The boot loader usually is in the first sector of the hard drive, usually called the Master Boot Record.

Which was the first bootloader program used in Linux?

The GRUB (Grand Unified Bootloader) is a bootloader available from the GNU project. A bootloader is very important as it is impossible to start an operating system without it. It is the first program which starts when the program is switched on.

Where is the bootstrap program stored?

Full bootstrap program is stored in the boot blocks at a fixed location on the disk. A disk which has a boot partition is called a boot disk. The code in the boot ROM basically instructs the read controller to read the boot blocks into the memory and then starts the execution of code.