Why do some 3 to 5 decoders only have certain inputs decoded?

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Certain 3 to 5 decoders only decode specific inputs due to design choices that prioritize certain output combinations while categorizing other states as "don't care." This approach allows for a more efficient design process by focusing on unique input combinations to generate required outputs. Multiple valid designs exist, such as using Y1 = I1 AND I0 to avoid conflicts. In applications where every input must be decoded, these shortcuts would not be applicable. The EN signal functions as an ENABLE line, further influencing the decoder's operation.
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I don't know why they circle the 001 .011. 100.101. 110 .because I saw the textbook showed for 3to 4 decoder. Y0 =1 since 00. Y1 = 1 since 01. Y2 =1 since 10,Y3 = 1 since 11.

But for this problem . I am so confused.
 

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My guess is that the need was for an output only for the listed 3-bit inputs. Other states come under the "don't care" category. All that is required in the 'design' process is to find a unique combination of bits on the input (the fewer the better) to generate each of those required outputs. There is more than one valid design. e.g., an alternative which won't conflict with any of the other decoding appears to be Y1=I1.and.I0

There would be plenty of applications where they'd want every input decoded to an output, so none of these shortcuts would be possible.

The EN signal looks like an ENABLE line.
 
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