.. mode: -*- rst -*-

VM for Digital Unix
===================

:Tag: design.mps.vmo1
:Author: David Jones
:Date: 1997-03-25
:Status: incomplete document
:Revision: $Id: //info.ravenbrook.com/project/mps/version/1.116/design/vmo1.txt#1 $
:Copyright: See `Copyright and License`_.
:Index terms:   pair: VM for Digital Unix; design


.. warning::

    As of 2013-05-26, the MPS is no longer supported on Digital Unix,
    so this document is only of historical interest.


Introduction
------------

_`.readership`: Any MPS developer.

_`.intro`: This is the design of the VM Module for Digital UNIX (also
known as OSF/1 and Tru64 Unix; see os.o1). In general aspects
(including interface) the design is as for design.mps.vm_.

.. _design.mps.vm: vm


Functions
---------

_`.fun.unmap`: ``VMUnmap()`` "unmaps" a region by replacing the
existing mapping with a mapping using the ``vm->none_fd`` file
descriptor (see mumble mumble, ``VMCreate()``), and protection set to
``PROT_NONE`` (that is, no access).

_`.fun.unmap.justify`: Replacing the mapping in this way means that
the address space is still reserved and will not be used by calls to
``mmap()`` (perhaps in other libraries) which specify
``MAP_VARIABLE``.

_`.fun.unmap.offset`: The offset for this mapping is the offset of the
region being unmapped in the VM; this gives the same effect as if
there was one mapping of the ``vm->none_fd`` from the base to the
limit of the VM (but "behind" all the other mappings that have been
created).

_`.fun.unmap.offset.justify`: If this is not done (if for example the
offset is always specified as 0) then the VM will cause the kernel to
create a new file reference for each mapping created with
``VMUnmap()``; eventually the kernel refuses the ``mmap()`` call because it
can't create a new file reference.


Document History
----------------

- 1997-03-25 David Jones. Incomplete document.

- 2002-06-07 RB_ Converted from MMInfo database design document.

- 2013-05-26 GDR_ Converted to reStructuredText.

.. _RB: http://www.ravenbrook.com/consultants/rb/
.. _GDR: http://www.ravenbrook.com/consultants/gdr/


Copyright and License
---------------------

Copyright © 2013-2014 Ravenbrook Limited <http://www.ravenbrook.com/>.
All rights reserved. This is an open source license. Contact
Ravenbrook for commercial licensing options.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

#. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.

#. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.

#. Redistributions in any form must be accompanied by information on how
   to obtain complete source code for this software and any
   accompanying software that uses this software.  The source code must
   either be included in the distribution or be available for no more than
   the cost of distribution plus a nominal fee, and must be freely
   redistributable under reasonable conditions.  For an executable file,
   complete source code means the source code for all modules it contains.
   It does not include source code for modules or files that typically
   accompany the major components of the operating system on which the
   executable file runs.

**This software is provided by the copyright holders and contributors
"as is" and any express or implied warranties, including, but not
limited to, the implied warranties of merchantability, fitness for a
particular purpose, or non-infringement, are disclaimed.  In no event
shall the copyright holders and contributors be liable for any direct,
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(including, but not limited to, procurement of substitute goods or
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