Alternatively referred to as bootstrapping, bootloader, or boot program, a bootstrap loader is a program that resides in the computer's EPROM, ROM, or another non-volatile memory. It is automatically executed by the processor when turning on the computer. The bootstrap loader reads the hard drives boot sector to continue to load the computer's operating system. The term bootstrap comes from the old phrase "Pull yourself up by your bootstraps."
When the computer is turned on or restarted, the bootstrap loader first performs the power-on self-test, also known as POST. If the POST is successful and no issues are found, the bootstrap loader loads the operating system for the computer into memory. The computer can then access, load, and run the operating system.