.. _struct:

Operating-System Structure
========================================================

Simple Structure
^^^^^^^^^^^^^^^^^^

.. figure:: dos_struct.png
   :align: center

   In MS-DOS, applications may bypass the operating system.

* Operating systems such as MS-DOS and the original UNIX did not have
  well-defined structures.

* There was no :ref:`mode` (user and kernel), and so errors in applications
  could cause the whole system to crash.

.. index:: monolithic kernel

.. _monolithic:

Monolithic Approach
^^^^^^^^^^^^^^^^^^^^

* Functionality of the OS is invoked with simple function calls within the
  kernel, which is one large program.

* Device drivers are loaded into the running kernel and become part of the
  kernel.

.. figure:: unix-mono-kernel.jpg
   :align: center

   A monolithic kernel, such as Linux and other Unix systems.

Layered Approach
^^^^^^^^^^^^^^^^^^

This approach breaks up the operating system into different layers.

* This allows implementers to change the inner workings, and
  increases modularity.

* As long as the external interface of the routines don't change,
  developers have more freedom to change the inner workings of the
  routines.

* With the layered approach, the bottom layer is the hardware, while the
  highest layer is the user interface.

    * The main *advantage* is simplicity of construction and debugging.

    * The main *difficulty* is defining the various layers.

    * The main *disadvantage* is that the OS tends to be less efficient than
      other implementations.

.. figure:: NTkernel.jpg
   :align: center

   The Microsoft Windows NT Operating System.  The lowest level is a monolithic
   kernel, but many OS components are at a higher level, but still part of the
   OS.

.. index:: microkernel
.. _microkernel:

Microkernels
^^^^^^^^^^^^^^^^^^

This structures the operating system by removing all nonessential portions of
the kernel and implementing them as system and user level programs.

    * Generally they provide minimal process and memory management, and a
      communications facility.

    * Communication between components of the OS is provided by message
      passing.

The *benefits* of the microkernel are as follows:

    * Extending the operating system becomes much easier.

    * Any changes to the kernel tend to be fewer, since the kernel is smaller.

    * The microkernel also provides more security and reliability.

Main *disadvantage* is poor performance due to increased system overhead
from message passing.

.. figure:: microkernel.jpg
   :align: center

   A Microkernel architecture.

.. note::

    * `Andrew Tanenbaum's <http://www.cs.vu.nl/~ast/>`_ `Minix
      <http://www.minix3.org/>`_ is an example of a microkernel system.  Minix
      was developed primarily to facilitate teaching graduate level operating
      system classes. Tanenbaum has authored several text books and is with `VA
      University <http://www.cs.vu.nl/>`_ in Amsterdam.

    * Another well known microkernel system is **Mach**, which was
      developed at Carnegie Mellon University in the mid-1980's.  Mach was used
      as the low-level part of Apple OS X.