.. _mem_hw:

###################################
Memory Management Hardware
###################################

Memory Hierarchy
=================================

.. index:: cache

Computers have several different types of memory.  This memory is often
viewed as a hierarchy as shown below.

.. figure:: memory_hierarchy.png
   :align: center

Our main concern here will be the computer's main or RAM memory.  The cache
memory is important because it boost's the speed of accessing memory, but it
is managed entirely by the hardware.  The rotating magnetic memory or disk
memory is used by the :ref:`virt_mem`.


.. _mmu:

Memory Management Unit
=================================

.. index:: Memory Management Unit

As a program runs, the memory addresses that it uses to reference its data is
the :term:`logical address`.  The real time translation to the
:term:`physical address` is performed in hardware by the CPU's :ref:`mmu`
(MMU).  The MMU has two special registers that are accessed by the CPU's
control unit.  A data to be sent to main memory or retrieved from memory is
stored in the `Memory Data Register` (MDR).  The desired logical memory
address is stored in the `Memory Address Register` (MAR).  The address
translation is also called address binding and uses a memory map that is
programmed by the operating system.  

.. note::

    The job of the operating system is to
    load the appropriate data into the MMU when a processes is started and to
    respond to the occasional :ref:`page_fault` by loading the needed memory and
    updating the memory map.

.. figure:: mmu.png
   :align: center

   Before memory addresses are loaded on to the system bus, they are
   translated to physical addresses by the MMU.