Reverse Engineering for Beginners

CHAPTER 21. STRUCTURES CHAPTER 21. STRUCTURES

#endif

#ifdef _MSC_VER
#include <intrin.h>
#endif

struct CPUID_1_EAX
{
unsigned int stepping:4;
unsigned int model:4;
unsigned int family_id:4;
unsigned int processor_type:2;
unsigned int reserved1:2;
unsigned int extended_model_id:4;
unsigned int extended_family_id:8;
unsigned int reserved2:4;
};

int main()
{
struct CPUID_1_EAX *tmp;
int b[4];

#ifdef _MSC_VER
__cpuid(b,1);
#endif

#ifdef GNUC
cpuid (1, &b[0], &b[1], &b[2], &b[3]);
#endif

tmp=(struct CPUID_1_EAX *)&b[0];

printf ("stepping=%d\n", tmp->stepping);

printf ("model=%d\n", tmp->model);

printf ("family_id=%d\n", tmp->family_id);

printf ("processor_type=%d\n", tmp->processor_type);

printf ("extended_model_id=%d\n", tmp->extended_model_id);

printf ("extended_family_id=%d\n", tmp->extended_family_id);

return 0;
};

AfterCPUIDfillsEAX/EBX/ECX/EDX, these registers are to be written in theb[]array. Then, we have a pointer to the
CPUID_1_EAXstructure and we point it to the value inEAXfrom theb[]array.

In other words, we treat a 32-bitintvalue as a structure. Then we read specific bits from the structure.

MSVC

Let’s compile it in MSVC 2008 with/Oxoption:

Listing 21.25: Optimizing MSVC 2008

_b$ = -16 ; size = 16
_main PROC
sub esp, 16
push ebx

xor ecx, ecx

mov eax, 1

cpuid

push esi

lea esi, DWORD PTR _b$[esp+24]

mov DWORD PTR [esi], eax

mov DWORD PTR [esi+4], ebx

mov DWORD PTR [esi+8], ecx

mov DWORD PTR [esi+12], edx

mov esi, DWORD PTR _b$[esp+24]