Simulation Backends

Use Verilator by default. Use Icarus Verilog for lightweight smoke tests, or where installing Verilator is impractical. Use xezim for a 4-state simulator that checks concurrent SVA without a C++ build step.

Choose a backend

Capability	Verilator	Icarus 12	xezim
SystemVerilog procedural code	Yes	Yes	Yes
Concurrent SVA and cover property	Yes	No	Yes
interface class frameworks	Yes	No	Yes
RTL Buddy line/toggle coverage	Yes	No	Yes (statement-level)
cocotb through VPI	Yes	Yes	Yes (VCS-flavour VPI library)
Default waveform	FST	VCD	FST, with --fst

Verilator compiles a cycle-based simv binary. Icarus compiles a .vvp snapshot and runs it with vvp; RTL Buddy generates a simv wrapper so the rest of the flow is the same.

xezim parses, elaborates and simulates in one invocation. The compile runs xezim --compile, which stops after elaboration, so syntax and elaboration errors fail the compile as with the other backends. RTL Buddy then writes a simv wrapper that reruns the same command line without --compile. Both point --cache-dir at the build directory, so runs reuse the elaborated design. The wrapper also adds --error-exit: without it xezim exits 0 after $error or a failed assertion, so the exit status could not fail a test, with or without a PASS/FAIL marker. xezim prints every error-severity message as ** Error: <message> followed by a Time: … Scope: … line: a bare failed assertion as ** Error: Assertion error., an assertion's else $error(...) action block or a plain $error with its own message. assertions: true counts each of these, as it counts Verilator's %Error: … Assertion failed lines. Note that:

• $dumpfile/$dumpvars do nothing without --wave. Put --wave, or --fst dump.fst, in the builder's compile-time options for the modes that need waves.
• xezim stops at --max-time, 100 ms of simulated time by default. Raise it in compile-time for longer tests.
• xezim is built from source: cargo build --release in a xezim checkout (Rust 1.92+). rb tool-check lists it as optional.

If a suite must run on Icarus, gate unsupported constructs. Use expected failures only when the failure mode is understood.

Select a builder

The builder comes from, in order: --builder <name>, the test's builder:, the suite's builder:, the platform default.

Set simulator-family on the cfg-rtl-builder entry, or use an executable name RTL Buddy can infer it from. See Selecting the simulator builder.

Open waveforms

Verilator writes dump.fst, Icarus writes dump.vcd, and xezim writes the file --fst names. rb wave opens the newest supported dump under artefacts/<test>/.

To convert an Icarus dump to FST after simulation, set wave-format: fst-postproc on the builder. If vcd2fst is missing, the VCD is kept.

Collect coverage

RTL Buddy coverage supports Verilator and xezim and follows the platform-selected builder. Pass --builder <name> or make that builder the platform default; a suite- or test-level override alone is not enough. See Coverage uses the platform builder. xezim coverage is statement-level and has no expression metric; see Collect coverage with xezim.