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4.1. About unaligned and mixed-endian support

The ARM1136JF-S processor executes the ARM architecture v6 instructions that support mixed-endian access in hardware, and assist unaligned data accesses. The extensions to ARMv6 that support unaligned and mixed-endian accesses include the following:

  • CP15 register c1 has a U bit that enables unaligned support. This bit was specified as zero in previous architectures, and resets to zero for backwards compatibility.

  • Architecturally defined unaligned word and halfword access specification for hardware implementation.

  • Byte reverse instructions that operate on general-purpose register contents to support signed/unsigned halfword data values.

  • Separate instruction and data endianness, with instructions fixed as little-endian format, naturally aligned, but with support for 32-bit word-invariant binary images and ROM.

  • A PSR endian control flag, the E-bit, cleared on reset and exception entry, that adds a byte-reverse operation to the entire load and store instruction space as data is loaded into and stored back out of the register file. In previous architectures this Program Status Register bit was specified as zero. It is not set in code written to conform to architectures prior to ARMv6.

  • ARM and Thumb instructions to set and clear the E-bit explicitly.

  • A byte-invariant addressing scheme to support fine-grain big-endian and little-endian shared data structures, to conform to a shared memory standard.

The original ARM architecture was designed as little-endian. This provides a consistent address ordering of bits, bytes, words, cache lines, and pages, and is assumed by the documentation of instruction set encoding and memory and register bit significance. Subsequently, big-endian support was added to enable big-endian byte addressing of memory. A little-endian nomenclature is used for bit-ordering and byte addressing throughout this manual.