memory barrier in UP

Hi All, I am reading about the barrier from linux kernel development.In this books, he says "On SMP kernels they are defined only as usual memory barriers.where as on UP kernels they are defined only as a compiler barrier" Does this mean in UP, processor won't reorder the instructions ? Could please give any idea on this. -- With Regards Subin Gangadharan I am not afraid and I am also not afraid of being afraid.