Question about UTCB

Volkmar Uhlig volkmar at
Fri Feb 24 00:22:35 CET 2006

> -----Original Message-----
> From: Jonathan S. Shapiro
> Sent: Thursday, February 23, 2006 5:34 PM
> To: Benno
> Cc: l4-hackers at
> Subject: Re: Question about UTCB
> On Fri, 2006-02-24 at 09:18 +1100, Benno wrote:
> > >When a process is preempted, where are its registers 
> > > stored? Are the stored in the UTCB, or are they stored 
> > > in some data structure that is only accessible to the 
> > > kernel?
> > 
> > They are stored in the thread's TCB, which is kernel private data.
> So, in particular, the saved registers are not accessable to 
> normal load and store instructions that might occur on a 
> second processor. Correct?

Actually, it depends.  At this point most implementations use the kernel
stack to store the user-level registers.  However, this is up to the
implementation and not architecturally defined.  Storing it in the UTCBs has
some important advantages.

However, for the SMP case registers should not be accessed on a remote
processor and typically there is no reason to do so.  Threads are pinned to
a particular CPU and migrated by application threads.  The synchronization
model in the kernel heavily depends on that assumption.

- Volkmar

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