Question

如何访问x86程序集中int变量中的特定位以检查它是0还是1？

我认为这会有用，但它不会

int main()
{
  int num = 3; //stored as a 32 bit binary I assumed something like ...011

  //now I need to test the 2nd bit of num
  __asm  
  {  
    test num, 1 // By the way, how would I check the ZF flag?
  }
}

Answer 1

我假设aex代表eax。

如果你只想测试eax的第0位，你可以这样做：

test eax, 1
; ZF will be set based on eax AND 1. eax remains unchanged
jnz bit0_is_set
; The jump wasn't taken, so bit 0 was clear
...
jmp done
bit0_is_set:
; The jump was taken, so bit 0 was set
...
done:

要测试其他位，请使用增加的2的幂：对于位1为2，对于位2为4，对于位3为8等。

如果你真的想将eax的第0位复制到ebx，你可以这样做：

mov ebx, eax
and ebx, 1

如何访问x86程序集中的int变量中的特定位以检查它是0还是1？

1 个答案: