How operating system addressing in hard disk.

1. CHS AND LBA ADDRESSING
Submitted To : Mr. Dinesh Kamble
Submitted By : Miss. Ayushi Beragi

2. INTRODUCTION
• A hard disk consists of the data present in the
computer. It is allocated on different spots in the hard
disk.
• Addressing or locating those files on the hard disk is
known as hard disk addressing –
• There are mainly two types of addressing:
• 1. CHS addressing
• 2. LHS addressing

4. CYLINDER HEAD SECTOR (CHS)
CHS is a method of giving addresses to each
physical block of data on a hard drive.
At that time, the hard disk drive has small capacity
and it is produced in a way similar to that of Floppy
disk,
So, the disk geometry, namely the cylinder head
sector value and the corresponding CHS addressing
are generated.

5. BASIC STRUCTURE OF HARD DISK
DRIVE
• Each hard disk consists of platter and read write heads.
The number of the platter is related to the capacity of a
hard drive.
• Each platter is divided into tracks that are concentric
circles.
• The read write heads read and write data along tracks
on the platters, and two sides of the platter can all
record data so, the number of read write heads is
double that of platters.

6. All concentric tracks with same radius on all platters are vertically
stacked into the cylinder.
so, the cylinder value is the number of the tracks on one side of each
platter. Surely, the track number each side of each platter are the
same.
The track is divided into many short segments, which are called
sectors.
Each sector normally has a capacity of 512 bytes. The sector
number on the each track are the same in early hard disk.
The inner sector with small physical area can have same capacity
with the outer sectors through different density arrangement.

7. •so, if we know the number of the cylinder- head-
sector, we can calculate formula is as follow :
•Head disk capacity = cylinder number x head
number x sector number x 512 bytes.

8. LOGICAL BLOCK ADDRESSING (LBA)
• LBA is a common scheme used for specifying the
location of blocks of the data stored on the computer
storage devices, generally secondary storage system
such as hard disks
• LBA has replaced the CHS scheme in order to
overcome certain of its limitations.
• LBA provides a simple linear address space to the host.
The host only need to provide the LBA address without
knowing anything of the physical sector positions.

9. • The LBA scheme replaces earlier scheme which exposes
the physical details of the storage device to the software of
the operating system.
• In CHS scheme, where blocks were addressed by the
means of a tuple which defined the cylinder, head and
sector at which they appeared on the hard disk.
• In LBA address does not stand for the actual physical
address [cylinder, head and sector] in an actual disk. LBA
transfers CHS from the 3-D addressing to 1-D linear
address.

10. •It converts C/H/S numbers of all the physical
sectors into linear number through certain rules,
which improves the efficiency of the system and
avoid the complicated C/H/S addressing when the
hard disk controller will transfer this logical address
into the physical address of the actual disk.

11. DIFFERENCE B/W CHS AND LBA ADDRESSING
•CHS addressing is fairly old and limited because of
the size of the numbers involved.
•LBA addressing was introduced to simplify matter
and increase the address space.
•LBA was first developed around SCSI hard drives.
Nowdays, it is the dominant form of hard disk
addressing, since its addressing mode is simpler
than that of CHS.

12. ADDRESSING MODE
•CHS :- From the full name of CHS, we can
predicate how it specifies the location of blocks of
data saved in the hard disk.
•For example: if the hard drives are accessed
through CHS, they are addressed by specifying its
cylinder, head and sector.

13. • LBA : with LBA each sector is assigned a unique
number rather than referring to a cylinder, head and
sector to access the hard drive.
• Using it, the hard disk is simply addressed as a single,
large device, which simple counts the existing blocks
starting at zero.
• In other words, LBA is a way by which a drive is
accessed by linearly addressing sector addresses.
Therefore, it seems that this addressing mode is
simpler than that of the CHS.

14. • LBA does not allow to address more sectors than CHS
style addressing does.
• In order to apply LBA, you should make sure it must be
supported by both the BIOS and OS.
• The hard drive itself must support LBA as well, luckily,
all newer hard drives do in fact support LBA.
• Even though CHS now no longer maintains a physical
relationship with the disk`s actual characteristics, CHS
is still used by many utilities.

15. THANK YOU