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a) The PC contains the address of the instruction we are now supposed to execute. The first thing we need to do is to read it from memory. These control lines initiate the appropriate read.

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.

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