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 within well-defined time constraints. Typically, a real-time OS responds to inputs in microseconds. Computing devices with less than 1 MB of memory need specific features in a tiny operating system called an RTOS.

In this section, you’ll learn about what an RTOS does, what makes an RTOS different from other operating systems, three RTOS types, and two RTOS architectures.

What an RTOS OS does

An RTOS operating system manages processor cycles, memory, peripherals, and interrupts. Embedded developers use an RTOS when an application’s real-time requirements or memory constraints make unsuitable a general purpose operating system, like Linux or Windows.

Difference between an RTOS and other OS?

Determinism, small footprint, quick response, concurrent jobs, safety, and security are key aspects of an RTOS.

RTOS Types

RTOS come in three types based on levels of responsiveness: hard, firm, and soft.

RTOS Architectures

Two categories of RTOS architectures are monolithic kernel and microkernel.

RTOS in Embedded Systems

An embedded RTOS runs applications, enables them to interoperate in the device, and communicate with the world.

In this section, you’ll learn about seven popular RTOSes used by embedded software developers as well as typical RTOS features.

RTOS Examples

Examples of open-source and commercial RTOS include:

RTOS Features

In this section, you’ll learn about six RTOS features to consider for your next embedded project: determinism, spatial and temporal separation, scheduling, preemptive and cooperative RTOS, memory management, and interprocess communications.

Deterministic RTOS

An RTOS that guarantees a task runs in a specified amount of time is called deterministic. Some RTOSes may guarantee determinism 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, allows tasks to run in their own private memory spaces. Temporal separation allows tasks to meet timing requirements without interference.

RTOS Scheduling

A scheduling algorithm decides the order in which tasks are run on processing units, so they can each meet their time constraints. A thread is a unit of work that can run concurrently with other threads. RTOS schedulers use priority, absolute and relative deadlines, and state to manage threads:

Preemptive or Cooperative Multitasking
Preemptive and cooperative multitasking are two methods an RTOS uses to execute multiple tasks or processes at once. A preemptive RTOS may stop the execution of a task to allocate the CPU to another process. By contrast, in cooperative multitasking, only the task itself can yield control.

RTOS Memory Management

An RTOS divides physical memory into sections, called stack and heap, to share it. The stack stores a function's call information and its local variables, which requires careful management. The heap allocates dynamic memory for global information for sharing. To manage memory effectively, an RTOS uses techniques, such as first-fit and best-fit.

Interprocess Communication (IPC)

Interprocess communication enables processes to synchronize and communicate with each other, such as passing messages and sharing memory. An RTOS enables this communication through queues, semaphores, and mutexes.

RTOS Best Practices

In this section, you’ll learn about POSIX compliance, security, and functional safety certification, which are three RTOS development best practices.

POSIX Compliance

POSIX pthreads are an industry-standard set of APIs for multithreading. This includes threads, semaphores, mutexes, condition variables, signals, timers. An RTOS supporting the POSIX threads (pthreads) API reduces developer training and effort, as most Linux distributions are POSIX-compliant.

Any application using a POSIX-compliant RTOS call can be ported to another compliant RTOS with minimal effort.

Examples below show one standards-based API call to create a semaphore, followed by three propriety API calls, each different:

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);

RTOS Security

RTOS security requires spatial separation between tasks to prevent a compromised component from affecting other components.

As a secure RTOS, PX5 RTOS provides security-specific enhancements for memory corruption detection and mitigation. Known as pointer data verification (PDV), this feature verifies all function pointers before calling them and verifies the return address on the stack. In addition, data objects and memory pools can be verified to prevent most sequential corruption. Central error processing is called when corruption is detected.

Safety-Certified RTOS

Embedded devices are often used in safety-critical situations that could cause human injury or property damage if they malfunction. Examples of safety-critical systems include medical, industrial, and automotive applications. Proving functional safety in embedded devices requires a fully tested and verified RTOS as well as certifications, such as compliance with the IEC 61508 international functional safety standard for electronic and programmable systems.

A safety-certified RTOS, also called a pre-certified RTOS, complies with one or more of the functional safety standards, such as IEC 61508 (industrial) or ISO 26262 (automotive). A safety-certified RTOS meets industry best practices. A safety-certified RTOS helps developers achieve 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 the certification artifacts provided by PX5 to save time and money when they safety-certify their application.

How to Choose the Best RTOS

Your choice of RTOS affects embedded system performance, development time, regulatory compliance, and total cost. The overhead an RTOS imposes on the processor and response time of RTOS functions affect system performance, such as:

Several more RTOS selection criteria include:

Bare Metal vs. Open Source vs. Commercial RTOS

When deciding whether to use a bare metal, open source RTOS, or commercial RTOS, consider the following:

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 RTOS cybersecurity and functional safety in these ways: