Difference in the datapath of Load Upper Immediate to Load Word in a 32 bit MIPS processor
I had these pictures sitting around except for the red lines. 

The two instructions differ in semantics:



$t is the destination register, $i is an immediate value.

LUI:



LW:



The images are all links to their full size versions.

In a real implementation the muxes to the ALU could likely produce 0 and 1 constants.

And along the bottom of those pipeline diagrams the $t should actually be t, the pointer to the target or destination register.