Cpu's big endian mode little endian mode comparison

First, the origin of big endian and little endian

The concept of big-endian and little-endian has an interesting historical background. It originated from Jonathan Swift's "Gulliver's Travels," where a conflict between two fictional nations, Lilliput and Blefuscu, was sparked by a dispute over how to break eggs. The Lilliputians preferred breaking the larger end, while the Blefuscu people broke the smaller end. This satirical tale mirrored real-world conflicts between Britain and France. Danny Cohen, a pioneer in network protocols, later used these terms to describe byte order in computing, which became widely accepted.

Second, what is big-endian and little-endian?

Big-endian and little-endian refer to the way multi-byte data is stored in memory. In big-endian mode, the most significant byte (MSB) is stored at the lowest memory address, while the least significant byte (LSB) is stored at the highest address. In little-endian mode, the LSB is stored at the lowest address, and the MSB is at the highest. For example, the 32-bit value 0x12345678 would be stored as follows:

Big Endian:

Low Address -----------------> High Address 0x12 | 0x34 | 0x56 | 0x78

Little Endian:

Low Address -----------------> High Address 0x78 | 0x56 | 0x34 | 0x12

This difference affects how data is interpreted across systems and is crucial for data exchange between different platforms.

Third, examples of storage in big-endian and little-endian modes

Let’s consider the 16-bit number 0x1234. In little-endian mode, it would be stored as 0x34 at address 0x4000 and 0x12 at 0x4001. In big-endian mode, the values would be reversed: 0x12 at 0x4000 and 0x34 at 0x4001. Similarly, for a 32-bit number like 0x12345678, the bytes are stored in reverse order in little-endian mode compared to big-endian.

Fourth, advantages and disadvantages of each mode

Little-endian mode simplifies data access for small byte sizes and allows direct reading without byte swapping. Big-endian mode makes it easier to determine the sign of a number, as the sign bit is always in the first byte. Each mode has its own strengths depending on the use case.

Fifth, determining the endianness of a machine

You can check the endianness of a system using simple code. A common approach is to store a known value in an integer and then check the first byte. If the byte matches the higher-order byte, the system is big-endian; otherwise, it is little-endian. Another method uses a union to compare the stored values directly.

Sixth, common endianness in systems and applications

Most operating systems today use little-endian, but many communication protocols, such as those used in networking, rely on big-endian. CPUs like x86 use little-endian, while PowerPC and IBM processors use big-endian. ARM processors can support both, depending on configuration.

Seventh, converting between endianness

To convert between endianness, you can swap the bytes of a number. For example, a 16-bit value can be converted using bitwise operations to swap the high and low bytes. For 32-bit values, more complex operations are required to ensure correct byte ordering.

Eighth, understanding endianness in software

In software development, especially for network communication, endianness must be carefully managed. Functions like ntohs() and htons() are used to convert between host and network byte orders. These functions are essential when transferring data between systems with different endianness.

Ninth, understanding endianness from a system perspective

From a hardware standpoint, endianness affects how data is stored and accessed. Big-endian systems store the most significant bits at lower addresses, while little-endian systems do the opposite. This impacts everything from memory access to peripheral interfaces, requiring careful design in both software and hardware.

Tenth, practical example: MODBUS communication

In MODBUS communication, data is typically sent in big-endian format. If a system uses little-endian, the data must be converted before transmission. For instance, if a 16-bit value is stored as 0x5634 in memory, it should be swapped to 0x3456 before sending to ensure the correct interpretation by the receiving device.