task switching and current pointer implemenation question

[subin gangadharan]

Hi Guys,

I have a question related to task switching.Basically when user space
application switches to kernel (through sys calls),
which stack will be used for the user app.Mainly I want to know how
the current pointer implementation works.
I saw that in arm they are masking the sp with ~(THREAD_SIZE -1) and
getting the current thread info and from there its
taking the task pointer.

So here I am a little confused about the stack pointer since when it
switches the kernel it will be supervisory mode,so which
sp it's referencing.I know it will be a supervisory stack,but how this
stack will be related with the user app.

If anybody could give a pointer on this,it would be of great help to me.

-- 
With Regards
Subin Gangadharan

I am not afraid and I am also not afraid of being afraid.