The Addressing of Hard Disk Drive Sectors (LBA, CHS)

The hard disk drive stores the information in fixed portions, which are called sectors. The sector is the smallest portion of the data that has a unique address and location on a magnetic disk. The exchange of information with the hard disk drive presupposes the address indication as a command parameter. Linear addressing used in hard disk drives was given the name LBA (Logical Block Addressing). On receiving a command the hard disk transmits LBA address to the physical address of the sector, i.e. the number of the cylinder, the head and the sector (CHS - Cylinder Head Sector).

Before ATA/ATAPI-6 version appeared, the 28-bit number was used for addressing of sectors that limited the maximum capacity of the hard disk to 128 Gb. In new versions of the standard the commands with 48-bit LBA value appeared that has increased the maximum possible capacity of the hard disk drive to 134217727 Gb. In the original version of Windows XP the support of the 48-bit addressing is absent. In Windows 2000 (SP3 or above) and also in Windows XP (SP1) it can be enabled by creating the parameter EnableBigLba (REG_DWORD) with value 1 in the registry section [HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\ATAPI\Parameters]. In Windows XP SP2 support of 48-bit addressing is enabled by default.