In this article, we will learn how to construct C code from x86 assembly for bitwise operations. The article is divided into two parts. In the first part, we will write a small piece of C code that uses bitwise operation and analyze the corresponding assembly code. In the second part, we will look into the x86 assembly code and try to construct the corresponding C code that uses bitwise operations.

Let’s first write a small piece of C code that uses bitwise operation.

#include <stdio.h>
int main() {
        int a = 1, b = 2, c;
        c = a | b;
        return 0;
}

The above piece of code defines two local variables a and b in the main function. After that, we perform one bitwise or operation on a and b and store the result in another variable c.

Now, let’s compile the above program and generate the corresponding object code using the following commands:

$ gcc -c bitwise_operations.c
$ objdump -M intel -d bitwise_operations.o

Please note that the “-M intel” option will generate the assembly code in the Intel syntax.

The corresponding assembly code will look like the following:

0000000000000000 <main>:
   push   rbp
   mov    rbp,rsp
   mov    DWORD PTR [rbp-0xc],0x1
   mov    DWORD PTR [rbp-0x8],0x2
   mov    eax,DWORD PTR [rbp-0xc]
   or     eax,DWORD PTR [rbp-0x8]
   mov    DWORD PTR [rbp-0x4],eax
   mov    eax,0x0
   pop    rbp
   ret    

The first two instructions are part of the function prologue and they are executed every time a function is called. The RBP …