3-710
INSTRUCTION SET REFERENCE
WRMSRWrite to Model Specific Register
Description
This instruction 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 the 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. For more information, refer to Section 3.7., Translation Lookaside Buffers
(TLBs) in Chapter 3, Protected-Mode Memory Management of the Intel Architecture Software
Developers Manual, Volume 3. MTRRs are an implementation-specific feature of the Pentium
®
Pro processor.
The MSRs control functions for testability, execution tracing, performance monitoring and
machine check errors. Appendix B, Model-Specific Registers, in the Intel Architecture Software
Developers 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. For more information, refer to Section 7.4.,
Serializing Instructions in Chapter 7, Multiple-Processor Management of the Intel Architecture
Software Developers Manual, Volume 3.
The CPUID instruction should be used to determine whether MSRs are supported (EDX[5]=1)
before using this instruction.
Intel Architecture Compatibility
The MSRs and the ability to read them with the WRMSR instruction were introduced into the
Intel Architecture with the Pentium
®
processor. Execution of this instruction by an Intel Archi-
tecture processor earlier than the Pentium
®
processor results in an invalid opcode exception
#UD.
Operation
MSR[ECX]
<
EDX:EAX;
Flags Affected
None.
Opcode
Instruction
Description
0F 30
WRMSR
Write the value in EDX:EAX to MSR specified by ECX
