Lpc2000

Robert,

I think you are mixing privilege with protection.  I refer to
privilege as a policy without reference to mechanisms to enforce this
policy.

Whether or not you need protection to enforce a particular policy
depends on requirements:

* In the context of general purpose operating systems (like that on a
desktop), the requirement is that policy be enforced assuming the
tasks are hostile.

* In the context of embedded systems, the OS (or kernel) assumes that
the tasks are cooperative -- that they are making a kernel call not
because they do not have the means to do this on their own.

Just because a task can do anything it does not follow that having a
task do whatever it can do without reference to other tasks (current
and in the future) is a good idea.

For the above reason I do not find your argument relevant to the
original proposition: to recognise that even in embedded systems, the
notion of user vs system can be exploited to make the system easier to
describe and manage.

Jaya

--- In lpc2000@yahoogroups.com, Robert Adsett <subscriptions@...> wrote:
>
> At 04:29 AM 4/19/2006 +0000, jayasooriah wrote:
> >--- In lpc2000@yahoogroups.com, Robert Adsett <subscriptions@> >
> >If this were true, the notion of privilege or levels of privilage in
> >operating systems would meaningless.
> 
> In those systems the privilege levels are supported by HW which
restricts 
> access.  Sometimes as 'simple' as a MMU but often with restrictions
on I/O 
> access as well.  Sometimes with restrictions on the way pointers to
memory 
> can be passed between privilege levels.
> 
> W/O this hardware support, which is not present  on the ARM 
> microcontrollers we are talking about, there is indeed no protection 
> offered.