uint32_t如果没4字节对齐直接就是 usagefault，这是 CM0.
看 Cortex-M3™ Revision: r0p0 Technical Reference Manua
14.4.1 Unaligned accesses
Unaligned data and debug accesses are converted into two or three (depending on the size and alignment of the unaligned access) aligned accesses. This stalls any subsequent accesses until the unaligned access has completed. For a description of unaligned accesses, see Access alignment on page 14-9.