Re: Is FREEDOS a real time OS?

Previously Allan Adler <ara@xxxxxxxxxxxxxxxxxxxx> wrote:
Arno Wagner <me@xxxxxxxxxxx> writes:

No. A Realtime OS gives you certain assureances with regard
to responsiveness. Depending on the OS these can be harder
or softer and they can be different for different operations.
Unless an OS is explicitely labeles as "realtime", it is not.
Even then, it may not be what you need.

One realtime version of Linux, that I have seen, allows you to
run some realtime tasks, while the rest of the system just
has the normal characteristics. Also kernel drivers can
have realtime properties, by locking everything else until
something has happened. Of course this time should be short,
if the rest of the system should stay responsive.

QNX comes to mind as a proper realtime OS. It has a pretty
similar API compared to Linux, except for the realtime
extensions.

I didn't know that Linux had an API.

Every OS has an API. It is the thing that lets you make system
calls ;-)

I just checked the website for qnx.
Apparently it needs a proprietary software to run, which has a 30 day
free demo.

Before you decide: Why do you need realtime? What kind of
realtime? Note that "realtime" does not mean "fast" or
"synchronous". It just means that some things cannot block
or take longer than a specific time.

As I mentioned I have a very old PC which has both a RH 7.3 partition
and FREEDOS partition. There is absolutely nothing on the PC of any
importance. I just want to use it to learn to interface to electronics
projects through various ports, including the parallel port. The first
thing I do is likely to be to turn an LED off and on. That obviously
doesn't require real time capabilities. But I would like to learn how
to handle projects that do involve real time stuff. I'm aware that there
are free systems such as FreeRTOS, which can be downloaded to various
single board computers so that they, at least, will perform in real time.
But it would be nice if an OS could be installed on the PC which will
also perform in real time, particularly if it interacts with a device
that requires it, whether that device be a SBC running FreeRTOS or
a stepper motor or a robot. I don't mind reformatting the HD and
repartitioning it so that I can add a new partition for the RT system
in question. Or maybe the RT system can be CD based or, probably even
better, live in a RAM disk.

Then I would recommend looking at the different RT extensions for Linux
and using one of them.

I'm aware of RTLinux. Maybe that is what I should get, but I have very
bad luck installing any form of Linux other than what is already on
there. There is only a couple of GB of HD and only about 32 MB RAM.
Even CD based live Linuxes are too big for it.

Hmm. My Linux installation is about 4GB, excluding some data storage
areas. Maybe install something that can be made minimialistic, such
as Debian, and not use the GUI (Xorg), but stay on the commandline.
You still get 6 virtual consoles, which is enough, once you have
gotten used to it.

Also, I have even worse
luck trying to do any kind of interfacing with Linux.

I see.

FREEDOS at least
has the advantage that I can run programs in GWBASIC or assembly language
without being overruled by the operating system.

Hehe. You do know that GWBASIC is not a real-time language, since it
has a garbage-collector?

Once I've done that,
and proved to myself that I really can communicate with an LED, I'll
be more receptive to trying to get Linux to do it.

Well, here is some code to control the serial and paralell lines
under Linux. Should be working with current Linuxes as well:
http://www.woschrader.de/sboxlinux.html

But ultimately I'd
like to be able to use the PC in real time, so I would like to have
at least some part of the machine, whether in RAM or in a partition
of the HD, which is devoted to a real time system, assuming I can
figure out how to use it. Presently I have LILO set up so that I have
a choice at boot time whether to boot Linux or FREEDOS. I would want
to modify that so that I also had a choice to boot the real time system.

So, what do I mean by real-time? Probably I mean that it is deterministic
in its timing and that I can reliably predict when things will get done.

You do not get that, unless you write it yourself. The only thing
an RTOS gives you is maximum reaction times on certain operations.
You do not get any minimum or exact times. However if you only need
a degree of precision that is less than the reaction time the RTOS
gives you, then you can emulate determinism.

The PC is a really old clunker, but even so it may be more powerful than
some of the available SBC's that FreeRTOS is intended to run on. So, why
not use the PC itself as a controller?

Indeed.

Arno
.

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