Before writing to a partition’s boot sector, Windows Setup ensures that the boot partition (the
Another of Setup’s roles is to prepare the Boot Configuration Database (BCD), which on BIOS systems is stored in the \Boot\BCD file on the root directory of the system volume. This file contains options for starting the version of Windows that Setup installs and any preexisting Windows installations. If the BCD already exists, the Setup program simply adds new entries relevant to the new installation. For more information on the BCD, see Chapter 3, “System Mechanisms,” in Part 1.
The BIOS Boot Sector and Bootmgr
Setup must know the partition format before it writes a boot sector because the contents of the boot sector vary depending on the format. For a partition that is in NTFS format, Windows writes NTFS-capable code. The role of the boot-sector code is to give Windows information about the structure and format of a volume and to read in the Bootmgr file from the root directory of the volume. Thus, the boot-sector code contains just enough read-only file system code to accomplish this task. After the boot-sector code loads Bootmgr into memory, it transfers control to Bootmgr’s entry point. If the boot-sector code can’t find Bootmgr in the volume’s root directory, it displays the error message “BOOTMGR is missing”.
Bootmgr is actually a concatenation of a .com file (Startup.com) and an .exe file (Bootmgr.exe), so it begins its existence while a system is executing in an x86 operating mode called
After Bootmgr enables protected mode, it is fully operational. However, it still relies on functions supplied by BIOS to access IDE-based system and boot disks as well as the display. Bootmgr’s BIOS-interfacing functions briefly switch the processor back to real mode so that services provided by the BIOS can be executed. Bootmgr next reads the BCD file from the \Boot directory using built-in file system code. Like the boot sector’s code, Bootmgr contains a lightweight NTFS file system library (Bootmgr also supports other file systems, such as FAT, El Torito CDFS, and UDFS, as well as WIM and VHD files); unlike the boot sector’s code, Bootmgr’s file system code can also read subdirectories.
Note
Bootmgr and other boot applications can still write to preallocated files on NTFS volumes, because only the data needs to be written, instead of performing all the complex allocation work that is typically required on an NTFS volume. This is how these applications can write to bootsect.dat, for example.
Bootmgr next clears the screen. If Windows enabled the BCD setting to inform Bootmgr of a hibernation resume, this shortcuts the boot process by launching Winresume.exe, which will read the contents of the hibernation file into memory and transfer control to code in the kernel that resumes a hibernated system. That code is responsible for restarting drivers that were active when the system was shut down. Hiberfil.sys is only valid if the last computer shutdown was hibernation, since the hibernation file is invalidated after a resume, to avoid multiple resumes from the same point. (See the section The Power Manager in Chapter 8, for information on hibernation.)