2.6.3.2 Restrictions when Using PC or SP

Many instructions have restrictions on whether you can use the Program Counter (PC) or Stack Pointer (SP) for the operands or destination register. See instruction descriptions for more information.

Bit[0] of any address you write to the PC with a BX, BLX, LDM, LDR, or POP instruction must be 1 for correct execution, because this bit indicates the required instruction set, and the Cortex-M3 processor only supports Thumb instructions.