I'm designing a 4 bit ALU as a project. I have circuits that can calculate addition, subtraction and bitwise logic operations - my question is, do I tie A & B (4 bit inputs) to every circuit that performs an operation/function, meaning every function is carried out at once, and use a multiplexer to select which output I want? Or is there a more efficient/elegant way of performing just the operation I want on A & B? I'm not sure (traditionally) how this is done.
Look up the detailed design of SN74181 from Texas Instruments. Every logic gate will be shown. Here is a portion, from http://www.ti.com/lit/ds/symlink/sn54s181.pdf
Waste of MUX to combine separate logic gates. MUX is the ultimate any gate. 4way pass-through such as 74CBT3253 make chain comparisons without ripple. From magnitude test we can instantly solve all slices of A-B's borrow, B-A's borrow, or A+B's carry.