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4.1. About unaligned and mixed-endian data

The processor has the following features to support unaligned and mixed-endian data access:

  • permanently enabled support for unaligned data access

  • architecturally defined unaligned word, unaligned halfword, and word-aligned doubleword access

  • byte-reverse instructions that operate on general-purpose register contents to support signed and unsigned halfword data values

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

  • EE bit in CP15 c1 Control Register 1 that controls load and store endianness during exceptions

  • ARM and Thumb instructions to change the endianness and the E flag in the Program Status Registers (PSRs)

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

Note

Instructions are always little-endian and must be aligned according to the size of the instruction:

  • 32-bit ARM instructions must be word-aligned with address bits [1:0] equal to b00.

  • 16-bit or 32-bit Thumb instructions must be halfword-aligned with address bit [0] equal to 0.

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