The Microsoft Windows Recovery Environment (Windows RE) is a simplified, scaled-back version of the Windows operating system. Windows RE is used to boot the system when Windows 8, Windows Server 2012 or Windows Server 2012 R2 cannot otherwise boot normally or run in a stable manner.
The principle of Windows RE is similar to the Windows Preinstallation Environment. When the recovery environment is invoked, a series of recovery tools can help to diagnose and repair problems with the operating system (OS). For example, automatic repair and troubleshooting tools can test disk and file integrity. They are often able to fix common disk problems and restore OS files damaged by disk failures, malware activity or user errors. Windows 8.x users can run a reset tool that will refresh the OS to its original state. Users can opt to keep or remove content such as applications and data. Similarly, Windows Server 2012 and 2012 R2 users can employ an image-recovery tool to restore the server's local hard drive from a backup.
Windows RE is typically invoked automatically in the wake of boot problems such as failed startup attempts, unexpected shutdowns, and security-related issues such as BitLocker or Secure Boot errors. Windows RE can also be invoked manually through several avenues. One common approach involves selecting Settings, choosing Power and then holding a Shift key while selecting Restart. Another common approach is to type the shutdown /r /o command in a run dialog. Once Windows RE starts, a boot option menu appears that allows users to start selected tools, boot from a bootable device (such as a flash drive) supported by a Unified Extensible Firmware Interface (UEFI), access the UEFI firmware or boot from a different OS (if more than one OS is available).
Windows RE is customizable. Organizations with specific recovery requirements can add different languages, device drivers and new diagnostic tools to the recovery environment using the Windows Imaging application programming interface (API) or the Deployment Image Servicing and Management (DISM) API. However, each customization increases the memory demands for Windows RE -- making the recovery environment more demanding on the host computer. Developers should try to minimize the number of customizations in order to keep the recovery environment as small and efficient as possible.