2.6.3.5 Address Alignment

An aligned access is an operation where a word-aligned address is used for a word, dual word, or multiple word access, or where a halfword-aligned address is used for a halfword access. Byte accesses are always aligned.

The Cortex-M3 processor supports unaligned access only for the following instructions:

• LDR, LDRT
• LDRH, LDRHT
• LDRSH, LDRSHT
• STR, STRT
• STRH, STRHT

All other load and store instructions generate a UsageFault exception if they perform an unaligned access, and therefore their accesses must be address aligned. For more information about UsageFaults, see 2.5.4 Fault Handling.

Unaligned accesses are usually slower than aligned accesses. In addition, some memory regions might not support unaligned accesses. Therefore, Arm recommends that programmers ensure that accesses are aligned. To trap accidental generation of unaligned accesses, use the UNALIGN_TRP bit in the Configuration and Control Register, see 2.7.2.7 Configuration and Control Register.