Unaligned and Mixed-endian Data Access Support
ARM DDI 0301H Copyright © 2004-2009 ARM Limited. All rights reserved. 4-18
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Mixed-endian data access
The operating-system typically has a required endian representation of internal data structures,
but applications and device drivers have to work with data shared with other processors, DSP or
DMA interfaces, that might have fixed big-endian or little-endian data formatting.
A byte-invariant addressing mechanism is provided that enables the load/store architecture to be
qualified by the CPSR E bit that provides byte reversing of big-endian data in to, and out of, the
processor register bank transparently. This byte-invariant big-endian representation is referred
to as BE-8 in this document.
Mixed-endian configuration supported on page 4-19 describes the effect on byte, halfword,
word, and multi-word accesses of setting the CPSR E bit when the U bit enables unaligned
support.
Byte data access
The same physical byte in memory is accessed whether big-endian, BE-8, or little-endian:
• unsigned byte load as Load unsigned byte, endian independent on page 4-6 describes
• signed byte load as Load signed byte, endian independent on page 4-6 describes
• byte store as Store byte, endian independent on page 4-6 describes.
Halfword data access
The same two physical bytes in memory are accessed whether big-endian, BE-8, or little-endian.
Big-endian halfword load data is byte-reversed as read into the processor register to ensure
little-endian internal representation, and similarly is byte-reversed on store to memory:
• unsigned halfword load as Load unsigned halfword, little-endian on page 4-7, LE, and
Load unsigned halfword, big-endian on page 4-7, BE-8 describe
• signed halfword load as Load signed halfword, little-endian on page 4-8, LE, and Load
signed halfword, big-endian on page 4-8, BE-8 describe
• halfword store as Store halfword, little-endian on page 4-9, LE, and Store halfword,
big-endian on page 4-9, BE-8 describe.
Word data access
The same four physical bytes in memory are accessed whether big-endian, BE-8, or
little-endian. Big-endian word load data is byte reversed as read into the processor register to
ensure little-endian internal representation, and similarly is byte-reversed on store to memory:
• word load as Load word, little-endian on page 4-10, LE, and Load word, big-endian on
page 4-10, BE-8 describes
• word store as Store word, little-endian on page 4-11, LE, and Store word, big-endian on
page 4-11, BE-8 describes.
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