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WRMSR           Write to Model Specific Register     Flags: Not altered
                                                      
WRMSR                                                CPU: Pent+ r

        Logic   if (CPL = 0)
                   MSR[ECX] . EDX:EAX
                else
                   #GP(0)
                endif

    Writes the contents of registers EDX:EAX into the 64-bit model
    specific register (MSR) specified in the ECX register. The high-
    order 32 bits are copied from EDX and the low-order 32 bits are
    copied from EAX. Always set undefined or reserved bits in an MSR to
    the values previously read.

    This instruction must be executed at privilege level 0 or in
    real-address mode; otherwise, a general protection exception #GP(0)
    will be generated. Specifying a reserved or unimplemented MSR
    address in ECX will also cause a general protection exception.

    When the WRMSR instruction is used to write to an MTRR, the TLBs are
    invalidated, including the global entries (see Translation Lookaside
    Buffers (TLBs) in Chapter 3, Protected-Mode Memory Management, of
    the Pentium Pro Family Developer's Manual, Volume 3).

    The MSRs control functions for testability, execution tracing,
    performance-monitoring and machine check errors. Appendix D in the
    Pentium Pro Family Developer's Manual, Volume 3 lists all the MSRs
    that can be written to with this instruction and their addresses.

    The WRMSR instruction is a serializing instruction (see Serializing
    Instructions in Chapter 7, Multiple Processor Management, of the
    Pentium Pro Family Developer's Manual, Volume 3).


    Note
    The CPUID instruction should be used to determine whether MSRs are
    supported (EDX[5]=1) before using this instruction.


    Protected Mode Exceptions
    #GP(0) If the current privilege level is not 0.
    If the value in ECX specifies a reserved or unimplemented MSR address.

    Real Address Mode Exceptions
    #GP If the current privilege level is not 0
    If the value in ECX specifies a reserved or unimplemented MSR address.

    Virtual 8086 Mode Exceptions
    #GP(0) The WRMSR instruction is not recognized in virtual 8086 mode.


    Opcode      Format
    0F 30       WRMSR


    Length and timing
    Operands    Bytes   8088    186     286     386     486     Pentium
    -           2       -       -       -       -       -       ??