ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.33 Unaligned data access in C and C++ code
It can be necessary to access unaligned data in memory, for example, when porting legacy code
from a CISC architecture where instructions are available to directly access unaligned data in
memory.
On ARMv4 and ARMv5 architectures, and on the ARMv6 architecture depending on how it is
configured, care is required when accessing unaligned data in memory, to avoid unexpected
results. For example, when a conventional pointer is used to read a word in C or C++ source
code, the ARM compiler generates assembly language code that reads the word using an LDR
instruction. This works as expected when the address is a multiple of four, for example if it lies
on a word boundary. However, if the address is not a multiple of four, the LDR instruction returns
a rotated result rather than performing a true unaligned word load. Generally, this rotation is not
what the programmer expects.
On ARMv6 and later architectures, unaligned access is fully supported.
5.33.1 See also
Concepts
• Types of data alignment on page 5-41
• The __packed qualifier and unaligned data access in C and C++ code on page 5-46.
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