Intel Pentium Instruction Set Reference

RDMSR - Read from Model Specific Register

Description

Loads the contents of a 64-bit model specific register (MSR) specified in the ECX register into registers EDX:EAX. The EDX register is loaded with the high-order 32 bits of the MSR and the EAX register is loaded with the low-order 32 bits. If less than 64 bits are implemented in the MSR being read, the values returned to EDX:EAX in unimplemented bit locations are undefined.

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.

The MSRs control functions for testability, execution tracing, performance-monitoring and machine check errors. Appendix B, Model-Specific Registers (MSRs), in the Intel Architecture Software Developer's Manual, Volume 3, lists all the MSRs that can be read with this instruction and their addresses.

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

Flags