What is the difference in power dissipation with the FPU disabled?
It is difficult to say. If the FPU is not in use, then the Dynamic Power Management circuitry has already shut it down, so the power savings have already been realized, and manually disabling it will not accomplish anything. If the FPU is in use, then software has to be executed to accomplish the same code using the Fixed Point Unit. This code also takes power to execute, and the tradeoffs are not well known. Possibly it will take more power to do the floating point calculations in software than in hardware. Finally, if the FPU is in use, and the customer decides to remove the floating point function from the system (how that would be is somewhat questionable :-). then the processor will be able to execute other instructions in the time that it was executing floating point instructions. While the floating point instructions are not being executed, and so do not use any power, the other instructions that are able to execute in that time do use power, so again there is a tradeoff that is
