.. _virt_mem:

Virtual Memory Management
=================================

.. index:: virtual memory

The :ref:`paged_mem` scheme gives rise to the notion of :term:`demand
paging` using :term:`virtual memory`.  The :ref:`virt_mem` system maintains
a copy of the memory for all programs on secondary storage, such as a hard
drive.  In fact, many pages for a process may only reside in virtual memory.
Loading only the page frames that are needed to run a program can make it
faster to load a program.  Some memory frames for a program may never be
needed while the program runs.


If a current copy of a frame of physical memory is held on disk in virtual
memory, then that frame may be removed from physical memory to free up
needed memory.  When a process references memory that is not loaded in
physical memory, the :ref:`mmu` of the CPU issues a :term:`page fault` trap.

------------------------

The tasks of the :ref:`virt_mem` system include:

#. Maintaining the virtual memory with a reasonably current copy of memory.

#. Keeping the page tables current.

#. Responding to page faults by loading the requested memory frames into
   physical memory.

------------------------

The performance of a virtual memory management system depends on the total
number of page faults, which depend on:

.. describe:: The page replacement algorithm

    Select which frames are to be replaced when a page fault occurs.
    (:ref:`page_replace`)

.. describe:: The frame allocation algorithm

    Decide how many frames to allocate to each process.
    (:ref:`frame_alloc`)


------------------------

**Contents:**

.. toctree::
   :maxdepth: 2

   not_swapped
   virt_address_space
   page_fault
   page_replace
   frame_alloc
   segmented