The Pentium® processor was the first IA32 processor to implement Model Specific Registers (MSRs) and the two instructions that are used to access them:
RDMSR instruction. The numeric ID of the MSR to be read is placed in ECX and the RDMSR instruction is then executed. The 64-bit content of the MSR is placed in the EDX:EAX register pair, with the 32 msbs in EDX and the 32 lsbs in EAX.
WRMSR instruction. The numeric ID of the MSR to be written to is placed in ECX, the data to be written is placed in the EDX:EAX register pair and the WRMSR instruction is then executed.
The RDMSR and WRMSR instructions can only be executed by programs running at privilege level 0.
The Pentium® implemented the MSRs described in Table 21-1 on page ...