VDC do an annual survey of the embedded system market which would no doubt answer your question; you have to pay for the full report, but you can get the executive brief for free if you register.
From other sources:
In 2006 of the commercial RTOS vendors the following led:
- VxWorks
- XP Embedded
- Windows CE
- DSP/BIOS
- Red Hat Linux
Now, these are from survey responses, and some of these I would not consider Real-Time, so if you weed out the non-real-time OSs, I would say:
- VxWorks
- Windows CE
- DSP/BIOS
- QNX
- RTX
But survey response is not divided by platform type; what is appropriate to an 8 bit system, or a deeply embedded system with no need for file-systems, networking, or display etc. are very different. And if you have never used a TI DSP, you'd never have even encountered DSP/BIOS before. So the question itself is probably too simplistic to answer, since to get meaningful results you probably have to specify the target platform and application complexity.
The survey also does not consider non-commercial RTOS such as FreeRTOS, RTEMS, and eCOS. These are very worthy of consideration and in some cases of superior quality to some commercial systems. In 2009 for example FreeRTOS was downloaded more than 77500 times, and eCOS has extensive support for file-systems and networking etc. Of course no one can tell how many such RTOSes are used in real products or simply by hobbyists or even downloaded and never used.
Another question is whether you are interested in volume of product or number of distinct projects? For example WidgetA may incorporate say Keil RTX bundled free with its ARM-MDK, and sell in millions, while PABX-Exchange-B, may use VxWorks costing tens of thousands in royalties and development licences, but sell in the few tens. The comparison is pointless.
