Update your 890FXA-GD65 BIOS to one compatible with the FX-6100.
You need v14 or later. See the CPU Support list for the 890FXA-GD65.
Alas I wish that was all there is to it,But I have the latest bios and no joy! Everything reads it as a six core but the performance monitor and some 3rd party apps show only 4 cores operating!MSI Tech support say's I have to do a clean reinstall of my operating system cross my fingers and hope that microsoft renews my licence yet again!