I just successfully flashed my 890GXM-G65 board using flashrom to the latest bios (7642v18). As the board is marked as OK but untested I have kept the logs of all the steps done using -V and no other option was needed:
flashrom -V | tee noargs.txt flashrom -Vr current.rom | tee read.txt flashrom -Vw A7642AMS.180 | tee write.txt flashrom -Vv A7642AMS.180 | tee verify.txt
And I must mention it: it has been the smoothest bios update I've ever done...
I hope it helps!