process to run) and able to support multi-threaded processes (the ability of separate sections of a process to run concurrently). In addition, there are five general attribute requirements associated with RTOSs, briefly described below.

Determinism is the ability of the operating system to perform a task in a determined amount of time. This attribute is probably most affected by the time required to acknowledge that an interrupt has occurred (i.e. interrupt latency). Responsiveness deals with how quickly the system can respond to an event. Interrupts here again are influential. The time after an interrupt has been acknowledged to handle the interrupt, execute it, and manage nested interrupts, plays a large role in the responsiveness of the system. User control in a RTOS is the ability of the user (the user writing applications for the RTOS, not the end-user) to set task and thread priorities. Reliability is the ability of the system to run for an extended period of time (maybe multiple years) without failure. Rebooting a GPOS is a common way to resolve a problem on a desktop computer. This simply is not acceptable for a real-time system. Finally, fail-soft operation is the ability of the operating system to keep running (with possibly a slight loss of performance) in spite of some internal corruption. Signaling an alarm, bringing a backup system on-line automatically, or gracefully shutting down the system (saving as much data as possible) are all more cost-effective alternatives than having the system just halt.

There are additional fundamental aspects of RTOS design. One is the ability of a particular process to communicate with another process (formally called interprocess communication or IPC). This allows separate applications to interact with each other and share the same data. The concept of IPC introduces another important operating system aspect, preventing a process from changing data while another process is using the data. This "mutual exclusion" of data is typically controlled in RTOSs by the use of semaphores (a variable that indicates the status of a shared resource). This method does not disable system interrupts which would hinder system responsiveness. Another important aspect of RTOSs is the preemptive scheduling of processes, scheduling such that the operating system can change the running process to another process at any given time. This allows higher priority tasks (normally those with the shortest deadline) to finish first.

Advantages

Real-time systems are typically embedded systems. The use of a commercial real-time operating system liberates the embedded applications programmer, saving the time and cost developing a hardware interface. The principle is the same as using a high-level language to write software, instead of assembly language. A possible tradeoff is some overhead in return for low development cost and effectiveness. However, certain industries (e.g., medical, avionics, etc.) may have standards that a commercial RTOS will not meet. In these cases, the embedded applications programmer can write only the hardware interface needed for the particular application, eliminating unnecessary operating system overhead. Another solution however emerges from the fact that many RTOSs are "scalable." This means applications programmers need use only the RTOS features required by the application, thus reducing the overall size and increasing the performance of the system.

Standards

POSIX is a standard for applications and operating systems that allows some assurance that applications conforming to the standard are able to be ported to operating systems that also conform to the standard. Originally, this standard did not cover attributes specifically relevant to real-time applications. Real-time extensions to the standard (POSIX 1003.1b) have enabled the appearance of POSIX-compliant real-time operating systems. More information can be found on the Internet at either the Portable Application Standards Committee (http://www.pasc.org/) or the official ISO POSIX site (http://anubis.dkuug.dk/JTC1/SC22/WG15/).

Commercial Products (listed alphabetically)

CMX-RTX - CMX Company
http://www.cmx.com/

CMX-RTX RTOS from CMX Company supports many 8, 16, and 32 bit embedded microcontrollers and microprocessors. The product provides over 65 kernel services allowing configuration of many operating system controls, including events, messages, resources, semaphores, cyclic timers, queues, fixed memory blocks, and UARTs. The vendor claims the operating system has fast context switch times, low interrupt latency and small ROM and RAM requirements for smaller applications. Support is included for automatic power down management and the ability to have cooperative or time sliced schedulers, in addition to preemption. CMX-RTX supports over 30 compiler versions. Source code is included and is royalty-free. Development hosts: Windows NT and Linux.

eCos - Cygnus Solutions
http://www.cygnus.com/

The source code for eCos, including all kernel components, Hardware Abstraction Layers (HAL), drivers, math and ISO C runtime libraries is available free for download from Cygnus Solutions. Two additional Cygnus products (which are not free), Source-Navigator and GNUPro Toolkit, are GUI based applications that provide simpler code browsing and the ability to build, debug, and configure eCos applications. eCos is targeted at high-volume embedded systems and was designed to reduce the cost and increase the quality of embedded system development. eCos supports simulation of embedded applications on the host, without the use of the target hardware. Among the features included are: support for nested interrupts, platform-independent HAL options, binary and counting semaphores, source-level debugging, alarm and cyclic handlers, and uITRON compatibility. Source code is included and is royalty-free. Development hosts: Windows NT, Windows 95, and Linux.

LynxOS - Lynx Real-Time Systems, Inc.
http://www.lynx.com/

LynxOS is a UNIX-compatible operating system designed for various levels of hard real-time applications. The preemptible RTOS kernel has 256 global user-thread priority levels; vendor claims fast context switching and short, deterministic blocking times. LynxOS has a modular design that allows the operating system to be highly scalable. For specialized embedded applications, the kernel only can be linked with an application. On the other hand, the full LynxOS consists, in part, of a development environment and has a graphical user interface. In addition, LynxOS supports various peripheral devices, networking protocols, and a wide range of microprocessors. Open POSIX and UNIX APIs. Development hosts: Sun Solaris, SunOS, RS6000, and LynxOS. POSIX-compliant (1003.1a, .1b, .1c).

NucleusPLUS - Accelerated Technologies Inc.
http://www.atinucleus.com/

NucleusPLUS is a small real-time kernel available for most processor families. The kernel is implemented as a C library, therefore only the portions used in a specific application are bound into the executable. In addition, applications written with NucleusPLUS are highly portable, as the kernel was written mostly in ANSI C. Features include task control, task communications, task synchronization, memory management, programmable timers, and dynamically created and deleted objects. Add-ons are available for Web, networking, and graphic support. Source code is included and is royalty-free. Development host: Windows NT.

QNX - QNX Software Systems Ltd.
http://www.qnx.com/

QNX is a scalable RTOS designed for hard real-time performance. Its modular design allows it to be used in small, embedded applications or to run a distributed network of several hundred processors. Modules can be independently loaded depending on applications‘ needs, and new modules can be created for extensibility. In part, QNX‘s modularity comes from that fact that operating system services run in their own protected address space. Access to these services is provided through message-passing interprocess communication mechanisms. Device drivers run in user space making the kernel more reliable. Additional features include nested interrupts, dynamically attachable and removable interrupt handlers, multiple timers per process, user-configurable system limits and resources, 32 priority levels, and preemptive, prioritized context switching. The vendor offers an add-on product named Photon microGUI, which is a complete embeddable windowing system that requires less than 500KB of flash memory or ROM. Development hosts: Windows NT, and QNX RTOS. POSIX-compliant (1003.1, 1003.1b).

VRTX - Mentor Graphics Corp.
http://www.mentor.com/

VRTX is a deterministic, preemptive multitasking, and scalable real-time operating system. Scalability is provided through a modular, library based architecture and the choice of two compatible, modular kernels. Separately the kernels are designed for use on microcontrollers with restrictive footprint requirements, or in an environment requiring a full-featured real-time operating system. Additional features include priority inheritance and preemptible system calls; additional modules support networking and file protocols. VRTX is supported by the VRTX Development System, which provides application simulation, debugging, and analysis. VRTX is FAA-certified. Development hosts: Windows NT, Sun Solaris, and HP-UX.

VxWorks - Wind River Systems, Inc.
http://www.wrs.com/

VxWorks is one of three components in the Tornado suite of tools from Wind River Systems. In addition to the VxWorks RTOS, Tornado includes development and debugging tools, and a range of communications software options (e.g., Ethernet, serial line, in-circuit/ROM emulator). VxWorks is scalable through the configuration of over 100 different options and the removal of individual functions from run-time libraries. Additional features include interrupt support, preemptive and round robin scheduling, network functionality, file system support, C++ support, 1800 application program interfaces (APIs), 256 priority levels, and incremental linking and loading of components. Interprocess communication is provided through the use of shared memory; message queues and pipes; binary, counting, and mutual exclusion semaphores. Development hosts: Windows NT, Windows 95, Sun Solaris, SunOS, and HP-UX. POSIX-compliant (1003.1b).

Useful Links

IEEE-CS Technical Committee on Real-Time Systems
http://cs-www.bu.edu/pub/ieee-rts/

Contains an archive of the committee‘s newsletter and links to numerous university and institute, research and special interest groups. The site houses information on conferences, workshops, books, and journals dealing with real-time systems, and as well, links to various research tools and commercial products.

Real-Time Concurrent Operating System Applications
http://www.dcs.shef.ac.uk/~prc/COM324-97/m6rfs/

A collection of RTOS Internet references compiled by a student from the University of Sheffield. Extracted articles and abstracts define real-time operating systems, show the difference between general-purpose operating systems, and contrast hard and soft systems.

Usenet FAQ - Real-time Computing
http://www.cis.ohio-state.edu/ hypertext/faq/usenet/realtime-computing/top.html

Contains definitions, publications, controversial questions, market information, and research resources on real-time systems. In addition there is an extensive list of real-time operating systems and company contacts. The FAQ is a helpful resource for both newcomers and experts in the field.

Real-Time Encyclopedia
http://www.realtime-info.be/

The home of Real-Time Magazine (subscription information available). Provides many real-time related resources including current press releases, publication information, and a large glossary of real-time system and general operating system (including RTOS) terminology. The site also contains a long list of commercial real-time operating systems with brief descriptions.

Real-Time and RTOS by EG3
http://www.cera2.com/navi/real.htm

Links to a wide variety of Internet resources on real-time and real-time operating systems.