Structural Design and Hierarchy

This chapter describes how to accomplish hierarchy within Verilog using lower-level subsystems. Structural design in Verilog refers to including lower-level subsystems within a higher-level module in order to produce the desired functionality. This is called hierarchy and is a good design practice because it enables design partitioning. A purely structural design will not contain any behavioral constructs in the module such as signal assignments, but instead just contain the instantiation and interconnections of other subsystems. A subsystem in Verilog is simply another module that is called by a higher-level module. Each lower-level module that is called is executed concurrently by the calling module.