Real-time operating systems have evolved over the years from
being simple executives using cyclic scheduling to the current
feature-rich operating environments. The standardization of POSIX
1003.1, ISO/IEC 9945-1 (real-time extensions to POSIX) has contributed
significantly to this evolution, however, the specification leaves plenty of
room for individual implementations to both interpret and
specialize their RTOSs. Accordingly, there has been a
proliferation of both commercial and free RTOSs, notably, the
$\mu$ITRON OS specification that is an industry standard among
Japanese embedded systems developers, the OSEK-VDX OS
specification that is has been adopted by the automobile industry
in Europe, popular commercial RTOSs like VxWorks, VRTX, LynxOS
and QNX, and free RTOSs like RT-Linux (RTAI) and Windows CE.
The goal of the work reported in this paper is to draw the real-time systems practitioner and researcher's attention to these choices and
bring out the similarities and differences among them. The paper notes
that these RTOSs are more alike than different in the
functionality they provide with some notable differences in
performance. Work is underway to, install, test and benchmark the
aforementioned OSs to draw a more objective assessment.
