Operating Software is the first program run on a computer when computer boots up.

The BIOS chip tells it to look in a fixed place, usually on the lowest - numbered hard disk (the boot disk) for a special program called a boot loader (under Linux the boot loader is called Grub or LILO). The boot loader is pulled into memory and started. The boot loader's job is to start the real operating system.

The first thing a computer has to do when it is turned on is start up a special program called an operating system. The operating system's job is to help other computer programs to work by handling the messy details of controlling the computer's hardware. ... The boot loader's job is to start the real operating system.