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(a) Describe the difference between parallel adders and serial adders; your answer should explain the significance of carry lookahead. What are the advantages and disadvantages of serial adders over parallel adders? [4 marks] (b) The diagram below shows a serial adder circuit. The two arguments are fed in at A and B, the sum returned at S.
(i) In what order should the bits of the arguments be fed in? [1 mark] (ii) Explain the purpose of the lowest of the three D flip-flops. [1 mark] (iii) What is the purpose of the input marked Z? Explain how the circuit achieves the purpose [2 marks] (c) Design a circuit that can be used to take as input two 4-bit numbers, and feed them bit by bit to the serial adder. Your circuit should take as inputs A0, A1, A2, A3 and B0, B1, B2, B3, the two numbers to be added, and S, a signal that is set high for one clock pulse to make the start of the addition then remains low for the rest of the computation. You may use shift registers for the inputs. [5 marks] (d) Design a circuit that takes as input the output of the serial adder, presumed to be the result of adding two 4-bit numbers, and collects it to yield a single 4-bit sum to be read in parallel. [2 marks] |
