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As discussed in the section on the standard Int13h BIOS interface, that older standard has an important limitation that has become a serious issue for PC upgraders over the last few years: it uses 24 bits of addressing information, and as such can only handle drives that contain up to approximately 16.5 million sectors, which at 512 bytes per sector yields a maximum capacity of 8.46 GB (or 7.88 GiB). As modern drives approached 8 GB in size, hardware and operating system makers all realized that they had a problem here: something had to be done to allow access to the larger hard disks of the future.
When a bridge is too narrow to handle increased traffic, the usual solution is to widen it, and that's exactly what was needed here: to widen the access path from 24 bits to something larger. Unfortunately, it was not possible to expand the existing Int13h BIOS interface. The reason is that if this were done, a lot of older hardware and software would stop working. Making changes that cause millions of older hardware and software products to stop working is not how you win friends in the PC world. :^)
Instead, a new interface was developed to replace Int13h: these routines are called Int13h extensions. This new interface uses 64 bits instead of 24 bits for addressing, allowing a maximum hard drive size of 9.4 * 10^21 bytes. That's 9.4 trillion gigabytes! I'm sure that when the original Int13h interface was developed, nobody ever expected us to hit 8 GB drives as fast as we did. Still, even with the rapid pace of technological advancement, I'd say we're pretty safe with 9.4 trillion gigabytes as a limit. If not, I'll be pleased as punch to move to a still newer interface in exchange for a hard drive that big. ;^)
There's a catch to these Int13h extensions of course: they are different from the old way of doing things, and therefore support for them must be incorporated into several key areas of the system. This includes the system BIOS and the operating system. For more information on this, see this section.
For more information on the Int13h interface limitation and the problems it causes, see the section on the Int13h interface size barrier.