PC Hardware in a Nutshell, 3rd Edition

The Maxtor Big Drive Initiative

The 28-bit LBA used by ATA/ATAPI-5 and earlier interfaces limits hard drive sizes to 128 GB (binary, although drive makers use decimal, and so refer to 128 GB as 137 GB). Before 2001 this was not an issue, but during 2001 drive makers began shipping SCSI hard drives larger than 128 GB. They wanted to produce larger ATA drives as well, but 28-bit LBA made that impossible unless those drives used sector sizes larger than the standard 512 bytes. Using nonstandard sector sizes introduces severe compatibility problems, so there was no easy solution.

Maxtor Corporation solved the problem with its Big Drive initiative, which was subsequently adopted in ATA/ATAPI-6. Big Drive specifies a new ATA interface that uses 48-bit LBA. Those additional 20 bits allow the new interface to address a binary million (2²⁰) times more sectors than a 28-bit ATA interface, or (2⁴⁸ × 512 bytes/sector) = 144,115,188,075,855,872 bytes = 128 PB. Drive makers, of course, call that “144 PB.”

Tip

Don’t bother telling us that binary values are actually kibibytes, mebibytes, and gibibytes, which use “bi” to indicate binary rather than decimal values. We’re aware of this ill-considered initiative, and we don’t know anyone who uses those words in real life. As an alternative, we propose everyone use the original definition of a kilobyte as 1,024 bytes, and substitute kidebyte, medebyte, or gidebyte when referring to decimal values. You heard it here first.

Compatibility between old and new ...