Counting physical CPU

[Sven]

regarding getting the cpuid and the APIC vector you could do also in C with:

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#include <intrin.h>

int cpu_apic_id() {

  int CPUInfo[4] = {-1};
  __cpuid(CPUInfo, 0x00000001);

  return (CPUInfo[1] >> 24) & 0xff;
}

See http://msdn.microsoft.com/en-us/library ... 80%29.aspx

But I don't understand where you found the information to extract the # of cores. [...]

I assume you already know that CPUID and the __cpuid() intrinsics function are specific for Intel and MSVC (>= 2005). Furthermore, to get a correct result the documentation requires to query the maximum allowed value for the second parameter by calling __cpuid(CPUInfo, 0) first, and to call __cpuid(CPUInfo, x) for x > 0 only if the first call returned a value >= x in CPUInfo[0]. The # of cores is found by something like:

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__cpuid(CPUInfo, 4);
return CPUInfo[0] >> 26;

where masking the result with 0xff is redundant since there are at most 6 nonzero bits left after shifting.

But since this "# cores" value is not available for all Intel processors (that's why the first call with 0 is necessary), one might want to maintain two functions: one boolean function telling whether the information about # of cores is available, and the other one relying on availability and returning the correct number. One might of course be lazy and put that into one function, returning a special value like 0 or -1 for "not available".

EDIT: See also the Visual Studio 2008 page for __cpuid. The link above is for VS2005. I guess it may even be necessary to use VS2008 at least for # of cores since the VS2005 intrinsics implementation might not be capable of that, although I did not test.

Sven