How to Access CPU Information using GCC in Linux: Assembly vs. `cpuid.h`?

How to Access CPU Information in Linux Using GCC

In the realm of x86 architecture, developers often rely on _cpuinfo() in Windows API to retrieve valuable information about their CPUs. Linux users, however, have their own set of tools at their disposal, one of which is the cpuid instruction.

One method to utilize cpuid in Linux using GCC involves inline assembly, a technique that intermixes assembly instructions with C/C code. Assemblers enable developers to control CPU operations directly, and you might have already attempted to write your own assembly routines for cpuid:

<code class="c++">// Accessing CPUID using assembly

#include <iostream>

int main()
{
  int a, b;

  for (a = 0; a < 5; a++)
  {
    __asm ( "mov %1, %%eax; "            // a into eax
          "cpuid;&quot;
          "mov %%eax, %0;&quot;             // eax into b
          :&quot;=r&quot;(b)                     // output
          :&quot;r&quot;(a)                      // input
          :&quot;%eax&quot;,&quot;%ebx&quot;,&quot;%ecx&quot;,&quot;%edx&quot; // clobbered register
         );
    std::cout << "The CPUID level " << a << " gives EAX= " << b << '\n';
  }

  return 0;
}</code>

Although this method grants you low-level access to cpuid, it necessitates assembly coding, which can be time-consuming and error-prone. Fortunately, there's a simpler way that eliminates the need for assembly.

GCC offers a powerful header file called cpuid.h that provides comprehensive support for cpuid operations. This header declares robust functions that enable you to retrieve CPU information without the complexity of inline assembly. Here's how you can leverage cpuid.h to retrieve CPU data:

<code class="c++">// Accessing CPUID using cpuid.h

#include <iostream>
#include <cpuid.h>

int main()
{
  unsigned int eax, ebx, ecx, edx;

  // Get the maximum supported CPUID level
  unsigned int max_level = __get_cpuid_max(0x0, NULL);

  // Retrieve CPUID data for level 0
  __get_cpuid(0, &eax, &ebx, &ecx, &edx);
  std::cout << "CPUID level 0:" << std::endl;
  std::cout << "  EAX: " << eax << std::endl;
  std::cout << "  EBX: " << ebx << std::endl;
  std::cout << "  ECX: " << ecx << std::endl;
  std::cout << "  EDX: " << edx << std::endl;

  // Repeat for other levels as needed
  // ...

  return 0;
}</code>

With the cpuid.h header, you can effortlessly retrieve CPU information without the complexities of assembly coding. It provides a convenient and reliable interface for accessing CPU-specific data in your Linux applications.

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