You're confusing the terms "banks" and "ranks."
Ulrich goes into quite a bit of detail about how an address is broken into a "row" and "column" component, but there is a third component, bank, internal to the DRAM.
A DIMM can be single-rank or dual-rank. When you write or read a single-rank DIMM, you talk to every memory component on the DIMM, every time. Dual-rank DIMMs have double the number of components as a single-rank DIMM, and when you write or read (activate or precharge, for that matter) you only talk to half of the components.
What any of this has to do with software, I don't know. I work with some absolutely lousy programmers who know quite a bit more about memory than I do. :)
 I know this holds for unbuffered DIMMs, but I don't know about fully-buffered DIMMs. I don't work with those.
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