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GATE 2022 CS – Question 40

Digital Logic · Combinational Circuits · 2 marks · Multiple choice

Consider a digital display system (DDS) shown in the figure that displays the contents of register X. A 16-bit code word is used to load a word in X, either from S or from R. S is a 1024-word memory segment and R is a 32-word register file. Based on the value of mode bit M, T selects an input word to load in X. P and Q interface with the corresponding bits in the code word to choose the addressed word. Which one of the following represents the functionality of P, Q, and T?

the code word is M | S-address | R-address; the S-address feeds P into S, the R-address feeds Q into R, and S, R and M feed T into X.
  1. P is 10:1 multiplexer; Q is 5:1 multiplexer; T is 2:1 multiplexer
  2. P is 10:$2^{10}$ decoder; Q is 5:$2^5$ decoder; T is 2:1 encoder
  3. P is 10:$2^{10}$ decoder; Q is 5:$2^5$ decoder; T is 2:1 multiplexer
  4. P is 1:10 de-multiplexer; Q is 1:5 de-multiplexer; T is 2:1 multiplexer

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Correct answer: (C) P is 10:$2^{10}$ decoder; Q is 5:$2^5$ decoder; T is 2:1 multiplexer

Explanation

A 10-bit address selecting one of 1024 words needs a 10:1024 decoder, and a 5-bit address selecting one of 32 registers needs a 5:32 decoder. T chooses between S and R using the mode bit M, so it is a 2:1 multiplexer.