How does context switching work?

[Richi Dubey]

Thanks a lot for your reply.

But that code is okay because the floating point unit is not used during
> that sequence. it is untouched and up to a point it doesn't matter when the
> floating point unit is restored as long as it's before the thread needs it.
> And RTEMS should not be using floating point operations except in very
> special places in the code base.
> not sure that helps but the floating point unit and the integer unit are
> separate and most of the time can be contact switched separately completely
> safely.

I understand. This is smart.

The _Context_Switch() function changes the thread stack and the
> non-volatile (callee save) registers. So, the value of the executing
> local variable changes with a call to _Context_Switch().

I understand. Thank you.

On Mon, Nov 2, 2020 at 11:48 AM Sebastian Huber <
sebastian.huber at embedded-brains.de> wrote:

> On 31/10/2020 16:50, Richi Dubey wrote:
>
> >
> > I want to learn more about how context switching works in RTEMS. I saw
> > the following lines in theaddispatch.c:
> >
> >     _Thread_Save_fp( executing );
> >     _Context_Switch( &executing->Registers, &heir->Registers );
> >     _Thread_Restore_fp( executing );
> >
> > I do not understand how it works. Here, the executing process saves
> > its context by calling _Thread_Save_fp( executing ), then if a
> > different process resumes execution after the context switch, why does
> > it get the context of previous process (executing)?
> The _Context_Switch() function changes the thread stack and the
> non-volatile (callee save) registers. So, the value of the executing
> local variable changes with a call to _Context_Switch().
>
> --
> Sebastian Huber, embedded brains GmbH
>
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>
>
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