CPU Signature
CPUID Function eax=$1
Returned in eax

This function returns the family, model, stepping, and processor type information in the format of the table in Chapter #7. If he processor serial number is active, then all 32 bits will be 32 bits of the 96-bit processor serial number.

In order to read the CPU signature when the CPU supports serial numbers, the serial number must be disabled

Update 2003/11/21: I have received reports that for CPUs that feature Processor Serial Numbers, this signature table is still valid and correct (presumably these figures have been integrated into the serial number itself). So therefore disabling the PSN is not required. As I do not possess a PSN-enabled CPU, I cannot confirm this directly.