I've also experienced Windows / DirectX API's use MMX, hence the floating point code I was using (x87 FPU) required a femms instruction before it to change back

I've also experienced Windows / DirectX API's use MMX, hence the floating point code I was using (x87 FPU) required a femms instruction before it to change back from MMX state. This just goes to show that if you need a particular state on a processor (CPU, FPU, GFX, etc) you need to set it explicitly before use.