ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.37 Unaligned pointers in C and C++ code
By default, the compiler expects conventional C and C++ pointers to point to naturally aligned
words in memory because this enables the compiler to generate more efficient code.
If you want to define a pointer that can point to a word at any address, you must specify the
__packed qualifier when defining the pointer. For example:
__packed int *pi; // pointer to unaligned int
When a pointer is declared as __packed, the compiler generates code that correctly accesses the
dereferenced value of the pointer, regardless of its alignment. The generated code consists of a
sequence of byte accesses, or variable alignment-dependent shifting and masking instructions,
rather than a simple LDR instruction. Consequently, declaring a pointer as __packed incurs a
performance and code size penalty.
5.37.1 See also
Reference
Compiler Reference:
• __packed on page 5-9.
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