Disable serial number on <P3...

[Peter_Vigren]

Hellu... I read a tutorial about the CPUID, which stated that when using the instruction with Eax=1, Pentium III will not return the CPU signature in Eax (as the processors before P3). Instead it will return 32 bits of the 96-bits serial number. The tutorial supplies the following code to disable the returning of the serial number instead of the signature:

Code: Select all

; Disable CPU serial number/Enable CPU signature

Mov Ecx,0x119
Rdmsr
And Eax,0xFFDFFFFF   ; Disable serial bit
Wrmsr

First of all, is that code correct? And can I use this code safely on processors below P3?

[Peter_Vigren]

Hm... The tutorial that I read may be wrong... Quoting the "Intel Processor Identification and the CPUID Instruction"-pdf:

"To access the most significant 32-bits of the processor serial number the program should set the EAX register parameter value to ?1? and then execute the CPUID instruction as follows:

MOV EAX, 01H
CPUID

After execution of the CPUID instruction, the EAX register contains the Processor Signature."

It seems that on all processors (even => Pentium III), what is returned in Eax is the Processor Signature. Therefore the disabling the serial code don't seem to be needed... since the signature is part of it and returned correctly. However, I can be wrong... and I don't think the author of the tutorial I read just came up with the code out of the blue... maybe I should mail him and ask...