10.5.2 Boot block boot blocks-operating system tutorial

A simple article clearly explains the startup process of the computer operating system

OPERATING SYSTEM Concepts
ABRAHAM Silberschatz
PETER BAER Galvin
GREG GAGNE

10.5.2 Boot Block

For a computer-to-start running-for instance, when it was powered up orrebooted-it must had an initial program to run. This initial bootstrap program tends to is simple. It initializes all aspects of the system, from CPUs registers to device controllers and the contents of main memory, and th En starts the operating system. To do it job, the bootstrap program finds the Operating-system kernel on disk, loads this kernel into memory, and jumps T Oan initial address to begin the Operating-system execution.

For most computers, the bootstrap are stored in read-only memory (ROM). This is convenient, because ROM needs no initialization and are at a fixed location that the processor can start E Xecuting when powered up or reset. And,since ROM is read only, it cannot are infected by a computer virus. The problem isthat Changing this bootstrap code requires changing the ROM hardware chips. For this reason, most systems store a tiny bootstrap loader program on the boot ROM whose only job was to bring in a full B Ootstrap program from disk. The full bootstrap program can be changed easily:a new version is simply written onto the disk. The full bootstrap program was stored in the ' boot blocks ' at a fixed location on the disk. A disk, a boot partition is called a boot disk or system disk .

The code in the boot ROM instructs the disk controller to the read the boot blocks into memory (no device drivers is loaded a T) and then starts executing that code. The full bootstrap program was more sophisticated than the bootstrap loader in the boot ROM. It is able to load the entire operating systemfrom a non-fixed where on disk and to start the operating system running. Even so, the full bootstrap code may small.

Let's consider as an example the boot process in Windows. First, note that Windows allows a hard disk to is divided into partitions, and one partition-identified as theboot partition-contains the operating system and devicedrivers. The Windows system places its boot code of the first sector on the harddisk, which it terms theMaster Boot Record, orMBR. Booting begins by running code which is resident in the system ' s ROM memory. This code directs the system to read the boot code from the MBR. In addition to containing boot code, the MBR contains a table listing the partitions for the hard disk and a flag Indicati ng which partition the system is to being booted from, as illustrated in Figure 10.9. Once The system identifies the boot partition, it reads the first sector from this partition (which is called theBoot Sector) and continues with the remainder of the boot process, which includes loading the various subsystems and system services.

10.5.2 Boot block boot blocks-operating system tutorial