Reverse Polarity: Understanding and Application

[EvLer]

So, we are just starting ABEL... and I don't quite understand reverse polarity usefullness or application. Here's the extract from my textbook (Wakerly, 3rd ed.):
"In this example, the reverse-polarity equation has one less product term than the normal polarity equation (whatever...the example), so the compiler will select this equation if the targeted device has selectable output inversion."

could someone explain or give an example how this concept is used...
ok, so i understand that compiler will go for a more optimal option, but what does not make sense is that it would select let's say F' over F if F' is more optimal, but the circuit should operate on F not F'! how is this making sense?
Maybe I am asking the wrong question...anyway, I'm confused.

thanks much.

 

[berkeman]

I believe it refers to the fact that you can write an equation using OR or AND functions to gather up terms. Like:

A & B = !(!A + !B)

In some equations, it may take fewer terms when you use one form or the other, which may help you fit in a smaller CPLD, for example.