Now with a proper on-paper design of the datapath, the implementation phase of the development can begin. At this point there are a lot of different options that can be explored for the actual implementation.
VHDL hardware description language provides a method in which to construct the basics of the datapath. It can allow the designer to construct the signals within the circuit and how they interact. Although this provides a good developing environment, there are still many decisions on how the datapath implementation should be structured. For example, the 6502 datapath could be entirely composed of a single behavioral description with all of the registers and transfers written as assignments in VHDL code. This clearly would have some definite benefits in that is a compact (a single file) representation, and would be very similar to a emulator style implementation. Also simulation on a single behavior description would be much faster than doing separate components.
The main disadvantage of a single file implementation is that results in a excessively large single description which does not particularly represent the breakdown of the hardware components. For example, the registers of the datapath all should be instances of the same component type. Furthermore, without a hierarchical and structural grouping of the datapath components, debugging the implementation requires looking at all of the signals of the datapath, not just a particular level of the hierarchy.
The most beneficial method for implementing the datapath is to define the structural components through a behavioral model and then instance them together through a structural definition. This provides the designer with benefits of both approaches. The behavioral components provide an easy implementation that can be self contained and only export the useful values, like the program counter value instead of all the other signals like the carry for the incrementor, etc. Also the structural description at the next level provides clean representation that matches the appearance of the on-paper design.
