Booting process of Windows NT

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Process by which several Microsoft Windows operating systems initialize

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The booting process of Windows NT is the process run to start Windows NT. The process has been changed between releases, with the biggest changes being made with Windows Vista. In versions before Vista, the booting process begins when the BIOS loads the Windows NT bootloader, NTLDR. Starting with Vista, the booting process begins with either the BIOS or UEFI load the Windows Boot Manager, which replaces NTLDR as the bootloader. Next, the bootloader starts the kernel, which starts the session manager, which begins the login process. Once the user is logged in File Explorer, the graphical user interface used by Windows NT, is started.

History[edit]

Windows Vista introduces a complete overhaul of the Windows operating system loader architecture.[1][2] The earliest known reference to this revised architecture is included within PowerPoint slides distributed by Microsoft during the Windows Hardware Engineering Conference of 2004 when the operating system was codenamed “Longhorn.”[3] This documentation mentions that the Windows operating system loader would be undergoing a significant restructuring in order to support EFI and to “do some major overhaul of legacy code.”[4] The new boot architecture completely replaces the NTLDR architecture used in previous versions of Windows NT.[2]

Most of the steps that follow the NT kernel being loaded, including kernel initialization and user-space initialization, are kept the same as in earlier NT systems.[5] Refactoring in Winlogon resulted in GINA being completely replaced by Credential Providers and graphical components in Windows Vista and later.[6]

BIOS/UEFI[edit]

On systems with a BIOS, the BIOS invokes MBR boot code from a hard disk drive at startup. The MBR boot code and the VBR boot code are OS-specific. In Microsoft Windows, the MBR boot code tries to find an active partition (the MBR is only 512 bytes), then executes the VBR boot code of an active partition. The VBR boot code tries to find and execute NTLDR for Windows XP and earlier, or the Windows Boot Manager for Windows Vista and later, from an active partition.[7]

On systems with a UEFI, the UEFI invokes bootmgfw.efi from an EFI system partition at startup, starting the Windows Boot Manager.

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Boot loader phase[edit]

The Windows NT startup process starts when the computer finds a Windows boot loader, a portion of the Windows operating system responsible for finding Microsoft Windows and starting it up. Prior to Windows Vista, the boot loader was NTLDR. Microsoft has also released operating systems for Intel Itanium processors which use IA-64 architecture. The boot loader of these editions of Windows is IA64ldr.efi (later referred as simply IA64ldr). It is an Extensible Firmware Interface (EFI) program.[8] Windows Vista and later use the Windows Boot Manager (bootmgr).

Operating system selection[edit]

Windows Boot Manager (BOOTMGR) with Windows 7 highlighted and options to load Windows Vista through BOOTMGR and XP through NTLDR.

The boot loader, once executed, searches for Windows operating systems. Windows Boot Manager does so by reading Boot Configuration Data (BCD), a complex firmware-independent database for boot-time configuration data. Its predecessor, NTLDR, does so by reading the simpler boot.ini. If the boot.ini file is missing, the boot loader will attempt to locate information from the standard installation directory. For Windows NT and 2000 machines, it will attempt to boot from C:WINNT. For Windows XP and 2003 machines, it will boot from C:WINDOWS.

Both databases may contain a list of installed Microsoft operating systems that may be loaded from the local hard disk drive or a remote computer on the local network. NTLDR supports operating systems installed on disks whose file system is NTFS or FAT file systems, CDFS (ISO 9660) or UDFS.[9] Windows Boot Manager also supports operating systems installed inside a VHD file, stored on an NTFS disk drive.[10]

In Windows 2000 or in later versions of Windows in which hibernation is supported, the Windows boot loader starts the search for operating systems by searching for hiberfil.sys. NTLDR looks into the root folder of the default volume specified in boot.ini. Windows Boot Manager looks up the location of hiberfil.sys in BCD. If this file is found and an active memory set is found in it, the boot loader loads the contents of the file (which is a compressed version of a physical memory dump of the machine) into memory and restores the computer to the state that it was in prior to hibernation by running winresume.exe.

Next, the boot loader looks for a list of installed operating system entries. If more than one operating system is installed, the boot loader shows a boot menu and allow the user to select an operating system. If a non NT-based operating system such as Windows 98 is selected (specified by an MS-DOS style of path, e.g. C:), then the boot loader loads the associated “boot sector” file listed in boot.ini or BCD (by default, this is bootsect.dos if no file name is specified) and passes execution control to it.

Otherwise, the boot process continues. For Windows Vista and after, this is done through a seperate program, winload.exe.

Loading the Windows NT kernel[edit]

The operating system starts when certain basic drivers flagged as “Boot” are loaded into memory. The appropriate file system driver for the partition type (NTFS, FAT, or FAT32) which the Windows installation resides in is amongst them. At this point in the boot process, the boot loader clears the screen and displays a textual progress bar (which is often not seen due to the initialization speed); Windows 2000 also displays the text “Starting Windows…” underneath.

NTLDR Bootloader’s Advanced Option Menu

If the user presses F8 during this phase, the advanced options menu is displayed, containing various special boot modes including Safe mode, with the Last Known Good Configuration, with debugging enabled, and (in the case of Server editions) Directory Services Restore Mode. Starting with Windows Vista, this menu was changed significantly. Once a boot mode has been selected (or if F8 was never pressed) booting continues.

Hardware information about the computer is gathered by NTDETECT.COM in Windows XP and earlier or by winload.exe in later versions. This information is stored in the HKLMHARDWAREDESCRIPTION key in the Windows Registry.

Next the Windows NT kernel (Ntoskrnl.exe), the Hardware Abstraction Layer (hal.dll), kdcom.dll (Kernel Debugger HW Extension DLL), bootvid.dll (the Windows logo and side-scrolling bar), and configsystem (one of the registry hives) are loaded.

For Windows XP and earlier, if multiple hardware configurations are defined in the Registry, the user is prompted at this point to choose one.

With the kernel in memory, boot-time device drivers are loaded (but not yet initialized). The required information (along with information on all detected hardware and Windows Services) is stored in the HKEY_LOCAL_MACHINESYSTEM portion of the registry, in a set of registry keys collectively called a Control Set. In Windows XP and earlier, multiple control sets are kept, in the event that the settings contained in the currently-used one prohibit the system from booting. HKEY_LOCAL_MACHINESYSTEM contains control sets labeled ControlSet001, ControlSet002, etc., as well as CurrentControlSet. During regular operation, Windows uses CurrentControlSet to read and write information. CurrentControlSet is a reference to one of the control sets stored in the registry.

Windows now picks the “real” control set being used based on the values set in the HKEY_LOCAL_MACHINESYSTEMSelect registry key:

  • Default will be the boot loader’s choice if nothing else overrides this
  • If the value of the Failed key matches Default, then the boot loader displays an error message, indicating that the last boot failed, and gives the user the option to try booting anyway, or to use the “Last Known Good Configuration”.
  • If the user choose (or has chosen) Last Known Good Configuration, the control set indicated by the LastKnownGood key is used instead of Default.

When a control set is chosen, the Current key gets set accordingly. The Failed key is also set to the same as Current until the end of the boot process. LastKnownGood is also set to Current if the boot process completes successfully.

Which services are started and the order which each group is started in are provided by the following keys:

  • HKLMSYSTEMCurrentControlSetServices
  • HKLMSYSTEMCurrentControlSetControlServiceGroupOrder

For the purposes of booting, a driver may be one of the following:

  1. A “Boot” driver that is loaded by the boot loader prior to starting the kernel. “Boot” drivers are almost exclusively drivers for hard-disk controllers and file systems (ATA, SCSI, file system filter manager, etc.); in other words, they are the absolute minimum that the kernel will need to get started with loading other drivers, and the rest of the operating system.
  2. A “System” driver which is loaded and started by the kernel after the boot drivers. “System” drivers cover a wider range of core functionality, including the display driver, CD-ROM support, and the TCP/IP stack.
  3. An “Automatic” driver which is loaded much later when the GUI already has been started.

With this finished, control is then passed from the boot loader to the kernel.

Kernel phase[edit]

The initialization of the kernel subsystem and the Windows Executive subsystems is done in two phases.

During the first phase, basic internal memory structures are created, and each CPU’s interrupt controller is initialized. The memory manager is initialized, creating areas for the file system cache, paged and non-paged pools of memory. The Object Manager,[11] initial security token for assignment to the first process on the system, and the Process Manager itself. The System idle process as well as the System process are created at this point.

The second phase involves initializing the device drivers which were identified by NTLDR as being system drivers.

Through the process of loading device drivers, a “progress bar” is visible at the bottom of the display on Windows 2000 systems; in Windows XP and Windows Server 2003, this was replaced by an animated bar which does not represent actual progress. Prior to Windows XP, this part of the boot process took significantly longer; this is because the drivers would be initialized one at a time. On Windows XP and Server 2003, the drivers are all initialized asynchronously.

Session manager[edit]

Once all the Boot and System drivers have been loaded, the kernel (system thread) starts the Session Manager Subsystem (smss.exe).

Before any files are opened, Autochk is started by smss.exe.[12] Autochk mounts all drives and checks them one at a time to see whether or not they were cleanly unmounted. If autochk determines one or more volumes are dirty, it will automatically run chkdsk and provides the user with a short window to abort the repair process by pressing a key within 10 seconds (introduced in Windows NT 4.0 Service Pack 4; earlier versions would not allow the user to abort chkdsk). Since Windows 2000, XP and 2003 show no text screen at that point (unlike NT 3.1 to 4.0, which displayed a blue text screen), the user will see a different background picture holding a mini-text-screen in the center of the screen and show the progress of chkdsk there.

At boot time, the Session Manager Subsystem:

  • Creates environment variables (HKLMSYSTEMCurrentControlSetControlSession ManagerEnvironment)
  • Starts the kernel-mode side of the Win32 subsystem (win32k.sys). This allows Windows to switch into graphical mode as there is now enough infrastructure in place.
  • Starts the user-mode side of the Win32 subsystem, the Client/Server Runtime Server Subsystem (csrss.exe). This makes Win32 available to user-mode applications.
  • Creates virtual memory paging files (HKLMSYSTEMCurrentControlSetControlSession ManagerMemory Management)
  • Performs any rename operations (HKLMSYSTEMCurrentControlSetControlSession ManagerPendingFileRenameOperations) that are queued up. This allows previously in-use files (e.g. drivers) to be replaced as part of a reboot.
  • Executes any programs listed in HKLMSYSTEMCurrentControlSetControlSession ManagerBootExecute such as autocheck and convert.
  • Starts the Windows Logon Manager (winlogon.exe). Winlogon is responsible for handling interactive logons to a Windows system (local or remote).

The Session Manager stores its configuration at HKLMSYSTEMCurrentControlSetControlSession Manager. The exact operation of most of these items is based on the configuration set in the registry.

Authentication[edit]

Winlogon starts the Local Security Authority Subsystem Service (LSASS) and Service Control Manager (SCM), which in turn will start all the Windows services that are set to Auto-Start.[13] It is also responsible for responding to the secure attention sequence (SAS), loading the user profile on logon, and optionally locking the computer when a screensaver is running.

The login process is as follows:[14][15]

  • The Session Manager Subsystem starts winlogon.exe.
  • Winlogon starts the Service Control Manager (services.exe).
    • Starts the auto-start services.
    • Updates the Control Sets; the LastKnownGood control set is updated to reflect the current control set.
  • (Windows XP and later) Winlogon starts UIHost (logonui.exe), a full-screen graphical UI.
  • (Windows XP and earlier) Winlogon loads GinaDll (msgina.dll)
    • (Optional) Login prompt is displayed by GINA, and the user presses the Secure Attention Sequence (SAS) (Control-Alt-Delete).
    • Winlogon checks if the system is configured to log into a specific account automatically (AutoAdminLogon).
    • Login dialog is displayed by GINA
    • User enters credentials (username, password, and domain)
    • GINA passes credentials back to Winlogon
  • Winlogon passes credentials to LSASS
  • LSASS tries to use cached data in the LSA database (SYSTEM hive)
  • If there is none, LSASS determines which account protocol is to be used by using the Security Packages listed in the key HKLM/SYSTEM/CurrentControlSet/Control/Lsa:
    • msv1_0.dll implements the NT LAN Manager protocols. This package is used in stand-alone systems and domain-member systems for backward compatibility.
    • Kerberos.dll provides remote login by using Active Directory.
  • LSASS enforces the local security policy (checking user permissions, creating audit trails, doling out security tokens, etc.).
  • Control is passed back to Winlogon to prepare for passing the control to the user.
    • Create Windows Stations (WinSta0)[18]
    • Create the desktops (Winlogon, Default and ScreenSaver)[19]
    • It then starts the program specified in the Userinit value which defaults to userinit.exe. This value supports multiple executables.

If the user is trying to log into the local host then the HKLM/SAM key will be used as database. If the user is trying to log into another host then the NetLogon service is used to carry the data.

msv1_0.dll<->netlogon<->remote netlogon<->remote msv1_0.dll<->remote SAM

On Windows XP, GINA is only shown if the user presses the secure attention sequence.

Winlogon has support for plugins that get loaded and notified about specific events and LSASS also supports plugins (security packages). Some rootkits bundle Winlogon plugins because they are loaded before any user logs in. Some keys allow multiple comma-separated values to be supplied that allow a malicious program to be executed at the same time as a legitimate system file. The hashing algorithms used to store credentials in the SAM database are weak and can be brute-forced quickly on consumer hardware.

Userinit is the first program that runs with the user credentials. It is responsible to start all the other programs that compose the user shell environment.

The shell program (typically Explorer.exe) is started from the registry entry Shell= pointed to by the same registry entry in key HKLMSOFTWAREMicrosoftWindows NTCurrentVersionIniFileMappingsystem.iniBoot; its default value is SYS:MicrosoftWindows NTCurrentVersionWinlogon, which evaluates to HKLMSOFTWAREMicrosoftWindows NTCurrentVersionWinlogon.[20]

  • Userinit loads the user profile. There are a few types of user profiles and it can be local or remote. This process can be very slow if the user profile is of the “roaming” type.
  • User and Computer Group Policy settings are applied.
    • Run user scripts
    • Run machine scripts
    • Run proquota.exe
  • Runs the startup programs before the shell gets started.
  • Starts the shell configured in registry, which defaults to explorer.exe.
  • Userinit exits and the shell program continues running without a parent process.

Userinit runs startup programs from the following locations:[13]

  • HKLMSOFTWAREMicrosoftWindowsCurrentVersionRunOnce
  • HKLMSOFTWAREMicrosoftWindowsCurrentVersionPoliciesExplorerRun
  • HKLMSOFTWAREMicrosoftWindowsCurrentVersionRun
  • HKCUSoftwareMicrosoftWindows NTCurrentVersionWindowsLoad
  • HKCUSoftwareMicrosoftWindows NTCurrentVersionWindowsRun
  • HKCUSoftwareMicrosoftWindowsCurrentVersionRun
  • HKCUSoftwareMicrosoftWindowsCurrentVersionRunOnce
  • %ALLUSERSPROFILE%Start MenuProgramsStartup (this path is localized on non-English versions of Windows before Vista)
  • %USERPROFILE%Start MenuProgramsStartup (this path is localized on non-English versions of Windows before Vista)

Advanced options[edit]

With the advent of the new boot manager in Windows Vista, many components have been changed; one is the Advanced Boot Options menu that provides options for advanced boot modes (e.g., Safe Mode). Due to the implementation of fast startup in Windows 8 and up, access to the Advanced Boot Options menu has been disabled by default. However, access is still possible with a BCD modification. These are the possible boot modes:

  • Repair Your Computer – Boots Windows Recovery Environment (WinRE or Windows RE)
  • Safe Mode – Loads Safe Mode, a boot mode with minimal drivers and resources intended for malware removal or replacing faulty drivers.
  • Safe Mode with Networking – Loads Safe Mode along with the network drivers.
  • Safe Mode with Command Prompt – Loads Safe Mode with the Command Prompt as the shell instead of Windows Explorer. Windows Explorer can still be loaded by typing explorer at the command prompt.
  • Enable Boot Logging – Enables writing of ntbtlog.txt, a file that will log the boot process; listing drivers that loaded and drivers that did not.
  • Enable low resolution video – Disables the default graphics driver and uses the standard VGA driver. Intended in case the user changed the resolution to an unusable level (i.e. 320×200 at low refresh rates <24 Hz, 60 Hz>)
  • Last Known Good Configuration – Loads configuration based on the last successful boot process. Intended for Registry corruptions. This mode is removed in Windows 8 and later versions of Windows.
  • Directory Services Restore Mode – Boot mode used to reboot the Domain Controller in case it is not working as intended.
  • Debugging Mode – Boots while loading the kernel debugger.
  • Disable automatic restart on system failure – Disables the auto-reboot function after a Blue Screen of Death is experienced.
  • Disable early launch anti-malware driver – ELAM prechecks boot required drivers for signatures and tampering. Disabling ELAM is intended to allow booting on false positive driver checks but could also allow a tampered driver to load.[21]
  • Disable Driver Signature Enforcement – Disables the kernel setting that prohibits unsigned drivers from loading.
  • Start Windows Normally

The ABO menu is accessible by rapidly pressing or holding the F8 key before Windows boots. Starting from Windows 8 on UEFI, it can only be accessed by clicking Restart while holding the Shift key.

Remote booting and installation[edit]

To successfully boot, the client must support PXE booting and the Windows Deployment Services (WDS) component must be installed on the server. It is not installed by default. WDS is the successor of Remote Installation Services (RIS).

The PXE program is found on the BIOS or on a ROM chip on the network card.

PXE booting is not a technology specific to Windows and can also be used to start a Linux system. In fact, a Linux system can act as a server to service DHCP or TFTP.

PXE can be used to start Windows Setup to install the system on the client computer or to run the operating system from RAM. The latter, called Remote Boot, was introduced by Windows XP Embedded SP1[22] and is only available for this flavor of Windows.[23]

The general process for both methods is as follows:

  • PXE boots
  • DHCP request broadcast
  • (Optional) DHCP router redirects to the server
  • The server sends the Network Bootstrap Program (NBP) (PXEboot.com)[24] through TFTP
  • The NBP program downloads the required files through the BINL protocol

The Boot Information Negotiation Layer (BINL) is a Windows 2000 service running on the server that communicates with the client after the NBP was already loaded by the PXE.

See also[edit]

References[edit]

  1. ^ “Inside the Windows Vista Kernel – Startup Processes”. Microsoft. Retrieved October 1, 2010.
  2. ^ a b Microsoft (February 4, 2008). “Boot Configuration Data in Windows Vista” (DOCX). Retrieved April 18, 2015.
  3. ^ “Microsoft Longhorn”. Experience Longhorn. Experience Longhorn. Archived from the original on April 21, 2014.
  4. ^ Ritz, Andrew (2004). “EFI and Windows ‘Longhorn’“. Microsoft. Archived from the original (PPT) on June 9, 2004. Retrieved April 18, 2015.
  5. ^ de Boyne Pollard, Jonathan. “The Windows NT 6 boot process”. Frequently Given Answers.
  6. ^ “Winlogon and GINA”. MSDN. Microsoft. Retrieved December 4, 2014.
  7. ^ “Boot Sequence of Windows Multi-Boot – Multibooters.com”. www.multibooters.com. Retrieved November 19, 2020.
  8. ^ “In Windows Server 2003, you may not be able to start a computer from a GPT disk when the computer has an Itanium processor (Revision: 2.2)”. Microsoft Support. Microsoft Corporation. Retrieved October 29, 2011.
  9. ^ “Unified Extended Firmware Interface support in Windows Vista (Revision: 1.5)”. Microsoft Support. Microsoft Corporation. October 26, 2007. Retrieved October 30, 2011.
  10. ^ “Boot from VHD in Win7”. TechNet Edge. Microsoft Corporation. February 20, 2009. Retrieved October 30, 2011.
  11. ^ “Windows, NT Object Manager”. Channel 9. Microsoft Corporation. June 3, 2005. Retrieved October 24, 2011.
  12. ^ “Resource Kit”. Microsoft Corporation. Archived from the original on March 11, 2007.
  13. ^ a b “Troubleshooting the Startup Process”. Windows XP Resource Kit. Microsoft Technet. November 3, 2005. Retrieved October 24, 2011.
  14. ^ Ionescu, Alex; Russinovich, Mark; Solomon, David A. (2012). Windows internals (6th ed.). Redmond, Wash.: Microsoft Press. pp. 555, 77. ISBN 978-0735648739.
  15. ^ Ionescu, Alex; Russinovich, Mark; Solomon, David A. (2012). Windows internals (6th ed.). Redmond, Wash.: Microsoft. pp. 522–527. ISBN 978-0735665873.
  16. ^ alvinashcraft. “Credential Providers in Windows 10 – Win32 apps”. learn.microsoft.com. Retrieved March 20, 2023.
  17. ^ “Create Custom Login Experiences With Credential Providers For Windows Vista”. MSDN Magazine. Microsoft Corporation. Retrieved April 13, 2014.
  18. ^ “Window Stations”. MSDN. Microsoft Corporation. Retrieved April 19, 2014.
  19. ^ “Desktops”. MSDN. Microsoft Corporation. Retrieved April 19, 2014.
  20. ^ “Different Shells for Different Users”. Microsoft Corporation. Retrieved March 16, 2014.
  21. ^ QuinnRadich. “Early launch antimalware – Win32 apps”. docs.microsoft.com. Retrieved December 14, 2021.
  22. ^ “Deploying Windows XP Embedded Remote Boot”. MSDN. Microsoft Corporation. Retrieved April 18, 2014.
  23. ^ “Remote Boot Overview”. MSDN. Microsoft Corporation. Retrieved April 19, 2014.
  24. ^ “Managing Network Boot Programs”. TechNet. Microsoft Corporation. Retrieved April 18, 2014.

Further reading[edit]

  1. Russinovich, Mark; Solomon, David A. (2005). “Startup and Shutdown”. Microsoft Windows Internals (4th ed.). Microsoft Press. pp. 251–273. ISBN 0-7356-1917-4.
  2. Minasi, Mark; Enck, John (June 1998). “Troubleshooting NT Boot Failures”. Administrator’s Survival Guide: System Management and Security. Windows IT Library. ISBN 1-882419-88-X. Retrieved February 15, 2006.
  3. “Description of PXE Interaction Among PXE Client, DHCP, and RIS Server (Revision 2.4)”. Microsoft Support. Microsoft Corporation. February 28, 2007. Retrieved October 24, 2011.
  4. “Definition of the RunOnce Keys in the Registry (revision 2.3)”. Microsoft Support. Microsoft Corporation. January 19, 2007. Retrieved October 24, 2011.
  5. “Available switch options for the Windows XP and the Windows Server 2003 Boot.ini files (revision 6.3)”. Microsoft Support. Microsoft Corporation. November 28, 2007. Retrieved October 24, 2011.

External links[edit]


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