Many instructions are unable to use, or 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.
When you update the PC with a
POP instruction, bit of any address must be 1
for correct execution. This is because this bit indicates the destination
instruction set, and the Cortex-M0+ processor only supports Thumb
instructions. When a
writes the value of bit into the LR it is automatically assigned
the value 1.