Logical block addressing in computing maps conceptual data storage onto secondary storage. LBA is used to overcome size limits of hard drives. Another less popular method is the Extended Cylinder-Head-Sector (ECHS ).

The idea is to give disk sectors linear numbers starting with 0, as opposed to classic cylinder-head-sector (CHS) addressing where disk sectors are described by their coordinates in terms of cylinders, heads and sectors, limited to 1024/4096/16384, 16/256 and 63 respectively. Original limits were 1024*16*63, but newer BIOSes can transform a coordinate system with more than 16 (virtual) disk heads into one which has more cylinders instead, or they can transform CHS coordinates in a coordinate system of up to 1024*255*63 size (limit for MS-DOS, usually corresponds to 8 GB disk size) into LBA addresses, internally communicating with the disk in terms of LBA space.

LBA addresses can be 28 bit or 48 bit wide, which results in a disk size limit of 128 GiB and 128 PiB, respectively, if you assume (very common for harddisks) 512 bytes per sector.

External links

• Logical Block Addressing (LBA) from the The PC Guide.
• Extended CHS (ECHS) / Large Mode from the The PC Guide.