Week 6 - The LOGICAL Structure of disks

Question 1

Desrcibe the basic logical structure of a disk

Answer 1

The OS doesn’t really care about the physical structure of the HDD, it needs logical representation.

Logical structure consists of:

MBR (Master Boot Record)

PARTITIONS

UNALLOCATED SPACE

Question 2

Describe the MBR

Answer 2

The master boot record occupies the first sector on a HDD (LBA 0). It is one sector in size (512 bytes)

It contains:

• the PARTITION TABLE (max 64 bytes)
• BOOT CODE (instructions for start up executed when the computer is switched on). Boot code is 446 bytes

SIGNATURE (2 bytes) 0x 55 AA - always at the end of the MBR (the last 2 bytes)

Question 3

What are partitions?

Answer 3

Partitions are areas of the disk that can be used to store a file system.

The file system is a set of structures that govern the location of data on a disk.

Common file systems are NTFS, FAT, EXT, HFS etc

Question 4

The Partition Table Structure

(the Partition Table Entry Structure is available in exams - remebmer these multi byte values will be in LE so need converting)

Answer 4

The partition table is 64 bytes.
Every partition needs an entry on the partition table, each entry is 16 bytes (per partition).
Means you can have up to 4 partitions.

But, these can be extended with Extended Boot Records EBRs, these have same structure as MBR

So there are really 3 types of partition

Question 5

Desribe the 3 types of partition

Answer 5

PRIMARY: Each primary partition stores a single file system. Details are in the MBR partition table.

EXTENDED: Used to overcome the 4 partition limit. Contain LOGICAL partitions.

LOGICAL: Each partition stores a single file system. Details are in the EBR