The CPUID instruction provides information to software about the vendor, family, model, and stepping of microprocessor on which it is running. An input value loaded into the EAX register for this instruction indicates what information should be returned by the CPUID instruction.
Following processing of the CPUID instruction with a zero in EAX, the EAX register contains the highest input value understood by the CPUID instruction. For the Pentium processor, the value in EAX will be a one. Also included in the output of this instruction with an input value of zero in EAX is a vendor identification string contained in the EBX, EDX, and ECX registers. EBX contains the first four characters, EDX contains the next four characters and ECX contains the last four characters. For Intel processors, the vendor identification string is "GenuineIntel" as follows:
EBX 756e6547h (* "Genu", with G in the low nibble of BL *) EDX 49656e69h (* "ineI", with i in the low nibble of DL *) ECX 6c65746eh (* "ntel", with n in the low nibble of CL *)
Following processing of the CPUID instruction with an input value of one loaded into the EAX register, EAX[3:0] contains the stepping id of the microprocessor, EAX[7:4] contains the model (the first model will be indicated by 0001B in these bits) and EAX[11:8] contains the family (5 for the Pentium processor family). EAX[31:12] are reserved, as well as EBX, and ECX. The Pentium processor sets the feature register, EDX, to 1BFH indicating which features the Pentium processor supports. A feature flag set to one indicates that the corresponding feature is supported. The feature set register is defined as follows:
EDX[0:0] FPU on chip EDX[6:1] Refer to the Intel documentation EDX[7:7] Machine Check Exception EDX[8:8] CMPXCHG8B Instruction EDX[31:9] Reserved
Software should determine the vendor identification in order to properly interpret the feature register flag bits. For more information on the feature set register, see the Intel documentation.