Big-Endian or Little-Endian?

Table of Contents

• 1 Big-endian or Little-endian?
• 2 Determining Endedness

1 Big-endian or Little-endian?

Computers store multibyte values in either big-endian or little-endian order.

What does this mean?

Suppose we are on a machine that has two-byte integers. And we do the following:

  int main()
  {
    int i; 
    i = 1;

    ...
    ...
    ...
}

How is the value 1 (or 00000000 00000001) stored in memory?

In what order are the bytes stored?

Memory Location	Value
000Ax	00000000
000Bx	00000001

Or

This is different

Memory Location	Value
000Ax	00000001
000Bx	00000000

Little-endian refers to the idea that the least significant byte (that's LSB) is stored in the lower memory location. (Mnemonic: Least-Low-Little)

Big-endian refers to the idea that the most significant byte (that's MSB) is stored in the lower memory location. (Mnemonic: Most-Low-Big)

Can you look at the tables above and identify which is big-endian and which is little-endian?

The first table shows a computer architecture that is big-endian, the second table, little-endian.

2 Determining Endedness

Determining whether or not we are big- or little-endian is not that hard, but your first guess of how to do it, may be wrong. Here's some code.

#include <stdio.h>
int main()
{
  int i, j;
  char *ptr;

  /* Will shifting work? */

  i = 1;
  j = i >> 1;

  if (j == 0){
    printf("Big-endian\n");
  }else{
    printf("Little-endian\n");
  }
/* This tells Big-endian --- but read on! */

  /* Casting gets us to the byte we want */
  i = 1;
  ptr = (char *) &i;

  /* ptr now points to the lower memory address */
  if (*ptr == 1){
    printf("Little-endian\n");
  }else{
    printf("Big-endian\n");
  }
  /* Prints 'Little-endian' --- that's correct */

}

The first attempt didn't work. That's because the shift operators work in a logical fashion, according to a logical representation of bit patterns.

As such, it's impossible to get to the actual layout of the hardware using them.

But by casting the integer to a char, we can get the first byte, and it becomes possible to determine if the machine is big- or little-endian.