Boot Block - Disk Management

A program at some fixed location on a hard disk, floppy disk or other media, which is loaded when the computer is turned on or rebooted and which controls the next phase of loading the actual operating system. The loading and execution of the boot block is usually controlled by firmware in ROM or PROM.
There is an initial program that is run whenever a computer starts running. This initial program initializes all aspects of the system, from CPU registers to device controllers and the contents of the main memory, and then starts the operating system.
Generally, the bootstrap is stored in read-only-memory (ROM) because ROM needs no initialization, and is at a fixed location that the processor can start executing when powered up or reset. The problem is that changing this bootstrap code requires changing ROM hardware chips. For this reason, most systems store a tiny bootstrap loader program in the boot ROM, whose only job is to bring in a full bootstrap program from disk.
The code in the boot ROM instructs the disk controller to read the boot blocks into memory, and then starts executing the code. The full bootstrap program is more sophisticated than the bootstrap loader in the boot ROM, and is able to load the entire operating system from a non-fixed location on disk, and to start the operating system running.