Configuring Startup and Troubleshooting Startup Issues : Understanding the Startup Process

After the POST, the settings that are stored in the nonvolatile memory, such as boot order, determine the devices that the computer can use to start an operating system. In addition to floppy disks or hard disks attached to Advanced Technology Attachment (ATA), Serial ATA, and small computer system interface (SCSI) controllers, computers can typically start an operating system from other devices, such as the following:

It is possible to specify a custom boot order, such as CDROM, Floppy, Hard Disk. When you specify CDROM, Floppy, Hard Disk as a boot order, the following events occur at startup:

There are exceptions in which code on bootable media transfers control to the hard disk. For example, when you start your system by using the bootable Windows DVD, Windows Setup checks the hard disk for Windows installations. If one is found, you have the option of bypassing DVD startup by not responding to the Press Any Key To Boot From CD Or DVD prompt that appears. This prompt is actually displayed by the startup program located on the Windows DVD, not by your computer's hardware.

If startup fails during the initial startup phase, you are experiencing a problem with the BIOS configuration, the disk subsystem, or the file system. The following error message is common during this phase. It indicates that none of the configured bootable media types was available.

Non-system disk or disk error
Replace and press any key when ready

If you changed the disk configuration recently, verify that all cables are properly connected and jumpers are correctly configured. If booting from the hard disk, verify that all removable media have been removed. If booting from a CD or DVD, verify that the BIOS is configured to start from the CD or DVD and that the Windows medium is present. If the disk subsystem and BIOS are configured correctly, the problem may be related to the file system. 

If you boot from the hard disk, the computer reads the boot code instructions located on the MBR. The MBR is the first sector of data on the startup hard disk. The MBR contains instructions (called boot code) and a table (called a partition table) that identify primary and extended partitions. The BIOS reads the MBR into memory and transfers control to the code in the MBR.

The computer then searches the partition table for the active partition, also known as a bootable partition. The first sector of the active partition contains boot code that enables the computer to do the following:

If an active partition does not exist or if boot sector information is missing or corrupt, a message similar to any of the following might appear:

If an active partition is successfully located, the code in the boot sector locates and starts Windows Boot Loader (WinLoad) and the BIOS transfers execution to it.

Startup for EFI computers initially differs from startup for BIOS computers. EFI computers have a built-in boot manager that enables the computer's hardware to choose from multiple operating systems based on user input. When you install Windows 7 on an EFI computer, Windows adds a single entry to the EFI boot manager with the title Windows Boot Manager. This entry points to the \Efi\Microsoft\Boot\Bootmgfw.efi 32-bit or 64-bit EFI executable program—the Windows Boot Manager. This is the same Windows Boot Manager that is eventually loaded on BIOS-based computers. Windows configures the EFI boot manager to display the EFI startup menu for only 2 seconds and then load the Windows Boot Manager by default to minimize complexity and startup time.

If you install a different operating system or manually change the EFI boot manager settings, EFI might no longer load the Windows Boot Manager. To resolve this problem, use the Startup Repair tool. Alternatively, you might be able to update the EFI boot manager settings manually using your computer's built-in EFI tools. For more information about configuring EFI, consult your computer's documentation.

The Windows Boot Loader reads control set information from the registry key HKEY_LOCAL_MACHINE\SYSTEM, which is stored in the file %SystemRoot%\System32\Config\System, so that the kernel can determine which device drivers need to be loaded during startup. Typically, several control sets exist, with the actual number depending on how often system configuration settings change.

The HKEY_LOCAL_MACHINE\SYSTEM subkeys used during startup are:

The Windows Boot Loader uses the control set identified by the \Select\Default value unless you choose the Last Known Good Configuration from the Advanced Boot Options menu.

The kernel creates the registry key HKEY_LOCAL_MACHINE\HARDWARE, which contains the hardware data collected at system startup. Windows supports an extensive set of devices, with additional drivers not on the Windows operating system DVD provided by hardware manufacturers. Drivers are kernel-mode features required by devices to function within an operating system. Services are features that support operating system and application functions and act as network servers. Services can run in a different context than user applications and typically do not offer many user-configurable options.

For example, the Print Spooler service does not require a user to be logged on to run and functions independently of the user who is logged on to the system. Drivers generally communicate directly with hardware devices, whereas services usually communicate with hardware through drivers. Driver and service files are typically stored in the %SystemRoot%\System32 and %SystemRoot%\System32\Drivers folders and use .exe, .sys, or .dll file name extensions.

Drivers are also services. Therefore, during kernel initialization, the Windows Boot Loader and Ntoskrnl use the information stored in the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Servicename registry subkeys to determine both the drivers and services to load. In the Servicename subkeys, the Start entry specifies when to start the service. For example, the Windows Boot Loader loads all drivers for which Start is 0, such as device drivers for hard disk controllers. After execution is transferred to the kernel, the kernel loads drivers and services for which Start is 1.

Table 1 lists the values (in decimal) for the registry entry Start. Boot drivers (those for which Start is 0) and file system drivers are always loaded regardless of the value of Start because they are required to start Windows.

Table 2 lists some of the values (in decimal) for the Type registry entry.

Some drivers and services require that conditions, also known as dependencies, be met. You can find dependencies listed under the DependOnGroup and DependOnService entries in the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Servicename subkey for each service or driver. The Services subkey also contains information that affects how drivers and services are loaded. Table 3 lists some of these other entries.

After all entries that have Boot and Startup data types are processed, the kernel starts the Session Manager (Smss.exe), a user process that continues to run until the operating system is shut down. The Session Manager performs important initialization functions, such as:

Session Manager searches the registry for service information contained in the following subkeys:

If startup fails during the kernel loading phase after another operating system was installed on the computer, the cause of the problem is likely an incompatible boot loader. Boot loaders installed by versions of Windows prior to Windows Vista cannot be used to start Windows Vista or Windows 7. Use System Recovery to replace startup files with Windows startup files.

Otherwise, if startup fails during the kernel loading phase, use boot logging to isolate the failing feature. Then use safe mode to disable problematic features (if possible) or use System Recovery to replace problematic files.