My instructor has been insisting that the multiplexor in the picture above is a 4:2 mux not a 4:1 mux and that my textbook (Digital Design and Computer Architecture by Harris) is incorrect and that there are typos in the text. From what I have learned, this is indeed a 4:1 mux, because there are four inputs and one output.
He also says that a 4:1 or (4:2) mux has four data inputs and one two bit selector. However, from what is shown above it also seems that it should have two separate 1 bit selectors.
I am asking which is correct because I would genuinely like to know and also because I need to implement one in JLS.
Best Answer
With 4 different combination of S1 and S2 (1bit selectors), you select one of the four inputs and get that selected input on the output. So 4 tells you that you can get four different values at the output, and 1 tells you that you can have just one at a time on the output. I hope its now clearer whats happening here.
Your picture is 4:1 multiplexor, and if I'm right 4:2 multiplexor looks like this
Used from The Multiplexer