Mbr uses this table to decide which partition has to

MBR uses this table to decide which partition has to be activated
depending on the choice exercised by the user at boot time. Of course, in some systems

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there may be no choice (e.g., there is a single partition or only one partition has an
operating system associated with it).
Partition Start address
{platter, track, sector}
End address
{platter, track, sector}
OS
1 {1, 10, 0} {1, 600, 0} Linux
2 {1, 601, 0} {1, 2000, 0} MS Vista
3 {1, 2001, 0} {1, 5000, 0} None
4 {2, 10, 0} {2, 2000, 0} None
5 {2, 2001, 0} {2, 3000, 0} None
Table 11.4: Partition Table Data Structure
Table 11.4 shows several partitions. Depending on the OS to be booted up, the MBR
program will activate the appropriate partition (in this case either partition 1 or 2). Note
that partitions 3-5 do not have any OS associated with it. They may simply be logical
“drives” for one or the other operating system.
Figure 11.18 shows the layout of information in each partition. Other than the very first
entry (boot block), the actual layout of information in the partition varies from file system
to file system. However, to keep the discussion concrete, we will assume a particular
layout and describe the functionality of each entry. The chosen layout of information is
close to traditional Unix file systems.
Figure 11.18: Layout of each partition

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Let us review each entry in the partition.
• Boot block is the very first item in every partition. MBR reads in the boot block of
the partition being activated. The boot block is simply a program (just like MBR)
that is responsible for loading in the operating system associated with this partition.
For uniformity, every partition starts with a boot block even if there is no OS
associated with a particular partition (entries 3-5 in Table 11.4).
• Superblock contains all the information pertinent to the file system contained in this
partition. This is the key to understanding the layout of the rest of this partition. The
boot program (in addition to loading the operating system or in lieu thereof) reads in
the superblock into memory. Typically, it contains a code usually referred to as the
magic number that identifies the type of file system housed in this partition, the
number of disk blocks, and other administrative information pertaining to the file
system.
• The next entry in the partition contains the data structures for storage management in
this partition. The data structures will be commensurate with the specific allocation
strategy being employed by the file system (see Section 11.2). For example, it may
contain a bit map to represent all the available disk data blocks (i.e., the free list). As
another example, this entry may contain the FAT data structure that we mentioned
earlier (see Section 11.2.4).
• The next entry in the partition corresponds to the per-file information maintained by
the file system. This data structure is unique to the specifics of the file system. For
example, as we mentioned earlier (see Section 11.3.1), in Unix file system, every file
has a unique number associated with it called the i-node number. In such a system,
this entry may simply be a collection of all the i-nodes in the file system.