CHA & LBA Addressing

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

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. THANK YOU