National Exams — December 2019 · 16-Elec-A4 Digital Systems & Computers.
Closed book · 3 hours · one of two approved calculators. Six questions; any five constitute a complete paper and all questions are worth 12 marks. All six are solved here as a study resource.
Reference texts: M. M. Mano & M. D. Ciletti, Digital Design (6th ed.); J. F. Wakerly, Digital Design: Principles and Practices (5th ed.); R. J. Tocci, N. S. Widmer & G. L. Moss, Digital Systems: Principles and Applications (12th ed.); Motorola/Freescale M68HC11 Reference Manual; V. C. Hamacher, Z. G. Vranesic & S. G. Zaky, Computer Organization (5th ed.).
Check — figure readings. The two circuit figures are redrawn below from the printed paper. Readings used below: Q1’s second and fifth product terms are $A\overline{B}C$ and $\overline{A}\,\overline{B}C\overline{D}$; Q2(a) asks for 8:1-multiplexer realizations and gives $ABC=011$ and $111$ as don’t-cares for $f_1$; Q3’s gates are three-input ANDs and flip-flop $B$ is wired as a toggle ($R_B=B$, $S_B=\overline{B}$), so the circuit is a binary up/down counter; Q4’s flip-flop clock is taken from decoder output ̅Y2 (third output down), not ̅Y0. Each correction is explained in the relevant answer.
