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Looking stuff up on the Internet, I ran into US patent 6529975. I'm no patent lawyer, but as far as I can tell it's a way of getting around the limited number of registers in the AC97 specification by writing to one register the address of an extended register to access, then reading or writing the contents of that extended register via another AC97 register. For those not familiar with computer hardware, this is an old* and well-known trick. However, presumably because it hasn't been applied to AC97 hardware before, it's still patentable. This is why the US patent system doesn't work.

(To be fair, they do list an alternative technique that splits one register up and uses half the bits as data and half as address, but that's still not exactly non-obvious.)

* By "old" I mean "the video address generator used in the BBC Micro and the original IBM PC was based on a variant of this technique" (and that's just the first likely example I happened to think of).