Leaf through almost any edition of PC User (or most other computer magazines or Windows help sites) and you're bound to come across a bunch of techniques and tips which involve editing the Registry. Each of those tips carries a rider about the inherent dangers of the process.

If you've been intimidated by the warnings and shied away, or if you've ignored the perils and tinkered with the Registry without really knowing much about it, now is a good time to become more familiar with this core component of Windows.

What is the Registry?

The Registry is the central repository for configuration data – user settings and machine-dependent settings – in Windows. It keeps track of installed programs; program interactions; hardware configuration; individual user preferences; and system customisations. Most of the information in the Registry is placed there automatically by Windows itself, and by utilities and applications via their setup routines and optional settings. It's also possible to edit Registry data directly.

The Registry, introduced with Windows 95, replaced the old INI files used to store configuration settings. INIs were handy because they were short text files, easily edited with Notepad. But they had many limitations including size restrictions, lack of internal organisation and lack of support for multiple users and alternative hardware configurations. The Registry does away with all these limitations and makes it easier to track changes and recover settings after a crash.

What form does it take?

The Registry is organised as a hierarchical database. That simply means the information is stored in a tree-like structure, somewhat similar to the hierarchy you're familiar with through using Windows Explorer:

FOLDER -> SUB-FOLDER -> FILE

In the Registry, the hierarchy goes:

KEY -> SUBKEY -> VALUE

with each key having one or more subkeys and each subkey containing a value.

Each value is stored with a value name and value data. The data also has a data type which identifies the kind of information – string, binary and so on – stored in the value.

There are six main keys, also known as the root keys. They are:

We'll look at each of these root keys shortly.

The Registry displayed in the Registry Editor.

The Registry displayed in the Registry Editor, with all root keys except HKEY_CLASSES_ROOT expanded to show the major subkeys. Click the image to see a full-size screenshot.

The physical structure

So much for the logical organisation of the Registry. Physically, the Registry is stored on your hard drive in two or more files:

In Windows Me, there's a third file called Classes.dat, which stores the HKEY_CLASSES_ROOT data. In network environments, there's often another registry file, Policy.pol, which contains network or company-wide settings established by the system administrator.

What's in the root keys?

Everything in the Registry branches out from the six root keys, whose names all begin with HKEY (it stands for handle to a key, which is programmer's terminology).

HKEY_CLASSES_ROOT contains information pertaining to OLE (object linking and embedding), file association mappings, Windows shortcuts and some other aspects of the user interface.

In HKEY_CLASSES_ROOT you'll find the names of all registered file types (such as .doc, .exe, .html and so on) and their associated properties. This key is, in fact, a pointer to another subkey, HKEY_LOCAL_MACHINE\Software\Classes.

HKEY_CURRENT_USER contains user-specific information. The contents of this key are derived from information in the HKEY_USERS key at logon time.

HKEY_LOCAL_MACHINE contains computer-specific information, including info about installed hardware and software settings. The information in this key is used by all users of the computer.

HKEY_USERS contains user information such as default application settings, desktop configuration and so on. Some of the information is user-specific and some is available to all users of the computer. The default, generic settings are stored in a subkey HKEY_USERS\.DEFAULT. A new subkey is created for each user who logs onto the computer. At first, that new subkey contains a copy of the contents of the .DEFAULT subkey. As the user changes desktop and application settings, those changes are stored in the user's subkey.

HKEY_CURRENT_CONFIG handles Plug & Play settings and information about multiple hardware configurations. Settings in this key are derived from a subkey of HKEY_LOCAL_MACHINE\Config.

HKEY_DYN_DATA maintains a dynamic record of the current status of the computer.

Registry aliases

Although we talk about the six root keys in the Registry, there are really only two keys – HKEY_LOCAL_MACHINE and HKEY_USERS. Three of the four 'non-real' keys are aliases, the Registry equivalent of Windows shortcuts, deriving their contents from subkeys of LOCAL_MACHINE and USERS. Anything you change in one key is reflected in its alias. The fourth 'non-real' key, HKEY_DYN_DATA, is created on-the-fly from device status information each time the computer boots.

Alias Branch from which alias is derived
HKEY_CLASSES_ROOT HKEY_LOCAL_MACHINE\Software\Classes
HKEY_CURRENT_USER HKEY_USERS\username
HKEY_CURRENT_CONFIG HKEY_LOCAL_MACHINE\Config\profile

Coming up

In the second part of this series (which I'll publish in the next few weeks) we'll delve more deeply into the Registry courtesy of Regedit, the Registry Editor.

© 2002  Rose Vines

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