Page 330 - DCAP103_Principle of operating system

Unit 10: Introduction of Windows and its Programming

The SAM (Security Account Manager) subkey contains the user names, groups, passwords, and Notes
other account and security information needed for logging in. The SECURITY subkey contains
general security policy information, such as minimum length for passwords, how many failed
login attempts are tolerated before the police is called, etc.
The SOFTWARE subkey is where software manufacturers store preferences etc. For example,
if a user has Adobe Acrobat, Photoshop and Premiere installed, there will be a subkey Adobe
here, and below that further subkeys for Acrobat, Photoshop, Premiere, and any other Adobe
products. The entries in these subdirectories can store anything the Adobe programmers want
to put there, generally system-wide properties such as the version and build number, how to
uninstall the package, drivers to use, and so forth. The registry saves them the trouble of having
to invent their own method for storing this information. User-specific information also goes in
the registry, but under HKEY_USERS.

The SYSTEM subkey holds most of the information about booting the system, for example,
the list of drivers that must be loaded. It also holds the list of services (daemons) that must be
started after booting up and the configuration information for all of them.
The next top-level key is HKEY_USERS, which contains the profiles of all the users. All the user-
specific preferences in a number of areas are stored here. When a user changes a preference using
the control panel, for example, the desktop colour scheme, the new settings are recorded here.
In fact, many of the programs on the control panel do little more than collect user information
and change the registry accordingly. Some of the subkeys under HKEY_USERS are shown in
Figure 10.5 and should need little additional comment. Some of the subkeys, such as Software,
contain surprisingly large numbers of subkeys, even if no software packages are installed.
The next top-level key, HKEY_PERFORMANCE_DATA contains neither data read in from
the disk nor data collected by the plug-and-play manager. Instead, it offers a window into
the operating system. The system itself contains hundreds of counters for monitoring system
performance. These counters are accessible via this registry key. When a subkey is queried, a
specified procedure is run to collect and return the information (possibly by reading one or more
counters and combining them in some way). This key is not visible using regedit or regedt32.
Instead one has to use the performance tools, such as pfmon, perfmon, and pview. There are
many such tools, some on the Windows 2000 CD-ROM, some in the resource kits, and some
from third parties.
The next three top-level keys do not actually exist. Each one is a symbolic link to some place
elsewhere in the registry. The HKEY_CLASSES_ROOT key is the most interesting. It points to
the directory that handles COM (Component Object Model) objects and also the associations
between file extensions and programs. When a user double clicks on a file ending in, say,
.doc, the program catching the mouse click looks here to see which program to run (probably
Microsoft Word). The complete database of recognized extensions and which program each one
is owned by is under this key.
The HKEY_CURRENT_CONFIG key links to the current hardware configuration. A user can
construct multiple hardware configurations, for example by disabling various devices to see if
they were the cause of strange system behaviour. This key points to the current configuration.
Similarly, HKEY_CURRENT_USER points to the current user so that user’s preferences can be
found quickly.
None of the last three keys really adds anything, since the information was available elsewhere
anyway (although less conveniently accessible). Thus despite the fact that regedit and regedt32
list five top-level keys, there are really only three top-level directories and one of them is not
shown among the five displayed.

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