AMD CS5535 - Manual (Page 11)

AMD Geode™ GX Processor/CS5535 Companion Device GeodeROM Porting Guide 11
2
Model Specific Registers 32430C
2.0
Model Specific Registers
There are two ways to read or write Model Specific Registers (MSRs) in a Geode™ GX processor system. Software run-
ning on the processor can use the RDMSR and WRMSR instructions, and modules within the processor can use the
GeodeLink™ MSR transactions. The second method allows debug modules, such as the System Navigator from FS
2 (First
Silicon Solutions), to program MSRs.
All MSRs are 64 bits wide. The MSR addresses are 32 bits, where each unique address refers to a 64-bit data quantity.
To communicate with modules on the GeodeLink interface, the address of that module must be known. Addresses are
obtained by either scanning the GeodeLink interface or having prior knowledge of the chip topology. This is discussed in
detail in Section 3.0 "GeodeLink™ Architecture" on page 13.
RDMSR:
Input
ECX - Address to read.
Output
EDX:EAX - 64 bits data returned.
WRMSR:
Input
ECX - Address to write.
EDX:EAX - 64 bits data written.
Output
None.
2.1 Example MSR Transaction
Read and write extended CPUID registers.
This example will change the CPUID.
RDMSR:
Load MSR specified by ECX into EDX:EAX.
WRMSR:
Write the value in EDX:EAX to MSR specified by ECX.
MSR_CPUID0 EQU 00003000h
MSR_CPUID1 EQU 00003001h
mov ecx, MSR_CPUID0
RDMSR ; get values
mov edx, ‘cdeR’ ; write edx:eax to MSR in ecx. No change to eax.
WRMSR
mov ecx, MSR_CPUID1
RDMSR
mov edx, ‘duol’ ; No change to eax
WRMSR
; Done