Real Time Operating Systems – A view from the shore

The Real Time Operating System from a common man’s perspective.

Computing systems can broadly be classified in to Generic computing systems, Embedded computing systems and High performance computing systems. The line  between generic computing systems such as desktop computing systems and the embedded computing systems is getting blurred day by day.

An operating system functions to be the mind of a given computing system. It creates and manages jobs, it deals with allocating and managing resources and much more. Often operating systems come bundled with user interfaces and programmer friendly libraries.

While desktop operating systems can be complex monsters, embedded operating systems usually tend to be much simpler and lighter. As modern embedded systems became much more powerful than earlier general purpose computing systems, the desktop operating systems such as Windows and Linux are finding their way in to embedded computing space, mostly in their miniature forms.

The embedded operating systems can be classified between real-time (RTOS) and non real-time systems. The non real-time systems can further be divided in to two further classes, round-robbin simple kernels and general purpose OS (GPOS) for embedded systems.

The RTOS is the key component in implementing systems with predictable responses. Imagine a mobile processing system, which has a very limited resources such as slow processor and often little RAM and storage facilities. Now, if one does not have a way to implement a predictable behavior software on top of the hardware, the resources are pretty small compred to the job they have to do: process the call data, process the control information, play a song or shoot a picture.

However, RTOS has it’s own set of limitations. First, RTOS cannot, at least at the heart, enforce security such as user/kernel space. Hence, typical RTOS systems cannot run user programs or third party applications. Though there are some exceptions, they run at low priority and consume extra resources. In either case, the application space is limited. Let’s look at that in some other post.

Besides, adding any more tasks can easily change the predictable behavior or the overall system. However, in most cases, the impacted are only the tasks below the new task’s priority. However, since the system is designed to be specific purpose system, complex interdependancies change their equations changing the overall behavior of the system.

On the other hand, a General Purpose Operating System or GPOS, does not have these or other such limitations, which RTOS has. However, GPOS cannot guarantee a predictable execution environment with constrained resources.

Let’s consider the case of a slower PC with Windows or Linux operating system. If you open a file or open the browser while you’re playing a video or even a song in the background, the video or the song will be interrupted for brief durations. In order to be able to run such multiple tasks smooth, the PC should have minimum resources, while a mobile can easily handle such tasks with even minimal resources.

The advantage with embedded GPOS is that it can allow user to build and deploy third party applications without worrying about security or other such issues. That might be the convincing reason behind mulitple processors, often, found in the smart phones.

Let’s take a quick look at RTOS components in the next post.

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