Volume 3: General-Purpose and System Instructions - Stanford ...

AMD64 Technology 24594 Rev. 3.10 February 2005instruction. If the value did not change, rFLAGS.ID is not writable, and the processordoes not support the CPUID instruction.The following code sample shows how to test for the presence of the CPUIDinstruction using 32-bit code.pushfd; save EFLAGSpop eax ; store EFLAGS in EAXmov ebx, eax ; save in EBX for later testingxor eax, 00200000h ; toggle bit 21push eax ; push to stackpopfd; save changed EAX to EFLAGSpushfd; push EFLAGS to TOSpop eax ; store EFLAGS in EAXcmp eax, ebx ; see if bit 21 has changedjz NO_CPUID ; if no change, no CPUIDStandard Function 0:Processor Vendor andLargest StandardFunction NumberAll software using the CPUID instruction must execute standard function 0. Thisfunction returns the largest standard function number and the processor vendor.Standard Function 0 EAX: Largest Standard Function Number. Standard function 0 loads EAXwith the largest CPUID standard function number supported by the processorimplementation.Standard Function 0 EBX, EDX, and ECX: Processor Vendor. Standard function 0 loads a12-character string into the EBX, EDX, and ECX registers identifying the processorvendor. For AMD processors, the string is AuthenticAMD. This string informs softwarethat it should follow the AMD CPUID definition for subsequent CPUID function calls.If the function returns a another vendor’s string, software must use that vendor’sCPUID definition when interpreting the results of subsequent CPUID function calls.Table 3-1 shows the contents of the EBX, EDX, and ECX registers after executingfunction 0 on an AMD processor.Table 3-1.Processor Vendor Return ValuesRegister Return Value ASCII CharactersEBX 6874_7541h “h t u A”EDX 6974_6E65h “i t n e”ECX 444D_4163h “D M A c”118 CPUID