Since the definition of an RTOS varies on the application, usually a computer pretending to be something much simpler, RISC OS is an RTOS for the medium-complex applications, and isn't necessarily for high-complex ones, although a highly-complex RTOS sounds like a contradiction in terms. The example by Mahmoud Almostafa RABBAH refers to no OS, and running a single-tasking program straight from the boot-loader, which isn't an RTOS either.
The only way reasonable way of making sense of this is to split the RTOS definition into three levels:
Low-complexity would be something like a washing machine or a data
logger, and you're probably better off with simpler hardware eg
Arduino or maybe a simpler MCU, or even just sequential logic, in the
first place. It'll consume less power, and there'd be a lot less to
worry about: Never make things more complicated than they have to be.
High-complexity would be something like a full multi-tasking system,
which a RTOS isn't. It'd probably be better to run your GUI on a
separate device, if you want that. High-complexity could also be
monitoring processes that call other processes, and some must be
prioritised, but again you're better off with some kind of parallel
processing there, or it fails the capability of responding in
Medium-complexity would be where you need the interfaces a normal OS
can provide, eg USB, and perhaps a small display output, but you want
to process a stream of data and not be interrupted by anything. This
sounds like the level of an automotive application.
For that, you
could compile something without an OS, using a host machine to
develop it, or you could use the version of RISC OS which boots
directly into BASIC and develop on the target machine, which is
That will run a single task which can be fast enough
to poll for a number of events, without being interrupted by other
things. Hardware interrupts would still run unless they're disabled
(fairly easy to do), and those are necessary in order to make the
display/USB etc work. Other hardware interrupts run timers and IO you
may not be using.
Another advantage of RISC OS in RTOS applications, is you can only use the modules you require, something that makes no sense in traditional GUI applications, and had been used by eg STD/AdvantageSix although they use the term "embedded systems" instead of "RTOS". The advantages this brings are simplified design, lower power requirements, lower memory usage, and faster boot times (some I/O device interfaces require mini booting themselves, and the OS has to participate in this, although the timescales are usually too short to notice).
I hope that both fills in some gaps in the info above, and makes clear the gaps in my own knowledge.
(Other examples from memory, are no longer available online.)