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The ATA Interface Limit (128 GiB / 137 GB) Barrier

To get around past hard disk barriers, most modern hard disks are now no longer addressed using discrete geometry (cylinder, head and sector numbers) but rather logical block addressing and a sector number. However, even if we go away from the problems associated with assigning some bits in an address to cylinder number and others for head number and sector number, we eventually reach the limit of the addressability of all the bits taken together. In the case of the ATA interface, 28 bits are used for the sector number interface between the operating system, BIOS and the hard disk. This means a hard disk can have a maximum of 2^28 or 268,435,456 sectors of 512 bytes. This puts the ATA interface maximum at 128 GiB or approximately 137.4 GB.

Of course, as of this writing in mid-2000, there aren't any ATA hard disks that are this large. As a result, most people don't think much about this particular barrier. But things change quickly in the hard disk world, and we'll be there before you know it. Based on the current rate of hard disk capacity improvement, I'd guess that we'll be pushing the limits of the ATA interface no later than 2002. So consider this one the "big hard disk size barrier of the future". :^)

Much as the Int13h interface barrier was a tough nut to crack, this one will be as well, and for similar reasons. Fairly significant changes will need to be made to the interface between the hard disk and the rest of the system. Seeing this barrier on the horizon, the T13 technical committee (which works on standards for the IDE/ATA interface) is working on a couple of different proposals for expanding ATA addressing from 28 bits to either 48 or 64, either of which would allow rather monstrous hard disk sizes (even the smaller 48-bit proposal would result in drive sizes a million times higher than the current limit).