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).
This page will be updated as we get closer to hitting this barrier, and more
information becomes available about how it will be addressed.
Next: BIOS Handling of "Oversized" Hard Disks
