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Compiler Command-line Options
2.1.161 --unaligned_access, --no_unaligned_access
These options enable and disable unaligned accesses to data on ARM
architecture-based processors.
Default
The default is --unaligned_access on ARM-architecture based processors that support
unaligned accesses to data. This includes:
• all ARMv6 architecture-based processors
• ARMv7-A, ARMv7-R, and ARMv7-M architecture-based processors.
The default is --no_unaligned_access on ARM-architecture based processors that do not
support unaligned accesses to data. This includes:
• all pre-ARMv6 architecture-based processors
• ARMv6-M architecture-based processors.
Usage
--unaligned_access
Use --unaligned_access on processors that support unaligned accesses to
data, for example --cpu=ARM1136J-S, to speed up accesses to packed
structures.
To enable unaligned support, you must:
• Clear the A bit, bit 1, of CP15 register 1 in your initialization code.
• Set the U bit, bit 22, of CP15 register 1 in your initialization code.
The initial value of the U bit is determined by the UBITINIT input
to the core.
The RVCT libraries include special versions of certain library functions
designed to exploit unaligned accesses. When unaligned access support
is enabled, the RVCT tools use these library functions to take advantage
of unaligned accesses.
--no_unaligned_access
Use --no_unaligned_access to disable the generation of unaligned word
and halfword accesses on ARMv6 processors.
To enable modulo four-byte alignment checking on an ARMv6 target
without unaligned accesses, you must:
• Set the A bit, bit 1, of CP15 register 1 in your initialization code.
• Set the U bit, bit 22, of CP15 register 1 in your initialization code.
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