Little-endian - Endianness and Hardware

Endianness and Hardware

Computer memory consists of a sequence of cells, usually bytes, and each cell has a number called its address that programs use to refer to it. If total number of bytes in memory is n then bytes addresses would be enumerated 0 to n-1. Multi-byte CPU registers are stored in memory as a simple concatenation of bytes. The simple forms are:

• increasing numeric significance with increasing memory addresses (or increasing time), known as little-endian, and
• decreasing numeric significance with increasing memory addresses (or increasing time), known as big-endian

Well-known processor architectures that use the little-endian format include x86 (including x86-64), 6502 (including 65802, 65C816), Z80 (including Z180, eZ80 etc.), MCS-48, 8051, DEC Alpha, Altera Nios II, Atmel AVR, SuperH, VAX, and, largely, PDP-11.

Well-known processors that use the big-endian format include Motorola 6800 and 68k, Xilinx Microblaze, IBM POWER, and System/360 and its successors such as System/370, ESA/390, and z/Architecture. The PDP-10 also used big-endian addressing for byte-oriented instructions. SPARC historically used big-endian until version 9, which is bi-endian, similarly the ARM architecture was little-endian before version 3 when it became bi-endian, and the PowerPC and Power Architecture descendants of POWER are also bi-endian (see below).