Apologies for this vague title, but my question is a little specific.
I have two concerns :
During my digital electronics class, I was told that the design of the processor is first carried on an FPGA board and verified. Only then it is finalized. In order to design the processors on FPGA board, you use Verilog and VHDL, and then let the tools do the heavy lifting. These tools then infer or deduce a schematic / design from our program. Also, simultaneously, another team of engineers is busy programming test benches and carry out verification and simulation etc.
My first question :
If you let the tools do the designing portion of your processor (or microprocessor),
then wont it always infer the same circuit, or at least nearly same circuit
for your processor ?
For example, as one of lab exercises, we were required to program a 4 bit ALU on xilinx ISE in verilog. As long as we had the same instructions / operations/ functionality for our ALU , if we let the tools design ALU, wont it always design the same circuit ? I believe this because ultimately at the transistor level, an AND gate (just an example )will always be composed of, lets say 5 mos transistors (as provided in the library of components ). Also ALU will always contain same blocks / nearly same blocks like adder, subtractor etc ( In other words, same design will be inferred).
My second question :
If I am correct above ( which obviously I doubt ), then how would the designs differ ?
In other words, where lies the role of a human / engineer if everything is
done by the tools .
As another example, Intel produces processors. If we for the time being assume that they produce on 28 nm, and will continue to do so for another 5 years ( just an example) . Then how is it that they come up with fast processors every year or so ? I am confused because at the end of the day, Intel itself will obviously be using these kind of tools (verilog / vhdl) to design their chips. So as long as they have a functionality which does not change, they should always come up with same design.
Please understand that this is not a broad question. It focus on the automation aspects / designing using tools only. Also I am not concerned with advances in computer science concepts, such as introduction of pipelines etc which improved performance. I am only interested in above mention aspects.
I am asking this because I am little interested in designing a small microprocessor before my graduation. But programming a couple of lines in Verilog / vhdl and letting the tools do the job does not seem to excite me at all.
Thank you.
Best Answer
No. There are several low-level details which will result in different circuitry. Suppose you design an adder. You are really only designing the functional specification of the adder, either in high level terms (e.g. verilog "+" operator) or lower level terms such as gates. However this design must be mapped onto the actual blocks or gates which will implement the design. On an FPGA you may have a macro-cell or a look-up table (LUT) structure which gets repeated in an array across the chip. For an ASIC you have raw gates.
As part of this there is typically some kind of "place and route" step which will map the functional blocks onto the two dimensional layout of the resources on the FPGA or ASIC. Placing an routing are not simple algorithms - my understanding is that they are "NP complete" as there is not a polynomial time algorithm to find the optimum layout and routing. As a typical part of a design you will specify timing constraints to be met, and the place and route will try to operate so that these constraints are met. Because the process is not exhaustive (it would take too much time) a "seed" value is typically used to determine how to start out, and then a solution is looked for that is close to where it needs to be. Different seeds will produce different layouts with the same functional characteristics but (usually slightly) different timings.
As other answers have mentioned, the actual functional design is not done by the tools. But there is still room for human interaction with the tools in determining the actual functioning. In addition to specifying timing requirements it is sometimes necessary to go back and tweak the design to accomodate these requirements. For instance, the tools might identify a critical path which limits the clock frequency at which the design can run. This might be slower than is desired, and the engineering would need to go back and figure out how to make it meet timing, such as pipelining, reducing fan-out, and so forth.