2.17 Saved Program Status Registers
A Saved Program Status Register (SPSR) stores the current value of the CPSR when an exception is taken so that the CPSR can be restored after handling the exception.
Each exception handling mode can access its own SPSR. User mode and System mode do not have
an SPSR because they are not exception handling modes.
The execution state bits, including the endianness state and current instruction set state
can be accessed from the SPSR in any exception mode, using the
MRS instruction. You cannot access the SPSR using
MRS in User or System mode.