Thread: Inline GCC Help with reading CR4 register

Hi, this is an attempt to read the CR4 register into the A register (rax for 64 bit) on fedora 11 64 bit. The movq operation produces a segment fault.

Code:

typedef unsigned long long UINT64;
typedef union
{
        UINT64 value;
        struct
        {
                 UINT64 VME0:1; 
                UINT64 PVI1:1; 
                UINT64 TSDB2:1;
                UINT64 DE3:1; 
                UINT64 PSE4:1;
                UINT64 PAE5:1;
                UINT64 MCE6:1;
                UINT64 PGE7:1; 
                UINT64 PCE8:1; 
                UINT64 OSFXSR9:1; 
                UINT64 OSXMMEXCPT10:1; 
                UINT64 bit1112:2; // reserved
                UINT64 VMXE13:1; 
                UINT64 SMXE14:1; 
                UINT64 bit1517:3; // reserved
                UINT64 OSXSAVE18:1; 
                UINT64 bit1931:13; // reserved
                UINT64 bit3264:32; // reserved should be 0
        }bits;
}CR4;

int main()
{
  unsigned long long result;
  //CR4 cr4_reg;
  printf("\n size of long long is %x ", sizeof(result));
  fflush(NULL);
  __asm__("movq   %%cr4, %%rax\n"
        : "=a"(result) : "a"
  );
 printf("\n Value of CR4 = %x", result);
 return 0;
}

Thanks for helping me out

Yes, you are right, I figured it later on after much sweating. It cannot be accessed from an application and can be accessed through the kernel.

thanks for the reply