How to detect what CPU is being used during runtime?

Question

How can I detect which CPU is being used at runtime ? The c++ code needs to differentiate between AMD / Intel architectures ? Using gcc 4.2.

Answer 1

The cpuid instruction, used with EAX=0 will return a 12-character vendor string in EBX, EDX, ECX, in that order.

For Intel, this string is "GenuineIntel". For AMD, it's "AuthenticAMD". Other companies that have created x86 chips have their own strings.The Wikipedia page for cpuid has many (all?) of the strings listed, as well as an example ASM listing for retrieving the details.

You really only need to check if ECX matches the last four characters. You can't use the first four, because some Transmeta CPUs also start with "Genuine"

• For Intel, this is 0x6c65746e
• For AMD, this is 0x444d4163

If you convert each byte in those to a character, they'll appear to be backwards. This is just a result of the little endian design of x86. If you copied the register to memory and looked at it as a string, it would work just fine.

Example Code:

bool IsIntel() // returns true on an Intel processor, false on anything else
{
  int id_str; // The first four characters of the vendor ID string

  __asm__ ("cpuid":\    // run the cpuid instruction with...
  "=c" (id_str) :       // id_str set to the value of EBX after cpuid runs...
  "a" (0) :             // and EAX set to 0 to run the proper cpuid function.
  "eax", "ebx", "edx"); // cpuid clobbers EAX, ECX, and EDX, in addition to EBX.

  if(id_str==0x6c65746e) // letn. little endian clobbering of GenuineI[ntel]
    return true;
  else
    return false;
}

EDIT: One other thing - this can easily be changed into an IsAMD function, IsVIA function, IsTransmeta function, etc. just by changing the magic number in the if.