Find MSB

Design a system that gets 8-bits input (X₇X₆…X₁X₀) and the output is also 8-bits wide which indicates the highest MSB that was ‘1’. You can use as many logic gates as you need.

For example: 

INPUT:       01101000    ,    00010100    ,    10000000

OUTPUT:  01000000   ,    00010000    ,    10000000

Answer

Here is a 4-bits input truth table that can easily be expanded to 8-bits input:

Notice: x is “don’t care”

From the truth table we can define the output:

Y₃ = X₃

Y₂ = X₂X₃’

Y₁ = X₁X₂’X₃’

Y₀ = X₀X₁’X₂’X₃’

Output of 8-bit system is defined similarly.

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