Re: Questions about copy-on-write

[Neal H. Walfield]

At Wed, 27 Oct 2004 21:33:09 +0100,
Sam Mason wrote:
> 
> Neal H. Walfield wrote:
> >physmem holds mappings
> >between virtual frame (per-task handles for physical memory blocks)
> >and physical frames.
> 
> Is this where most of the memory protections stem from?  Tasks can't
> ask for arbitrary pages of memory but they can ask physmem for a new
> (otherwise unused) page of memory or they can ask for a specific page
> of memory out of a container.

physmem deals is what we call virtual frames.  These are different
from normal frames in that they provide a level of indirection:
normally, frames are addressed by their physical address.  If we were
to do this, it would make rearranging physical rather difficult.

pages need not be in memory (they may be swapped out).  Tasks provide
the mapping between pages and frames.  (And pages may float between
different frames as no frame is, in general, better than any other.)

With respect to memory protection, L4 provides the mechanisms to limit
access to read-only or read/write, etc.  Is that what you mean?