Inside the Windows NT Registry

HKEY_DYN_DATA
HKDD is a fake key--it doesn't really exist. It serves as a convenient doorway to device driver, Win32 application, and native NT performance counters via the Registry API. When a Win32 program queries a value or key in HKDD, the request gets routed as an I/O request to the appropriate driver or Win32 program, which returns information that looks like the result of an authentic Registry access. The Performance Monitor (Perfmon) program accesses this root key to provide the intricate performance information it displays.

HKEY_CURRENT_CONFIG

HKCC, a new root key in NT 4.0, is a link to HKLM\SYSTEM\CurrentControlSet\Hardware Profiles\Current, which contains the configuration data for the hardware profile in use on the system. Microsoft added HKCC to NT to let applications that access this key run on both Windows 95 and NT. To create, configure, and change hardware profiles, you can use Control Panel's System, Services, and Device applets.

Registry Gems
Now that you understand the basic structure of the Registry, let's look at a few handy settings for data stored in the Registry. As always, before you try any of the following suggestions, back up your Registry. Editing Registry entries incorrectly can cause systemwide problems that may require you to reinstall NT to correct them.

Auto-logon. If you have a private NT system (e.g., a machine you use at home), you can configure NT to automatically log you on when you boot NT. To enable auto-logon, you must modify four values in HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon. First, set the AutoAdminLogon value to 1. Then specify appropriate strings for DefaultDomainName, DefaultPassword, and DefaultUserName. The next time you reboot, you'll automatically be logged on.

Tuning a workstation for server-like workloads. NT Workstation and NT Server have vastly different performance characteristics because of the internal tuning NT performs. You cannot access most tuning parameters, but you can find a few in the Registry. If you run NT Server and double-click the Server entry of the Services tab in Control Panel's Network applet, you get a dialog box that lets you determine what type of applications you want to tune the machine for. You can choose among Minimize Memory Used, Balance, Maximize Throughput for File Sharing, and Maximize Throughput for Network Applications. Systems running NT Workstation do not present this dialog box. The options change two Registry values: HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\LargeSystemCache and HKLM\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters\Size. Table 7 lists the value data you need to enter in the Registry of a system running NT Workstation to achieve the same tuning options the NT Server dialog box offers.

Previewing bitmaps in their icons. How many times have you wished you could get a quick look at what's in a bitmap file without opening it? You can, with a simple Registry setting. Just change the value of HKCR\Paint.Picture\DefaultIcon to "%1". Reboot for the change to take effect.

After the Tour
These few tips conclude a whirlwind tour of the Registry. If you want to learn more about the Registry, get Microsoft Windows NT Workstation Resource Kit or Microsoft Windows NT Server Resource Kit, which include extensive online documentation about the Registry. If your interest lies in programming the Registry API, refer to the Win32 software development kit (SDK).