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 C or C++ source code uses a conventional pointer 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
On ARMv6 and later architectures, unaligned access is fully supported.