The hierarchy technique involves building small circuits that perform specific functions and then combining them inside of a larger design. In other words; making big, complex, high level circuits by plugging together lots of small, simple, low level circuits. (High level means high up in the hierarchal design, low level means the opposite.)

A good example is the ALU that you will build in lab 4. Most of the operations that the ALU is to perform are simple enough that they could easily be included in the ALU module itself. However, one of these operations, addition, is relatively complex. Thus, it is better to first design a smaller, separate module that performs the addition operation on its own. It is then easy to instantiate that module inside of the ALU module. This prevents high level modules from turning into a mess!

Here is a simple visual example of a larger module (the outer gray box) with inputs (usually on the left side) and outputs (usually on the right side) that is made up of smaller modules (the smaller white boxes). The highest level in a hierarchal design is called the Top Level.

Doing this in Verilog is simple. Simply instantiate the smaller modules inside of the larger module. See Using Modules on how to do this.

The cascading technique involves making a large circuit that can operate on multiple bits by hooking up several instances of a smaller circuit that is designed to operate on single-bit inputs. Each instance of the smaller circuit hooks up to the next instance to cascade all the necessary information along.

For example, instead of building a circuit to add two 3-bit numbers, build a circuit to add just two 1-bit numbers together. Then instantiate that circuit three times, hooking up each instance properly to perform the complete operation.

Here is a simple visual example similar to the ripple-carry adder found in the textbook. Each box represents an operation being performed on only 1 bit. ¹⁾ The results combine together to form the final 3-bit result. (This example is positioned from right to left simply so that the LSB is on the right side like it is in normal numbers.)