Re: EMC C Code

paul@... wrote:

> > From: "Robert N. Ash" <esccmail@...>
> >
> > Thanks
> >
> > My misunderstanding as to the real meaning of "code portability of C and
> > C++". JAVA version would be a platform independent software maybe but still
> > need a RT OS???
>
> The Java program did not do real time machine control.
>
>
> This does not work for something like EMC which relies on real time
> scheduling support from the OS. This is not something that can be
> grafted on later, but must be part of the running OS.

Well, not exactly. The real time patch for Linux isn't really part of the
operating system. the only OS feature that it really uses is the ability
to have loadable modules which run at kernel priority. This is mostly
used for loadable device drivers. What the real time code does is
build an entire new real-time environment underneath the operating
system, and allow the entire Linux system to run as one of the
real-time processes, although the lowest on the priority list.
Other real-time processes can be created, but they run pretty much
OUTSIDE Linux. They have no ability to allocate/free memory,
do I/O, or any of the things Linux programs usually are able to
do. All communication with the rest of Linux and user programs
is through a shared memory area which is created by telling
the Linux boot loader program LILO to leave a chunk of memory
unused at the top of the system memory. The real time patch
has functions to create memory windows and fifos in this
area.

So, in fact, the real time feature WAS grafted on later to Linux.
It was only possible, of course, because the source to the Linux
kernel is available to anyone who wants to read it (or change
it). The real time extensions to Windows NT was done in a
very similar way, but Microsoft charged the outfit that created
it a bunch to see what they needed to change, and they then
have to charge the customer to cover this cost.

Jon