Working with BCD in Windows 10

What admins need to know about BCD data, structures, and organization in Windows 10.

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Reuters/Robert Galbraith

In the world of the Windows OS, BCD stands for Boot Configuration Data. This critical information in the Windows runtime environment tells the Windows boot loader where to look for boot information. It also establishes boot priority and timing when, as is typically the case, a PC can access two or more bootable partitions as it’s starting up. For example, a default Windows OS clean install actually writes two bootable partitions to the drive that’s designated as the install target. The primary and most frequently used of these partitions is the operating system partition. But a secondary, recovery partition also gets written and may be accessed at boot time to support repair and recovery. This produces a disk layout like the one shown in Figure 1.

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Figure 1: The default Windows 10 boot disk layout from a clean install.

In this disk layout, the partitions may be described as follows, from left to right (1-4):

1.     450 MB: The default Windows Recovery (WinRE) partition, which includes a basic Windows runtime and various repair and recovery tools (see How to create a repair/recovery partition in Windows 10).

2.     100 MB: This is the EFI (Extensible Firmware Interface) partition where Windows stores a copy of the boot loader, BCD and related logs, and other files for boot configuration and boot-up use.

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