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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 begins its existence while a system is executing in an x86 operating mode called real mode. In real mode, no virtual-to-physical translation of memory addresses occurs, which means that programs that use the memory addresses interpret them as physical addresses and that only the first 1 MB of the computer’s physical memory is accessible. Simple MS-DOS programs execute in a real-mode environment. However, the first action Bootmgr takes is to switch the system to protected mode. Still no virtual-to-physical translation occurs at this point in the boot process, but a full 32 bits of memory becomes accessible. After the system is in protected mode, Bootmgr can access all of physical memory. After creating enough page tables to make memory below 16 MB accessible with paging turned on, Bootmgr enables paging. Protected mode with paging enabled is the mode in which Windows executes in normal operation.

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 a mode in which services provided by the BIOS can be executed, called real mode. Bootmgr next reads the BCD file from the \Boot directory using built-in file system code. Like the boot sector’s code, Bootmgr contains read-only NTFS code (Bootmgr also supports other file systems, such as FAT, El Torito CDFS, UDFS, and WIM files); unlike the boot sector’s code, however, Bootmgr’s file system code can read subdirectories.

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 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 will be valid only if the last time the computer was shut down it was hibernated.

If there is more than one boot-selection entry in the BCD, Bootmgr presents the user with the boot-selection menu (if there is only one entry, Bootmgr bypasses the menu and proceeds to launch Winload.exe). Selection entries in the BCD direct Bootmgr to the partition on which the Windows system directory (typically \Windows) of the selected installation resides. This partition might be the same as the system partition, or it might be another primary or extended partition.

Entries in the BCD can include optional arguments that Bootmgr, Winload, and other components involved in the boot process interpret.

The Bcdedit.exe tool provides a convenient interface for setting a number of the switches. Some options that are included in the BCD save to the registry value HKLM\SYSTEM\ CurrentControlSet\Control\SystemStartOptions if they correspond to command-line switches; otherwise, they are kept only in the BCD.

If the user doesn’t select an entry from the selection menu within the timeout period the
BCD specifies, Bootmgr chooses the default selection specified in the BCD (if there is only one entry, it chooses this one). Once the boot selection has been made, Bootmgr loads the boot loader associated with that entry, which will be Winload.exe for Windows installations. Winload.exe also contains code that queries the system’s ACPI BIOS to retrieve basic device and configuration information. This information includes the following:

• The time and date information stored in the system’s CMOS (nonvolatile memory)

• The number, size, and type of disk drives on the system

• Legacy device information, such as buses (for example, ISA, PCI, EISA, Micro Channel Architecture [MCA]), mice, parallel ports, and video adapters are not queried and instead faked out

This information is gathered into internal data structures that will be stored under the HKLM\HARDWARE\DESCRIPTION registry key later in the boot.

Next, Winload begins loading the files from the boot volume needed to start the kernel initialization. The boot volume is the volume that corresponds to the partition on which the system directory (usually \Windows) of the installation being booted is located. The steps Winload follows here include:

1. Loads the appropriate kernel and HAL images (Ntoskrnl.exe and Hal.dll by default) as well as any of their dependencies. If Winload fails to load either of these files, it prints the message “Windows could not start because the following file was missing or corrupt”, followed by the name of the file.

2. Reads in the VGA font file (by default, vgaoem.fon). If this file fails, the same error message as described in step 1 will be shown.

3. Reads in the NLS (National Language System) files used for internationalization. By default, these are l_intl.nls, c_1252.nls, and c_437.nls.

4. Reads in the SYSTEM registry hive, \Windows\System32\Config\System, so that it can determine which device drivers need to be loaded to accomplish the boot.

5. Scans the in-memory SYSTEM registry hive and locates all the boot device drivers. Boot device drivers are drivers necessary to boot the system. These drivers are indicated in the registry by a start value of SERVICE_BOOT_START (0). Every device driver has a registry subkey under HKLM\SYSTEM\CurrentControlSet\Services. For example, Services has a subkey named fvevol for the BitLocker driver.

6. Adds the file system driver that’s responsible for implementing the code for the type of partition (NTFS) on which the installation directory resides to the list of boot drivers to load. Winload must load this driver at this time; if it didn’t, the kernel would require the drivers to load themselves, a requirement that would introduce a circular dependency.

7. Loads the boot drivers, which should only be drivers that, like the file system driver for the boot volume, would introduce a circular dependency if the kernel was required to load them. To indicate the progress of the loading, Winload updates a progress bar displayed below the text “Starting Windows”. If the sos option is specified in the BCD, Winload doesn’t display the progress bar but instead displays the file names of each boot driver. Keep in mind that the drivers are loaded but not initialized at this time— they initialize later in the boot sequence.

8. Prepares CPU registers for the execution of Ntoskrnl.exe.

This action is the end of Winload’s role in the boot process. At this point, Winload calls the main function in Ntoskrnl.exe (KiSystemStartup) to perform the rest of the system initialization.

Source of Information : Microsoft Press Windows Internals 5th Edition


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