Logical Block Addressing definition storage
(LBA) A hard disk sector
addressing scheme used on all SCSI
hard disks, and on ATA-2
hard disks. The addressing conversion is performed by the hard disk firmware.
Prior to LBA, combined limitations of IBM PC BIOS
restricted the useful capacity of IDE hard disks on IBM PCs and compatibles to 1024 cylinders * 63 sectors per track * 16 heads * 512 bytes per sector = 528 million bytes = 504 megabytes. Modern BIOSes select LBA mode automatically, and work around the 1024-cylinder BIOS limit by representing a hard disk to the OS as having e.g. half as many cylinders and twice as many heads. However, there is still an unbreakable BIOS disk size limit of 1024 cylinders * 63 sectors per track * 256 heads * 512 bytes per sector = 8 gigabytes, but modern OSes (including Windows 9x
, Windows NT
) are not affected by it, since they issue direct LBA-based calls, bypassing the BIOS hard disk services completely.