Structure of the Windows Registry

By opening the Registry Editor (by typing “regedit” in the run window), the Registry can be seen as one unified “file system”.  The left-hand pane, also known as the key pane contains an organized listing of what appear to be folders.  The five most hierarchal folders are called “hives” and begin with “HKEY” (an abbreviation for Handle to a Key).  Although five hives can be seen, only two of these are actually “real”, HKEY_USERS (HKU) and HKEY_LOCAL_MACHINE (HKLM).  The other three are shortcuts or aliases to branches within one of the two hives.  Each of these five hives is composed of keys, which contain values and subkeys.  Values are the names of certain items within a key, which uniquely identify specific values pertaining to the operating system, or to applications that depend upon that value. 

A common analogy that is often used to help understand the structure of the Windows Registry is a comparison between it and the Windows Explorer file system, both are very similar in their structures.  The key pane of the Registry is much like the hierarchical structure of the left-hand pane in the Windows Explorer file system.  The keys and subkeys located within the five main hives are similar to folders and subfolders of Windows Explorer, and a key’s value is similar to a file within a folder.  In the right-hand pane of the Windows Registry – a value’s name is similar to a file’s name, its type is similar to a file’s extension, and its data is similar to the actual contents of a file.
 

Figure 1 – Structure of the Windows Registry
 

Root Key Functions:

 Below are listed the five hierarchical hives seen in Figure 1, with a very basic description of each.  Beside the root key is their commonly referred to abbreviation in parenthesis, which will frequently be referred to as throughout the paper.


1.HKEY_CLASSES_ROOT (HKCR)
Information stored here ensures that the correct program opens when it is executed in Windows Explorer.  It also contains further details on drag-and-drop rules, shortcuts, and information on the user interface.  Alias for: HKLM\Software\Classes
 

2.HKEY_CURRENT_USER (HKCU)
Contains configuration information for the user who is currently logged into the system, including user’s folders, screen colors, and Control Panel settings.  Alias  for a user specific branch in HKEY_USERS.  The generic information usually applies to all users and is HKU\.DEFAULT. 
 

3.HKEY_LOCAL_MACHINE (HKLM)
Contains machine hardware-specific information that the operating system runs on.  It includes a list of drives mounted on the system and generic configurations of installed hardware and applications.
 

4.HKEY_USERS (HKU)
Contains configuration information of all user profiles on the system, which concerns application configurations, and visual settings.

5.HKEY_CURRENT_CONFIG (HCU)
Stores information about the systems current configuration.  Alias for: HKLM\Config\profile