Page 252 - DCAP103_Principle of operating system

Unit 7: Secondary Storage Structure

The controller automatically does the ECC processing whenever a sector is Notes
read or written. Most hard disks are low-level formatted at the factory as a
part of the manufacturing process.

7.5.2 Boot Block
For a computer to start running-for instance, when it is powered up or rebooted-it needs
to have an initial program to run. This initial bootstrap program tends to be simple. It
initializes all aspects of the system, from CPU registers to device controllers and the contents
of main memory, and then starts the operating system. To do its job, the bootstrap program
finds the operating system kernel on disk, loads that kernel into memory, and jumps to an
initial address to begin the operating-system execution. For most computers, the bootstrap
is stored in read-only memory (ROM). This location is convenient, because ROM needs no
initialization and is at a fixed location that the processor can start executing when powered
up or reset. And, since ROM is read only, it cannot be infected by a computer virus. The
problem is that changing this bootstrap code requires changing the 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 full bootstrap program can be
changed easily: A new version is simply written onto the disk. The full bootstrap program
is stored in a partition called the boot blocks, at a fixed location on the disk. A disk that has
a boot partition is called a boot disk or system disk.
The code in the boot ROM instructs the disk controller to read the boot blocks into memory (no
device drivers are loaded at this point), and then starts executing that code. The full bootstrap
program is more sophisticated than the bootstrap loader in the boot ROM; it is able to load the
entire operating system from a non fixed location on disk, and to start the operating system
running.
Even so, the full bootstrap code may be small. For example, MS-DOS uses one 512-byte block
for its boot program.

Be careful during the disk management because the disk is very important
term in computer system.

7.5.3 Bad Blocks

Because disks have moving parts and small tolerances (recall that the disk head flies just above
the disk surface), they are prone to failure. Sometimes the failure is complete, and the disk needs
to be replaced, and its contents restored from backup media to the new disk. More frequently,
one or more sectors become defective. Most disks even come from the factory with bad blocks.
Depending on the disk and controller in use, these blocks are handled in a variety of ways.
On simple disks, such as some disks with IDE controllers, bad blocks are handled manually.
For instance, the MS-DOS format command does a logical format and, as a part of the process,
scans the disk to find bad blocks. If format finds a bad block, it writes a special value into the
corresponding FAT entry to tell the allocation routines not to use that block. If blocks go bad
during normal operation, a special program (such as chkdsk) must be run manually to search
for the bad blocks and to lock them away as before. Data that resided on the bad blocks usually
are lost.

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