RTOSes range in their functionality, as well as the architecture and size of the processor they're best suited to. On the smaller side, we have smaller 8–32-bit MCU-focused RTOSes, such as FreeRTOS, Keil RTX, Micrium µC, ThreadX, and many more.  This class of RTOS is suitable for use on microcontrollers and provides a compact real-time kernel as the most basic offering. When moving from MCUs to 32- and 64-bit application processors, you'll tend to find RTOSes such as Wind River VxWorks and Wind River Linux, Green Hills' Integrity OS, and even Linux with PREEMPT_RT kernel extensions. These full-blown OSes offer a large selection of software, providing solutions for both real-time scheduling requirements as well as general computing tasks. Even with the OSes we've just rattled off, we've only scratched the surface of what's available. There are free and paid solutions (some costing well over USD$10,000) at all levels of RTOSes, big and small.

So, why would you choose to pay for a solution when there is something available for free? The main differentiating factors between freely available RTOS solutions and paid solutions are safety approvals, middleware, and customer support.  Because RTOSes provide a highly deterministic execution environment, they are often used in complex safety-critical applications. By safety critical, we generally mean a system whose failure could harm people or cause significant damage. These systems require deterministic operation because they must behave in a predictable way all the time. Guaranteeing the code responds to events within a fixed amount of time is a significant step toward ensuring they behave consistently. Most of these safety-critical applications are regulated and have their own sets of governing bodies and standards, such as DO-178B and DO-178C for aircraft or IEC 61508 SIL 3 and ISO 26262 ASILD for industrial applications. To make safety-critical certifications more affordable, designers will typically either keep code for these systems extremely simple (so it is possible to prove mathematically that the system will function consistently and nothing can go wrong) or turn to a commercial RTOS solution, which has been through certification already, as a starting point. WITTENSTEIN SafeRTOS is a derivative of FreeRTOS that carries approvals for industrial, medical, and automotive use.

Middleware can also be an extremely important component in complex systems. Middleware is code that runs between the user code (code that you, the application programmer, write) and lower layers, such as the RTOS or bare metal (no RTOS). Another value proposition of paid solutions is that the ecosystem offers a suite of pre-integrated high-quality middleware (such as filesystems, networking stacks, GUI frameworks, industrial protocols, and so on) that minimizes development and reduces overall project risk. The reason for using middleware, rather than rolling your own, is to reduce the amount of original code being written by an in-house development team. This reduces both the risk and the total time spent by the team—so it can be a worth-while investment, depending on factors such as project complexity and schedule requirements.

Paid solutions will also typically come with some level of customer support directly from the firmware vendor. Engineers are expensive to hire and keep on staff. There's nothing a manger dreads more than walking into a room full of engineers who are puzzling over their tools, rather than working on the real problems that need to be solved. Having expert help that is an email or phone call away can increase a team's productivity dramatically, which leads to a shorter turnaround and a happier workplace for everyone.

FreeRTOS has both paid support and training options, as well as paid middleware solutions, that can be integrated. However, there are also open source and/or freely available middleware components available, some of which will be discussed in this book.