What is Endianness?

1. What is Endianess?

Endianness is a term used to describe the order in which a sequence of bytes is arranged into larger numerical values when stored in computer memory. Specifically, it determines how multi-byte data types (such as integers or floating-point numbers) are stored and read from memory.

Endianness is more about memory than the CPU. While the CPU architecture dictates how endianness is implemented, the concept of endianness specifically refers to the order in which bytes are stored in and accessed from memory.

2. Types of Endianness

2.1 Little Endian:

In little-endian systems, the least significant byte (LSB) of a multi-byte value is stored at the lowest memory address.
For example, consider the 32-bit hexadecimal value 0x12345678. In a little-endian system, it would be stored in memory as follows:

Memory Address |  Value
--------------------|--------
        0x00             |  0x78
        0x01             |  0x56
        0x02             |  0x34
        0x03             |  0x12
Little-endian is commonly used by x86 architecture CPUs (such as Intel and AMD processors).

2.2 Big Endian:

In big-endian systems, the most significant byte (MSB) of a multi-byte value is stored at the lowest memory address.
Using the same 32-bit value 0x12345678, in a big-endian system, it would be stored in memory as follows:

Memory Address    |  Value
----------------------|--------
         0x00               |  0x12
         0x01               |  0x34
         0x02               |  0x56
         0x03               |  0x78
Big-endian is often used in network protocols and some older or specialized CPU architectures.

3. Why It's More About Memory:

• Memory Representation: Endianness defines how multi-byte data (like integers or floating-point numbers) is arranged in memory. The sequence of bytes in memory determines how data is read or written when accessed by software or hardware.
• Data Access: When a program accesses multi-byte data, it reads from memory according to the system's endianness. For example, reading a 32-bit integer from memory will yield different results depending on whether the system is little-endian or big-endian.

4. Role of the CPU:

• CPU Interpretation: The CPU must correctly interpret the byte order when fetching data from memory. However, the concept of endianness itself is more about how data is organized in memory rather than how the CPU processes it internally.
• Endianness Control: Some CPUs can operate in both little-endian and big-endian modes (bi-endian), and in such cases, the CPU can control how it interacts with memory. However, even in these cases, the primary concern is how data is stored and retrieved from memory.

Summary:

Memory is the primary context in which endianness is discussed and observed because it directly affects how data is stored and accessed.
CPU determines and respects the memory's byte order when performing operations, but the CPU’s internal workings are generally independent of endianness.