Logical Block Addressing

storage (LBA) A hard disk sector addressing scheme used

on all SCSI hard disks, and on ATA-2 conforming IDE hard

disks. The addressing conversion is performed by the hard

disk firmware.

Prior to LBA, combined limitations of IBM PC BIOS and

ATA 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 and Linux) are

not affected by it, since they issue direct LBA-based calls,

bypassing the BIOS hard disk services completely.

(2000-04-30)