Expert: John OConnor - 3/3/2010

Question
It is pleasant to talk with you sir,
sir we know boot strap will boot firstly when switch on the computer next it load operating system and rest things and run.
But my question is how the boot strap loader itself going to load for the first time.

Answer
Hi there, and thanks for your questions.

When a computer is first turned on, the BIOS does a system check (called the POST, or Power On System Test).  Once all of the checks are done, the BIOS looks for the first hard drive in its register that is marked as "bootable".  Marking a drive bootable is done when you format it (using fdisk or a tool like it).

Once it finds the drive marked "bootable", the BIOS looks at the Master Boot Record of the bootable disk by default.  If there is code there, the BIOS executes it.  This is the point at which the bootloader takes control.

The MBR (Master Boot Record) starts at sector 0 of a disk and is about 512 bytes in size.  From there, you're technically in the boot loader (in Grub, it's called "stage 1").  From here, the initial (stage 1) bootloading code can boot the drive directly, or it can point to another, more advanced bootloader (ie: Grub stage 1.5 in Linux).

Hope this answers your questions!