Logical Block Addressing
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.