“wrteei 0” is an instruction used to update of MSR[EE] without affecting other MSR bits (MSR is core register).
Pay attention to page 750 of Programmer’s Reference Manual:
http://www.nxp.com/files/32bit/doc/ref_manual/EREF_RM.pdf
wrteei 1 sets MSR[EE] i.e. enables external interrupt recognition
wrteei 0 clears MSR[EE] i.e. disables external interrupt recognition
