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SubjectRe: Difference between an RTOS and a OS like Linux ...

On Mon, 17 Feb 1997, W. Reilly Cooley wrote:

> # Could somebody tell me the difference between an RTOS and a UN*X clone like
> # Linux .?
> # Why cant Linux be used to simulate an RTOS ? are there any .. ? What's the
> # important
> # thing about an RTOS? Can I get some help ... please ?
> #
> # vimalkumar.
> See the article in the February issue of "Linux Journal". The primary
> difference, as I understand it, is that the real-time software, which
> is collecting data or whatever, is guaranteed the resources it needs
> to operate in a timely, consistent manner. This is in contrast to the
> standard way in which the scheduler runs.
> Wil

The difference is subtle yet significant. :-)

A standard multitasking kernel, such as Linux, generally places no limits
on the amount of time that a syscall can take (in whatever appropriate
unit). On those calls that DO attempt to make some sort of approximated
"promise" about their execution time, they will work even if they take
slightly longer than "promised." On an RT kernel, a syscall will ALWAYS
*fail* if it takes longer than gauranteed. As you can see, the task of
designing and implementing an RT kernel is not considerably harder than a
normal OS, you just design a paradigm for failing any sort of resource
usage that falls outside of documented parameters. The real chore in RT
is designing *libraries* and *applications* that can actually deal with
the possibility of any arbitrary syscall failing simply because the
time/cycles constraint was broken.

I worked for about a year and a half on a RT nuclear reactor simulation
for a former employer of mine. Yuck, yuck, yuck (on top of having to code
in COBOL-2).

+ -- Finger: for my PGP public key -- +

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