What Is a Real-Time Operating System (RTOS)?

What Is an RTOS?

A Real-Time Operating System (RTOS) is a software platform for real-time computing applications that process events and data with well-defined time constraints. Unlike general-purpose operating systems (GPOS), an RTOS must schedule processing and data sharing between applications on limited hardware resources. Typically, these applications respond to inputs within microseconds and run on small embedded devices with less than 1 MB of memory. In this section, you’ll learn more about what an RTOS is as well as RTOS types and architectures.

RTOS Definition

An RTOS manages processor resources, including processor cycles, memory, peripherals, and interrupts. Embedded developers typically use an RTOS in situations where the application’s real-time requirements or memory constraints make a desktop operating system unsuitable.

5 Characteristics of an RTOS

RTOS Types

There are three types of RTOSes as classified by levels of responsiveness. They are used in different types of embedded systems:

RTOS Architectures

RTOS architectures tend to fall into two categories: monolithic kernel and microkernel. A monolithic kernel RTOS houses all OS components and operating processes within the same memory space, which speeds up operations. A microkernel RTOS houses core functions separately from application-specific functions, which fosters modularity, fault tolerance, and security.

RTOS in Embedded Systems

An embedded RTOS is the underlying software mechanism by which embedded applications run, interoperate with each other, and communicate with the outside world. In this section, you’ll learn about popular RTOSes that embedded software developers use and the embedded hardware platforms they run on.

Most Popular RTOSes

There are many choices between popular open-source and commercial RTOS suppliers:

What to Expect in an RTOS

An RTOS uses various mechanisms to manage tasks, events, and memory to ensure time constraints are met in embedded systems. In this section, you’ll learn about RTOS features to consider when building your embedded applications, such as determinism, spatial and temporal separation, scheduling, memory management, and interprocess communications.

Deterministic RTOS

An RTOS that guarantees a task runs within a specified amount of time is called deterministic. Depending on the RTOS, determinism may be guaranteed even with unexpected inputs. PX5 RTOS is an example of a deterministic RTOS with submicrosecond context switching and API calls.

Spatial vs. Temporal Separation

Spatial separation, or spatial isolation, refers to tasks running in their own private memory space. Temporal separation ensures tasks run independently in their own time. These techniques protect memory and other resources by preventing two or more tasks from accessing the same memory space simultaneously.

RTOS Scheduling

An RTOS employs a scheduling algorithm to determine the order in which tasks are run on processing units, which ensures they meet their time constraints. The basic unit of concurrency in a real-time system is a thread, and RTOS schedulers use three parameters to schedule threads:

Preemptive RTOS and Cooperative RTOS
Preemptive and cooperative multitasking are two methods an RTOS uses to execute multiple tasks or processes simultaneously. A preemptive RTOS allows the execution of the currently running task to stop while the RTOS allocates the CPU to another process. In cooperative multitasking, only the task itself can yield control to another task, such as when it has completed its job or is blocked.

RTOS Memory Management

An RTOS divides physical memory into different sections, such as stack and heap. The stack stores a function's call information and its local variables, which requires careful management to avoid out-of-bounds memory errors. The heap allocates dynamic memory for global information and data sharing between parts of an application.

To manage memory effectively, an RTOS uses techniques, such as first-fit and best-fit, to allocate segments without overwriting other segments.

Interprocess Communication (IPC)

Interprocess communication is the mechanism by which an RTOS allows processes to synchronize and communicate with each other. Two common examples of IPC are passing messages between processes and sharing memory between processes.

Three common communication types that RTOS are:

RTOS Best Practices

When writing applications on an RTOS, there are additional considerations. Here, you’ll learn RTOS development best practices, such as POSIX compliance, security, and functional safety certification.

POSIX RTOS Compliance

An RTOS supporting the POSIX threads (pthreads) API reduces developer training and effort, as POSIX is a well-known industry standard. Using the pthreads API allows embedded developers to port code to other POSIX-compliant platforms, such as embedded Linux, which helps to protect your investment in software.

Here are examples of standards-based API calls and proprietary API calls used by different RTOSes to create a semaphore:

Standards-based IEEE POSIX API used to create a semaphore (including PX5 RTOS)

int sem_init(sem_t* semaphore_handle, int pshared, unsigned int value);

RTOSes that use proprietary APIs to create a semaphore

ThreadX RTOS API

UINT tx_semaphore_create(TX_SEMAPHORE* semaphore_handle, CHAR* name, ULONG initial_count);

FreeRTOS API

SemaphoreHandle_t xSemaphoreCreateCounting(UBaseType_t uxMaxCount, UBaseType_t unInitialCount);

Zephyr RTOS API

int k_sem_init(struct k_sem* sem, unsigned int initial_count, unsigned int limit);

By contrast, any application using a POSIX-compliant RTOS call can be ported to another compliant RTOS with minimal effort.

RTOS Security

The key aspects of RTOS security are maintaining spatial separation between tasks, so a compromised component does not impact other components in an embedded system, and protecting the system from external threats.

An advanced RTOS like PX5 RTOS provides security-specific enhancements for memory corruption detection and mitigation. Known as pointer data verification (PDV), this feature checks function pointers, system objects, global data, and memory pools for corruption to prevent remote execution attacks.

Safety-Certified RTOS

A safety-certified RTOS or pre-certified RTOS complies with one or more functional safety standards, such as IEC 61508 or ISO 26262. For developers of safety-critical and non-safety-critical devices, a safety-certified RTOS offers a stable, reliable foundation built to industry best practices, which supports greater product reliability, security, and quality as well as faster time to market.

As an example, PX5 RTOS is pre-certified to the highest levels of the IEC 61508, IEC 62304, ISO 26262 and EN 50128 functional safety standards. Embedded software developers who use PX5 RTOS can re-use its certification artifacts to save time and money during application certification.

How to Choose the Best Embedded RTOS

Your choice of RTOS affects embedded system performance, development time, regulatory compliance, and total cost. The main selection criteria for an RTOS are usually the overhead it imposes on the processor and response time of RTOS functions, such as:

On the best RTOSes, context-switching and API call overhead is less than a few microseconds. In situations where greater responsiveness is required, an ultrasmall high-performance RTOS, such as PX5 RTOS, is used to minimize overhead.

RTOS Selection Criteria

Other RTOS selection considerations are:

Bare Metal vs. Open Source vs. Commercial RTOS

When deciding whether to use a bare metal, open source RTOS, or commercial RTOS, there are several considerations:

How Can PX5 RTOS Help You?

PX5 RTOS is one of the smallest, highest performance RTOSes ever developed. Ideal for resource-constrained devices, PX5 RTOS delivers sub-microsecond context switching and API calls, and it requires less than 1 KB of flash memory to run. Designed as a native implementation of the POSIX pthreads API standard, PX5 RTOS enables code portability with embedded Linux and other POSIX RTOSes.

Secure and Safety-Certified RTOS

PX5 RTOS helps embedded developers meet system requirements for cybersecurity and functional safety in these ways: