VV is an interactive simulator for Verilog that visualises Verilog's simulation semantics as described by the SystemVerilog-2023 standard, in particular, the stratified event queue of the semantics. See the paper "The Simulation Semantics of Synthesisable Verilog" (OOPSLA'25) for further context.

VV is web-browser-based and a live demo is available at andreasloow.github.io/vv. See in particular the dropdown menu (top left) in the demo, which contains a collection of (commented) example Verilog modules illustrating various aspects of Verilog's semantics.

The example Verilog modules available in the demo are available as files in the directory examples, such that they can, e.g., be loaded into a traditional Verilog simulator to compare the execution in the simulator with the execution in VV.

VV is implemented in ReScript, an OCaml dialect of JavaScript. The front-end uses React. For parsing Verilog files, Ohm is used (whose error reporting leaves much to be desired). After parsing, some simple checks and elaboration are done by src/Elaborator.res and then the resulting parse tree is compiled to an internal representation by src/Compiler.res; the internal representation is unstructured (gotos) instead of structured (as Verilog: if, etc.) to make the implementation of pausing execution easier. The core of the simulator is located in src/Verilog.res. The front-end of the simulator is in src/Simulator.res.

Before building VV for the first time and when example modules are updated, you need to execute examples/compile.hs (e.g., runhaskell compile.hs) to generate a new/updated src/Templates.res file.

Now, to build and run VV locally, use the following commands:

npm i
npm run res:build
npm run dev

The test suite runs every example module in examples through the simulator, always executing the first active event in the event queue, and compares the resulting output and final state against the expected results recorded in test/expected.txt:

npm test

The suite needs npm run res:build to have been run first, since it imports the compiled simulator, but it does not need an up-to-date src/Templates.res, as it reads the example files directly.

When a change to VV intentionally changes simulation behaviour, regenerate the expected results and inspect the resulting diff before committing:

npm run test:update