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.