Technical information

10.7 DOS/Windows Boot Sequence

The first program executed in the boot sequence is built into your computer's motherboard. This

program is called the BIOS. The BIOS search for the next program to execute. It will look in the place

you want it to – Floppy, CD, hard drive, etc.

If you are booting an OS from a hard drive then the next program is, unsurprisingly, on the hard drive.

It is always right at the very beginning of the drive, starting on the very first byte of the very first sector.

This program is commonly called the MBR (Master Boot Record), but this is misleading because the

MBR contains the boot-program and the partition table. The most common name for the boot-program

part of the MBR is the Initial Program Loader (IPL). Just like the BIOS program the IPL is not usually

specific to any OS. The Microsoft IPL is very small and quite limited and its main job is just to find and

start that next program in the chain. It looks at the partition table to see if any of the entries there has a

flag to indicate an active partition. If it found one then it goes to the very first byte of that partition and

starts the little program that it finds there.

The third little program in the chain is at the very beginning of the partition. This one is called the

Partition Boot Record (PBR). Now the PBR will do its job and then start the next program. However,

unlike the BIOS and IPL, the PBR is operating system specific and needs to know the name and

location of the file it has to start. This next file will be different for various operating systems, so during

the install of an OS the PBR will be written with the information necessary to find the correct file. For

WinNT before Vista this will be ntldr, which will always just be in the root of the partition. That is it will

not be inside any folder or directory, but just right there on its own, next to the Windows and Program

Files folders.

For all WinNT before Vista the ntldr will be the 4th and last program in the boot sequence chain. It's

called the boot-loader and it is the one that does the actual job of starting Windows from the System32

folder.

For Win 2K/XP etc. the boot sequence is: - BIOS - IPL - PBR - ntldr - Windows

In these graphic the MBR is shown as a separate section at the very start of the hard drive. It is

indeed separate and not connected in any way to the following partitions. Convention is to reserve a

small section of the drive specifically for the MBR to reside on. I’ve shown the PBR as a separate

section but it is actually a part of the partition it is in. Windows reserves the first 16 sectors of its

partition to be used exclusively for the partition boot record.