GATE CS 2022 Set 1 — Question 40
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Digital Logic → Combinational Circuits → Encoders & Decoders
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Question
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?
Which one of the following represents the functionality of P, Q, and T?
Correct answer
(C) P is 10:2¹⁰ decoder; Q is 5:2⁵ decoder; T is 2:1 multiplexer
Solution
1.Analyze S (Memory Segment): S is a 1024-word memory. To address 1024 words, we need address bits. The component P takes the S-address (from the code word) and enables the specific word in S. This is the function of a decoder. Specifically, a decoder takes 10 input bits and activates one of the output lines corresponding to the rows/words in the memory.
2.Analyze R (Register File): R is a 32-word register file. To address 32 registers, we need address bits. The component Q takes the R-address and selects the specific register. This is the function of a decoder (or decoder).
3.Analyze T: T takes the data output from S and the data output from R and selects one of them to load into register X, based on the mode bit M. Since it selects 1 out of 2 inputs, T is a 2:1 multiplexer.
Therefore:- P is a decoder.
- Q is a decoder.
- T is a 2:1 multiplexer.
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