Next let's look at the conditional jumps, those whose RTL use if statements such as JZ:

if Z=1 then PC <- x

The Z wire is ANDed with the instruction wire for JZ coming from the operation decoder. Only if both the JZ wire and Z are high will PC-LD and PC-MUX be set to 1, thereby allowing IR's 12 bits to be copied into PC. Since PC is 16 bits long, the upper 4 flip-flops are always loaded from a wire that always has logic 0. PC could have been made to be only 12 bits wide, but sometimes it is simpler to reuse basic components and just rig them up to act special in special cases, such as the PC.