AMD's SB800-series fan speed controller exposed
 

The SB800 family of south-bridge silicon has an integrated microcontroller that, with a little help from firmware built into a system's BIOS, can offer more control over that system's fans than has ever graced a motherboard. The microcontroller can manage fan speeds using three algorithms: step, linear, and non-linear. In step mode, fan speeds jump from one value to the next as temperatures rise. The linear algorithm smooths things out, allowing fan speeds to ramp, well, linearly between predefined minimum and maximum values. Combine the two, and you have a non-linear approach that's really more multi-linear. This method drapes a string of linear slopes across multiple step points.

http://techreport.com/articles.x/19856