b) What is being added is the contents of the PC register, plus zero, plus 1. Thus we compute PC+1. The PC register contents comes from PCout; the Zero A causes a zero to be added into the mix; and the Set Carry-In supplies the 1. We need to compute the value of PC+1 at some point to store into the PC register so that the next time this fetch sequence is executed, the subsequent instruction in memory will be read.