In User mode, the control bits of the CPSR are protected from change, so only the condition code flags of
the CPSR can be changed. In other (privileged) modes the entire CPSR can be changed.
The SPSR register which is accessed depends on the mode at the time of execution. For example, only
SPSR_fiq is accessible when the processor is in FIQ mode.
R15 shall not be specified as the source or destination register.
A further restriction is that no attempt shall be made to access an SPSR in User mode, since no such register
exists.