Last Known Good Configuration

“Last Known Good Configuration” is a recovery option Microsoft built into all versions of Windows, and can be a valuable asset when attempting to recover a PC that isn’t working correctly. Last Known Good Configuration is found in the Advanced Boot Options menu.

Contents

About “Last Known Good Configuration”

Background information about the Windows Registry

Windows stores almost all of system-related configuration and settings in the Windows registry. The Windows registry is split into 4 primary sections: HKEY_CURRENT_USER which stores settings related to the logged-in user account, HKLM\SAM1 which stores information about Windows usernames and passwords, HKLM\SECURITY which contains settings related to the security configuration of your machine, and HKLM\SYSTEM which stores all global settings for your PC.

The HKLM\SYSTEM registry hive, as mentioned above, contains most of the settings and configurations that Windows uses to determine what drivers to load, which settings to use, what policies are configured, etc. Inside HKLM\SYSTEM, all configurations and settings that have to do with your hardware, the drivers to communicate with that hardware, and system services are found in HKLM\CurrentControlSet.

CurrentControlSet

To protect against system errors and provide an easy recovery option, each time Windows is successfully booted, the old CurrentControlSet and all its contents is copied and set aside. In reality, CurrentControlSet is actually a shortcut/pointer to one of CurrentControlSet001, CurrentControlSet002, or Clone. When your computer first starts up, Windows copies HKLM\SYSTEM\CurrentControlSet to HKLM\SYSTEM\Clone. CurrentControlSet is then “pointed” at CurrentControlSet001, to which it serves as a shortcut. Any changes made to HKLM\SYSTEM\CurrentControlSet are in actuality being made to CurrentControlSet001.

Last Known Good Configuration

This finally brings us to the topic of the day: Last Known Good Configuration. If changes made to CurrentControlSet end up causing Windows not to work properly, the configuration that was used last time for Windows is still available in the backed-up copy. When Windows fails to start, it is possible to to request Windows boot into “Last Known Good Configuration.” Here, instead of pointing CurrentControlSet to CurrentControlSet001 (as is the default behavior), Windows will instead make SYSTEM\CurrentControlSet point to CurrentControlSet002 — which (since we used it before) we know is good and working.

Booting your PC into Last Known Good Configuration

Directions on how to configure Windows to boot into Last Known Good Configuration have been moved to a separate article, as this detailed description of how Last Known Good Configuration operates underneath the hood contains advanced details, knowledge of which is not required to actually make use of LKGC.

See Booting your PC into Last Known Good Configuration for directions on restoring your PC to a working copy of CurrentControlSet via CurrentControlSet002.

Propose an edit

  1. HKLM is an abbreviation you’ll see often in discussions pertaining to the Windows registry, and it stands for HKEY_LOCAL_MACHINE and contains global settings regarding your PC. ↩