Endianness - Diagram For Mapping Registers To Memory Locations

Diagram For Mapping Registers To Memory Locations

Using this chart, one can map an access (or, for a concrete example: "write 32 bit value to address 0") from register to memory or from memory to register. To help in understanding that access, little and big endianness can be seen in the diagram as differing in their coordinate system's orientation. Big endianness's atomic units (in this example the atomic unit is the byte) and memory coordinate system increases in the diagram from left to right, while little endianness's units increase from right to left.

A simple way to remember is "In Little Endian, The Least significant byte goes into the Lowest value slot". So in the above example, D, the least significant byte, goes into slot 0.

If you are writing in a western language the hex value 0x0a0b0c0d you are writing the bytes from left to right, you are implicitly writing Big-Endian style. 0x0a at 0, 0x0b at 1, 0x0c at 2, 0x0d at 3. On the other hand the output of memory is normally also printed out bytewise from left to right, first memory address 0, then memory address 1, then memory address 2, then memory address 3. So on a Big-Endian system when you write a 32-bit value (from a register) to an address in memory and after that output the memory, you "see what you have written" (because you are using the left to right coordinate system for the output of values in registers as well as the output of memory). However on a Little-Endian system the logical 0 address of a value in a register (for 8-bit, 16-bit and 32-bit) is the least significant byte, the one to the right. 0x0d at 0, 0x0c at 1, 0x0b at 2, 0x0a at 3. If you write a 32 bit register value to a memory location on a Little-Endian system and after that output the memory location (with growing addresses from left to right), then the output of the memory will appear reversed (byte-swapped). You have 2 choices now to synchronize the output of what you are seeing as values in registers and what you are seeing as memory: You can swap the output of the register values (0x0a0b0c0d => 0x0d0c0b0a) or you can swap the output of the memory (print from right to left). Because the values of registers are interpreted as numbers, which are, in western languages, written from left to right, it is natural to use the second approach, to display the memory from right to left. The above diagram does exactly that, when visualizing memory (when "thinking memory") on a Little-Endian system the memory should be seen growing to the left.