Fundamental Hardware Elements of Computers: Boolean identities
Sometimes a very complex set of gates can be simplified to save on cost and make faster circuits. A quick way to do that is through boolean identities. Boolean identities are quick rules that allow you to simplify boolean expressions. For all situations described below:
A = It is raining upon the British Museum right now (or any other statement that can be true or false) B = I have a cold (or any other statement that can be true or false)
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It is raining AND It is raining is the same as saying It is raining |
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It is raining AND It isn't raining is impossible at the same time so the statement is always false |
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2+2=4 OR It is raining. So it doesn't matter whether it's raining or not as 2+2=4 and it is impossible to make the equation false |
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1+2=4 OR It is raining. So it doesn't matter about the 1+2=4 statement, the only thing that will make the statement true or not is whether it's raining |
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It is raining OR It is raining is the equivalent of saying It is raining |
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It is raining OR It isn't raining is always true |
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1+2=4 AND It is raining. It is impossible to make 1+2=4 so this equation so this equation is always false |
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2+2=4 AND It is raining. This statement relies totally on whether it is raining or not, so we can ignore the 2+2=4 part |
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It is raining OR I have a cold, is the same as saying: I have a cold OR It is raining |
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It is raining AND I have a cold, is the same as saying: I have a cold AND It is raining |
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It is raining OR (It is raining AND I have a cold). If It is raining then both sides of the equation are true. Or if It is not raining then both sides are false. Therefore everything relies on A and we can replace the whole thing with A. Alternatively we could play with the boolean algebra equation: Using the identity rule |
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It is raining AND (It is raining OR I have a cold). If It is raining then both sides of the equation are true. Or if It is not raining then both sides are false. Therefore everything relies on A and we can replace the whole thing with A. Alternatively we could play with the boolean algebra equation: Using the identity rule |
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Examples of manipulating and simplifying simple Boolean expressions.
Example: Simplifying boolean expressions Let's try to simplify the following:
Using the rule
Trying a slightly more complicated example:
dealing with the bracket first as as |
Exercise: Simplifying boolean expressions
Answer:
Answer:
Answer:
Answer:
Answer:
Answer:
Answer:
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Sometimes we'll have to use a combination of boolean identities and 'multiplying' out the equations. This isn't always simple, so be prepared to write truth tables to check your answers:
Example: Simplifying boolean expressions
Where can we go from here, let's take a look at some identities
Now for something that requires some 'multiplication'
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Exercise: Simplifying boolean expressions
Answer:
multiplying out
Answer: This takes some 'multiplying' out:
Answer: This takes some 'multiplying' out: treat the brackets first and the AND inside the brackets first multiply it out as as
Answer:
as as take A out as the common denominator as
Answer: This takes some 'multiplying' out:
Answer: This takes some 'multiplying' out: multiplied out as as
Answer: Take the common factor, from both sides: As Then As Then
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