Introducing the Windows Boot Environment

Windows computers can use several different processor architectures, several different types of firmware, and several different disk partitioning styles. Generally, computers with 32-bit x86-based processors use the Master Boot Record (MBR) disk partitioning style and BIOS. Computers with Itanium-based IA64 processors use the GUID Partition Table (GPT) disk partitioning style and Extensible Firmware Interface (EFI). Computers with 64-bit x64-based processors use the MBR disk partitioning style and Unified Extensible Firmware Interface (UEFI) which is wrapped around either BIOS or EFI.

Every computer has firmware. Firmware is implemented in motherboard chipsets. Just as Windows has an interface, so does firmware. The interface between the platform firmware and the operating system that handles the startup process. The way a firmware interface works and the tasks it performs depend on the type of firmware interface. Windows computers can use different types of firmware. Generally, computers with 32-bit x86-based processors use BIOS. Computers with Itanium-based IA64 processors use EFI. Computers with 64-bit x64-based processors use UEFI, which is wrapped around either BIOS or EFI. For the purposes of this discussion, a computer that uses

UEFI wrapped around BIOS is BIOS-based and a computer that uses UEFI wrapped around EFI is EFI-based.

If you are familiar with the way Windows XP and earlier versions of Windows start, you know these versions of Windows uses Boot.ini to initialize the startup environment and Ntldr to load the operating system. Windows Vista and Windows 7 don't use these boot facilities. Instead, startup is controlled using the parameters in the BCD data store:

• Entries in the BCD data store identify the boot manager to use during startup and the specific boot applications available.

• Windows Boot Manager controls the boot experience and enables you to choose which boot application is run.

• Boot applications load a specific operating system or operating system version. For example, a Windows Boot Loader application loads Windows 7.

Because BCD abstracts the underlying firmware, you can boot BIOS-based and EFI-based computers in much the same way—just as you can computers based on other firmware models. The BCD store is contained in a file called the BCD registry. On BIOS-based operating systems, the BCD registry file is stored in the \Boot\Bcd file of the hidden System Reserved partition that Windows 7 creates. This partition is visible in the Disk Management MMC Snap-In, but it is not assigned a drive letter by default. On EFI-based operating systems, the BCD registry file is stored on the EFI system partition.

The BCD store contains multiple entries. On a BIOS-based computer, you'll have one Windows Boot Manager entry. There is only one boot manager, so there is only one boot manager entry. You'll also have Windows Boot Loader application entries, with one entry for each instance of Windows Vista, Windows 7 or a later version of Windows installed on the computer.

On a computer with Windows XP or earlier operating systems installed in addition to Windows Vista, Windows 7 or later, you'll have one legacy operating system entry. The legacy entry is not for a boot application. This entry is used to initiate Ntldr and Boot.ini so that you can boot into a pre-Windows Vista operating system. If the computer has more than one pre-Windows Vista operating system, you'll be able to select the operating system to start after selecting the legacy operating system entry.

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