Always set undefined or reserved bits to the value previously read.

WRMSR is used to write the content of Model-Specific Registers that control functions for testability, execution tracing, performance monitoring, and machine check errors. Refer to the Pentium(TM) Processor Data Book for more information.

The values 3H, 0FH, and values above 13H are reserved. Do not execute WRMSR with reserved values in ECX.