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Aman Prasad
Aman Prasad

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Understanding Endianness: Little-Endian vs Big-Endian

Endianness refers to the order in which bytes are arranged and stored in computer memory.

In simple terms, endianness decides which byte is stored at the lowest memory address: the most significant byte (MSB) or the least significant byte (LSB).

A simple analogy:

Think of endianness like reading direction.

Some languages read left to right, while others read right to left. Both convey the same information but only if you know the rule beforehand.

Similarly, computers need a defined rule to interpret multi-byte values correctly.

The Two Main Types of Endianness

Most modern systems use one of two byte-ordering schemes.

To illustrate, consider storing the 4-byte hexadecimal value: 0x12345678.

little endian and big endian with memory address diagram

Big-Endian

In Big-Endian systems, the most significant byte (MSB) is stored at the lowest memory address.
This format is often considered more human-readable because it matches how we write numbers.

Memory Address Stored Byte
Address +0 0x12 (MSB)
Address +1 0x34
Address +2 0x56
Address +3 0x78 (LSB)

It is used in Networking protocols, older mainframes and legacy systems.

Little-Endian (LE)

In Little-Endian systems, the least significant byte (LSB) is stored at the lowest memory address. This layout aligns well with how CPUs perform arithmetic internally.

Memory Address Stored Byte
Address +0 0x78 (LSB)
Address +1 0x56
Address +2 0x34
Address +3 0x12 (MSB)

It is used by Intel (x86), AMD, and most modern desktops, laptops and embedded MCUs.

Bi-Endianness

Many modern processors, like ARM, are actually Bi-endian. This means they can be configured to operate in either Big-Endian or Little-Endian mode depending on the operating system's requirements.
In practice, most modern ARM systems run in little-endian mode.

Why Does It Matter?

In high-level languages like Python or Java, endianness is usually hidden.

However, it becomes critical in the following cases:

  • Networking: The internet uses big-endian. Without proper byte conversion, data sent from a little-endian system will be misinterpreted.
  • Binary File Sharing: Opening a binary file created on a big-endian system on a little-endian machine can corrupt values unless handled correctly.
  • Low-Level Programming: In C, assembly, or embedded systems, incorrect assumptions about byte order lead to subtle and dangerous bugs.

Detecting Endianness Using C

#include <stdio.h>
#include <stdint.h>

int main() {
    uint16_t x = 1;
    if (*(uint8_t*)&x == 1){
        printf("Little Endian");
    }
    else{
        printf("Big Endian");
    }
    return 0;
} 
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Step by step explanation

  1. The Variable uint16_t x = 1

    • uint16_t is a 16-bit (2-byte) integer
    • The numeric value is 1, but memory must store it using two bytes

    Possible memory layouts:

    Endianness Memory Bytes
    Big-Endian 00 01
    Little-Endian 01 00
  2. The pointer cast (uint8_t*)&x

    This line does three things:

    1. &x → gets the memory address of x
    2. (uint8_t*) → treats that address as a pointer to a single byte
    3. Dereferencing reads the first byte in memory
  3. if *((uint8_t)&x == 1)*

  4. If the first byte is 1: the least significant byte is stored first. The system is Little-Endian.

  5. If the first byte is 0: the most significant byte is stored first. The system is Big-Endian.

A Subtle Advantage of Little-Endian

One often-mentioned but rarely explained advantage of little-endian systems is:

The same value can be read from memory at different widths using the same base address.

This works because, in little-endian memory, the least significant byte is stored at the lowest address.

Example

uint32_t x =0x12345678;
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Little-endian memory layout:

Memory Address Stored Byte Significance
&x + 0 0x78 Least Significant Byte (LSB)
&x + 1 0x56
&x + 2 0x34
&x + 3 0x12 Most Significant Byte (MSB)

&x always points to the lowest memory address (&x + 0).

Now, reading from the same address:

Read size Expression Result
8-bit *(uint8_t*)&x 0x78
16-bit *(uint16_t*)&x 0x5678
32-bit *(uint32_t*)&x 0x12345678

The starting address never changes only the read size does. Increasing the width naturally reveals more significant bytes.

Why Do We Care About the Lower Bytes?

This raises an important question:

If big-endian systems expose the upper part of a number first, why do CPUs and programmers care so much about the lower bytes?

The answer lies in how arithmetic works.

In any positional number system, the least significant bits form the foundation of the value, while higher bits only add scale.

For example: 0x12345678

  • The lower byte (0x78) controls changes of ±1
  • The upper byte (0x12) only affects large magnitude

All arithmetic operations addition, subtraction, multiplication start from the least significant byte and propagate upward using carry.

Addition and Subtraction

When adding multi-byte numbers, the CPU must process the least significant byte first to determine whether a carry occurs.

Example:

  Carry:           1 1
  Value A:   0x0 0 F F
+ Value B:   0x0 0 0 1
  --------------------
  Result:    0x0 1 0 0
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  1. LSB addition: 0xFF + 0x01 = 0x00 (carry = 1)
  2. Next byte uses the carry to produce 0x01

In little-endian systems, the LSB is fetched first, allowing computation to begin immediately while higher bytes are fetched in parallel. This was especially important on early 8-bit and 16-bit processors and strongly influenced CPU and compiler design.

Why Big-Endian Still Exists

If little-endian fits computation so well, why does big-endian persist?

The reason is legacy and standardization, not performance.

Big-endian is used in:

  • Networking (TCP/IP)
  • File formats like JPEG and PNG
  • Older architectures and mainframes

Once a format or protocol is defined, changing its byte order would break compatibility with existing data and software.

Modern Reality

On modern systems:

  • CPU arithmetic happens in registers (no endianness)
  • Caches and pipelines hide memory order
  • High-level languages abstract it away

Today, endianness is mostly a data-format concern, not a CPU performance concern except in networking, embedded systems, and low-level code.

Reference

This discussion is inspired by community explanations on Stack Overflow:

https://stackoverflow.com/questions/13926760/the-reason-behind-endianness

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