A dynamically controllable variable displacement axial piston pump is described. In an embodiment, the pump comprises a rotating cylinder with hydraulic pistons that contact the face of a swash plate.The angle of the swash plate can be controlled to thereby control the movement of the pistons, the displacement of the pump, and the power generated by the pump. The dynamically controllable variabledisplacement axial piston pump may be used in combination with a rotary steerable apparatus, including such an apparatus as described herein that uses hydraulic pistons to actuate the deflection of the bit, or in combination with other downhole tools and devices. When used down hole in a drill string with a drilling mud powered turbine, the dynamically controllable variable displacement pump limits and regulates the power provided to the tool over a wide range of drilling mud weights and flow rates.