Windows NT Startup Process

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Windows NT startup processFrom Wikipedia, the free encyclopedia

Simplified VersionThis is the (simplified) boot sequence for Windows NT, 2000, XP and 2003: BIOS: performs Power On Self Test (POST) BIOS: loads MBR from the boot device specified/selected by the BIOS MBR: contains a small amount of code that reads the partition table, the first partition marked as active is determined to be the system volume MBR: loads the boot sector from the system volume BOOT SECTOR: reads the root directory of the system volume at loads NTLDR NTLDR: reads BOOT.INI from the system volume to determine the boot drive (presenting a menu if more than 1 entry is defined) NTLDR: loads and executes NTDETECT.COM from the system volume to perform BIOS hardware detection NTLDR: loads NTOSKRNL.EXE, HAL.DLL, BOOTVID.DLL (and KDCOM.DLL for XP upwards) from the boot (Windows) volume NTLDR: loads \WINDOWS\SYSTEM32\CONFIG\SYSTEM which becomes the system hive HKEY_LOCAL_MACHINE\System NTLDR: loads drivers flagged as "boot" defined in the system hive, then passes control to NTOSKRNL.EXE NTOSKRNL.EXE: brings up the loading splash screen and initializes the kernel subsystem NTOSKRNL.EXE: starts the boot-start drivers and then loads & starts the system-start drivers NTOSKRNL.EXE: creates the Session Manager process (SMSS.EXE) SMSS.EXE: runs any programs specified in BootExecute (e.g. AUTOCHK, the native API version of CHKDSK) SMSS.EXE: processes any delayed move/rename operations from hotfixes/service packs replacing in-use system files SMSS.EXE: initializes the paging file(s) and the remaining registry hives ** before this step completes, bugchecks will not result in a memory dump as we need a working page file on the boot (Windows) volume ** SMSS.EXE: starts the kernel-mode portion of the Win32 subsystem (WIN32K.SYS) SMSS.EXE: starts the user-mode portion of the Win32 subsystem (CSRSS.EXE) SMSS.EXE: starts WINLOGON.EXE WINLOGON.EXE: starts the Local Security Authority (LSASS.EXE) WINLOGON.EXE: loads the Graphical User Identification and Authentication DLL (MSGINA.DLL by default) WINLOGON.EXE: displays the logon window

WINLOGON.EXE: starts the services controller (SERVICES.EXE) ** at this point users can logon ** SERVICES.EXE: starts all services markes as automatic ---------

Full ExplanationNOTES: The SYSTEM volume is the partition from which the boot process starts, containing the MBR, boot sector, NTLDR, NTDETECT.COM & BOOT.INI The BOOT volume is the partition which contains the Windows folder - this can be a logical partition

The Windows NT startup process is the process by which Microsoft's Windows NT, Windows 2000, Windows XP and Windows Server 2003 operating systems initialize. In Windows Vista and later, this process has changed slightly (see Windows Vista startup process).


1 Boot Sequence 2 Boot loader phase 3 Kernel loading phase 4 Session Manager 5 Winlogon 6 Remote booting & installation 7 Additional information 8 See also 9 References 10 External links

Boot Sequence

Upon poweron, the CPU runs the instructions located at the real mode memory location 0xFFFF0 of the BIOS. 0xFFFF0 usually contains a jump instruction (jmp in assembly language) pointing to the BIOS startup programming. In any case, the startup code runs the POST to check & initialize required devices.

The BIOS checks a preconfigured list of nonvolatile storage devices (boot device sequence) for the first bootable device on the list. The last 2 bytes of the first sector contains 0xAA55 (a boot signature) indicates a bootable device. Once identified, BIOS loads the boot sector contents (from the MBR in the case of a HDD) into memory & transfers control to (executes) this boot code. This boot code scans for a partition flagged as bootable (often called "active") in its partition table. The boot code loads the boot sector code (the volume boot record) from that partition and executes (jumps to) it. The boot sector code is mostly OS specific, which loads and executes the kernel. If this is no partition flagged as active, or the active partition's boot sector is invalid, the MBR loads a secondary loader to select the partition via user input.

Boot loader phaseFor more details on this topic, see NTLDR. The boot loader phase varies by platform. Since the earlier phases are not specific to the OS, the boot process is considered to start:

For x86 or x64: when the partition boot sector code is executed in real mode and loads NTLDR For IA-64: when the IA64ldr.efi EFI program is executed (later referred as simply IA64ldr)

From that point, the boot process continues as follows: An NTLDR file, located in the root folder of the boot disk, is composed of two parts. The first is the StartUp module and immediately followed by the OS loader (osloader.exe), both stored within that file. When NTLDR is loaded into memory and control is first passed to StartUp module, the CPU is operating in real mode. StartUp module's main task is to switch the processor into protected mode, which facilitates 32-bit memory access, thus allowing it to create the initial Interrupt descriptor table, Global Descriptor Table, page tables and enable paging. This provides the basic operating environment on which the operating system will build. StartUp module then loads and launches OS loader. NTLDR's OS loader includes basic functionality to access IDE-based disks formatted for NTFS or FAT file systems, or CDFS (ISO 9660), ETFS or UDFS in newer operating system versions. Disks are accessed through the system BIOS, through native ARC routines on ARC systems, or via network using TFTP protocol. All BIOS calls are done through virtual 8086 mode beyond this point, as the BIOS can not be directly accessed within protected mode. If the boot disk is a SCSI disk and the SCSI controller is not using real-mode INT 0x13, an additional file, Ntbootdd.sys is loaded to handle disk access in place of the default routines. This is a copy of the same SCSI miniport driver that is used when Windows is running.

The boot loader then reads the contents of boot.ini to locate information on the system volume. 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. At this point, the screen is cleared, and in the Windows 2000 or later versions of NTLDR and IA64ldr which support system hibernation, the root directory default volume as defined in boot.ini is searched for a hibernation file, hiberfil.sys. If this file is found and an active memory set is found in it, the contents of the file (which will match the amount of physical memory in the machine) are loaded into memory, and control is transferred into the Windows kernel at a point from which hibernation can be resumed. The file is then immediately marked as non-active, so that a crash or other malfunction cannot cause this (now-outdated) memory state to be re-loaded. If a state resume fails, the next time NTLDR runs it will ask the user whether to try resuming again or to discard the file and proceed with normal booting. If boot.ini contains more than one operating system entry, a boot menu is displayed to the user, allowing the user to choose which operating system is to be loaded. 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 NTLDR loads the associated "boot sector" file listed in boot.ini (by default, this is bootsect.dos if no file name is specified) and passes execution control to it. If an NT-based operating system is selected, NTLDR runs, which gathers basic information about the computer's hardware as reported by the BIOS. At this point in the boot process, NTLDR clears the screen and displays a textual progress bar, (which is often not seen on XP or 2003 systems, due to their initialization speed); Windows 2000 also displays the text "Starting Windows..." underneath. If the user presses F8 during this phase, the advanced boot 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. Once a boot mode has been selected (or if F8 was never pressed) booting continues. If an x64 version of Windows is being booted (Windows XP Professional x64 Edition or Windows Server 2003 x64 Editions), the CPU is switched into long mode, enabling 64-bit addressing. Next, the Windows kernel Ntoskrnl.exe and the Hardware Abstraction Layer hal.dll are read into memory. If either of these files fails to load, the message "Windows could not start because the following file was missing or corrupt" is displayed to the user, and the boot process comes to a halt. 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). This information (along with information on all detected hardware and Windows Services) is stored in

the HKLM\SYSTEM portion of the registry, in a set of registry keys collectively called a Control Set. Multiple control sets (typically two) are kept, in the event that the settings contained in the currently-used one prohibit the system from booting. HKLM\SYSTEM 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 picks the "real" control set being used based on the values set in the HKLM\SYSTEM\Select registry key: Default

will be NTLDR or IA64ldr's choice if nothing else overrides this. If the value of the Failed key matches Default, then NTLDR or IA64ldr 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 has chosen Last Known Good Configuration from the boot menu, 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. For the purposes of booting, a driver is either a "Boot" driver that is loaded by NTLDR or IA64ldr prior to starting the kernel and started before system drivers by the kernel, a "System" driver, which is loaded and started by ntoskrnl.exe after the boot drivers or an "Automatic" driver which is loaded much later when the GUI already has been started. "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 ntoskrnl.exe will need to get started with loading other drivers, and the rest of the operating system. "System" drivers cover a wider range of core functionality, including the display driver, CD-ROM support, and the TCP/IP stack. The appropriate file system driver for the partition type (NTFS, FAT, or FAT32) which the Windows installation resides on is also loaded. With this finished, control is then passed from NTLDR or IA64ldr to the kernel. At this time, Windows NT shows the famous "blue screen" displaying number of CPUs and the amount of memory installed, whilst Windows 2000, XP and 2003 switch into a graphical display mode to display the Windows logo, unless the /noguiboot or /sos switches are present in boot.ini.

Kernel loading phase1. 2. 3. 4. 5. ntoskrnl.exe (the kernel) hal.dll (type of hardware abstraction layer) kdcom.dll (Kernel Debugger HW Extension DLL) bootvid.dll (for the windows logo and side-scrolling bar) config\system registry 1. HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\BootExecute

2. Process services in the order provided 3. *HKLM\SYSTEM\CurrentControlSet\Control\ServiceGroupOrder 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,[1] 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 ManagerOnce 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 [1] is started by smss.exe. Autochk mounts all drives and checks them one at a time whether they were not shut down cleanly before. In that case it will automatically run chkdsk, however just before the user can abort this process by pressing any key within 10 seconds (this was implemented in Windows NT 4.0 Service Pack 4, in earlier versions you could not skip chkdsk). Since Windows 2000, XP and 2003 show no text screen at that point (unlike NT, which still shows the blue text screen), they will show 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 (HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment) 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 (HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management)

Any rename operations queued up are performed. This allows previously in-use files (e.g. drivers) to be replaced as part of a reboot. Starts the Windows Logon Manager (winlogon.exe). Winlogon is responsible for handling interactive logons to a Windows system (local or remote). The Graphical Identification aNd Authentication (GINA) library is loaded inside the Winlogon process, and provides support for logging in as a local or Windows domain user.

The Session Manager stores its configuration atHKLM\SYSTEM\CurrentControlSet\Control\Session Manager.

The exact operation of most

of these items is based on the configuration set in the registry.


"Begin logon" dialog box in Windows XP. For more details on this topic, see Winlogon. 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 [2]. 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 logon process is as follows:

Winlogon calls GINA (Optional) Logon prompt is displayed by GINA, and the user presses the secure attention sequence (Control-Alt-Delete) Logon dialog is displayed by GINA User enters credentials (username, password, and domain) GINA passes credentials back to Winlogon Winlogon passes credentials to LSASS, which determines which account database is to be used: o Local SAM o Domain SAM o Active Directory LSASS enforces the local security policy (checking user permissions, creating audit trails, doling out security tokens, etc.).

After a user has successfully logged in to the machine, Winlogon does the following:

Updates the Control Sets; the LastKnownGood control set is updated to reflect the current control set. User and Computer Group Policy settings are applied. Starts the shell program (typically Explorer.exe) from the registry entry Shell= pointed to by the same registry entry in key is

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\IniFileMapping\system.ini\Boot [3]; its default value SYS:Microsoft\Windows NT\CurrentVersion\Winlogon, which evaluates to HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon.

Startup programs are run from the following locations [4]:o o o o o o o o o HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Explorer\ Run HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run HKCU\Software\Microsoft\Windows NT\CurrentVersion\Windows\Load HKCU\Software\Microsoft\Windows NT\CurrentVersion\Windows\Run HKCU\Software\Microsoft\Windows\CurrentVersion\Run HKCU\Software\Microsoft\Windows\CurrentVersion\RunOnce %ALLUSERSPROFILE%\Start Menu\Programs\Startup\ (please note that this

path is localized on non-English versions of Windows before Vista) %USERPROFILE%\Start Menu\Programs\Startup\ (please note that this path is localized on non-English versions of Windows before Vista)

Winlogon's responsibilities have changed significantly from the above in Windows Vista.

Remote booting & installation