NTLDR - Startup Process

Startup Process

For more details on this topic, see Windows NT startup process.

When a PC is powered on its BIOS follows the configured boot order to find a bootable device. This can be a harddisk, floppy, CD/DVD, network connection, USB-device, etc. depending on the BIOS. In the case of a floppy the BIOS interprets its boot sector (first sector) as code, for NTLDR this could be a NTLDR boot sector looking for the ntldr file on the floppy. For a harddisk the code in the Master Boot Record (first sector) determines the active partition. The code in the boot sector of the active partition could then be again a NTLDR boot sector looking for ntldr in the root directory of this active partition. In a more convoluted scenario the active partition can contain a Vista boot sector for the newer Vista boot manager with an {ntldr} entry pointing to another partition with a NTLDR boot sector.

When booting, the loader portion of NTLDR does the following in order:

1. Accesses the file system on the boot drive (either FAT or New Technology File System, NTFS).
2. If Windows was put in the hibernation state, the contents of hiberfil.sys are loaded into memory and the system resumes where it left off.
3. Otherwise, reads boot.ini and prompts the user with the boot menu accordingly.
4. If a non NT-based OS is selected, NTLDR loads the associated file listed in boot.ini (bootsect.dos if no file is specified or if the user is booting into a DOS based OS) and gives it control.
5. If an NT-based OS is selected, NTLDR runs ntdetect.com, which gathers information about the computer's hardware. (If ntdetect.com hangs during hardware detection, there is a debug version called ntdetect.chk that can be found on Microsoft support.)
6. Starts Ntoskrnl.exe, passing to it the information returned by ntdetect.com.