Waveform Debug

rwave — agent skill

rwave is a single binary for querying RTL simulation waveforms from the terminal. It natively reads VCD, FST, and GHW (prefer FST — typically 10× smaller than VCD). On linux-amd64 it also provides experimental support for WLF (Questa/ModelSim) and FSDB (Verdi) by calling into each vendor's own reader library interface. Seven query commands cover inspection, search, comparison, and summary. Always pass --json from an agent. This file covers what is unique to driving the tool from an agent — see the repo README for the full reference.

Install

Prebuilt binaries are attached to every tagged release (each with a .sha256). All four read VCD/FST/GHW; only rwave-linux-amd64 includes experimental WLF and FSDB support. Pick the one matching the runtime and chmod +x:

curl -fsSL -o ~/.local/bin/rwave \
  https://github.com/neveltyc/RWaveAnalyzer/releases/latest/download/rwave-linux-amd64
chmod +x ~/.local/bin/rwave
~/.local/bin/rwave --version

Vendor formats — experimental (linux-amd64 only)

On linux-amd64, rwave provides experimental support for Questa .wlf and Verdi .fsdb by calling into each vendor's own reader library interface. Point rwave at the library from the user's licensed installation via an env var, then query as usual:

export RWAVE_WLF_LIB=/path/to/questa/linux_x86_64/libwlf.so          # for .wlf
export RWAVE_FSDB_LIB="$VERDI_HOME/share/NPI/lib/linux64/libNPI.so"  # for .fsdb (needs a Verdi-Ultra license)
rwave --json info dump.fsdb

If the env var is unset, the library/license is missing, or the build is not linux-amd64, .wlf/.fsdb fail with a one-line Error: — fall back to converting the dump to VCD or FST first.

Pick the right command

User wants to know...
├─ "What's in this file?"
│   └─ info           file overview, signal count, time span, scopes
├─ "What signals exist?" / "Find signals matching X"
│   └─ list           signal paths with width and type
├─ "What happened between T1 and T2?"
│   └─ dump           value-change events in time order
├─ "Which signals are active/static?"
│   └─ summary        per-signal change count, edges, unique values
├─ "What is the value of X at time T?"
│   └─ snapshot       all known signal values at one time point
├─ "What changed between T1 and T2?"
│   └─ compare        diff of signal values at two time points
└─ "When does condition C hold?" / "Find handshakes"
    └─ search         condition-based, three sub-modes:
        ├─ interval   time ranges where condition is true (no --show, no --changed)
        ├─ segment    intervals + observed values         (with --show)
        └─ event      fires when one signal transitions   (--changed SIG)

search's JSON top-level key depends on the mode: intervals / segments / events. Always check mode before parsing. --changed takes one signal pattern, not comma-separated. To catch both edges, run two searches: !=0 for rising, =0 for falling.

Condition syntax (search only)

Comma-separated AND list. Each item is SIG=VAL, SIG==VAL, or SIG!=VAL.

• Signal pattern must resolve to exactly one signal. If ambiguous, the error lists candidates — use a more specific path.
• Values: decimal (5), hex (0xff), binary (b1010 / 0b1010), 4-state (b1x0z), or bare x/z.
• != does not match x/z ("unknown is not evidence of difference"). To find unknowns, ask explicitly with sig=x.
• No OR. Run two searches and merge.

Command quick reference

<F> is the input file. See the repo README for the full surface; the table below is the agent-side cheat sheet of the JSON-form arguments and the fields you'll usually parse out.

For dump, always pass --begin/--end and --filter — running it unbounded on a large dump streams the whole file. For snapshot and compare on large files, always pass --filter — unfiltered scans emit every signal.

Filter patterns: substring (clk), suffix glob (*_valid), prefix glob (top.u_dma.*). list shows all aliases of matched signals, not only the matching paths. A signal hit once may surface dozens of alias rows — use --verbose to group by id. For one signal = one row, filter precisely and use --verbose — same id means same signal.

Batch mode (one load, many queries)

For a pre-planned multi-step investigation of one file — especially a large .fsdb/.wlf that is slow to open — use --batch to load the file once and run a list of commands from stdin, instead of paying the open cost on every call:

printf '%s\n' \
  'info' \
  'list --filter clk,state' \
  'search --condition valid=1,ready=1 --show data  #handshake' \
  | rwave --batch --json sim.fsdb

• One command per line — exactly what you'd type after rwave, minus the file (the file is given once on the --batch line). Pass --json: output is one NDJSON object per line, {"id","ok","result"} or {"id","ok","error"}, in input order.
• id is the trailing #label if present, else a 1-based line number. Correlate by input order (authoritative) or id. Blank and #-comment lines are skipped; [global-opts] on the --batch line are per-command defaults.
• Each result is byte-identical to the equivalent single-command --json output — parse it exactly the same way.
• A failing command is "ok":false and does not stop the batch; the process still exits 0. Check each line's ok. Only a bad file or an unreadable stream is fatal (non-zero exit).
• Plan the full list up front — batch does not let you see one result before choosing the next. For adaptive, read-then-decide flows, use separate calls.

Workflow patterns

(all assume --json)

First contact with a waveform file

1. info                        learn time range, scopes, timescale
2. list --filter <suspect>     find the signals of interest
3. summary --filter <window>   spot active vs static signals
4. dump or search              drill into specifics

"What happened at time T?"

1. snapshot --at T
2. dump --begin T-Δ --end T+Δ
3. compare --at T-Δ,T+Δ

Protocol transaction extraction (AXI, AHB, etc.)

1. list --filter '*valid,*ready,*addr,*data,*len'
2. search --condition "arvalid=1,arready=1" --show araddr,arlen
3. search --condition "wvalid=1,wready=1" --show wdata,wstrb

search segment mode is the primary tool here — one row per sub-interval, with --show capturing the field values you care about.

Hunt an unexpected state

1. search --condition "state=x"          when does it go unknown?
2. search --condition "error!=0"         when does it assert?
3. snapshot --at <first_hit>             full picture at that moment
4. dump --begin <pre> --end <hit> --filter <relevant>

Clock/reset sanity

summary --filter clk,rst,reset
# clk should toggle with balanced rise/fall
# rst should be static after the initial assertion

Event-driven signal investigation

Use search --condition --show to bulk-extract field values across events — one call replaces multiple snapshot calls. Catch both edges with complementary search --changed (rising: !=0, falling: =0). Then drill down with compare for jump deltas, dump --limit 0 for full traces, and snapshot for precise checkpoints. When a transition is visible in a different signal's trace, use dump --limit 0 + external post-processing — not search --changed.

dump with multiple signals interleaves their events chronologically — see e.g. a push flag and data bus transition side-by-side in one timeline.

Agent-side gotchas

• Output truncation. Default --limit is 200. If truncated: true, there are more rows — either re-run with --limit 0 (unlimited) or a larger value. total_is_exact: false means total is a lower bound, not the true count.
• search mode discriminator. The output's top-level array key depends on the mode (intervals / segments / events). Always read the mode field first.
• Exit code is non-zero on errors. Errors are a single line on stderr starting with Error:. Catch and parse them.
• --json everywhere. Mixing text-mode parsing in is the most common source of fragility. Pass --json on every invocation.

Documented behaviors that may surprise

• dump's ordering of simultaneous events follows declaration order (not VCD writer-emission order). Set of events, timestamps, values are identical to the reference; only intra-timestamp order can differ.
• comments is always [] and synthesized_buses is always 0
• A zero-width search window (--begin T --end T) yields no rows.
• Value format. Multi-bit logic values print as 0x<hex> (lower-case, leading zeros stripped — 0x4, not 0x00000004); 1-bit as 0/1/x/z; a bus with any unknown bit as b<bits> (e.g. b01x0); real/string verbatim. Width is in the signal metadata, not the value — convert hex→int yourself if you need decimal.

For everything else (time syntax, filter syntax, value formatting, format quirks, the FST parameter-value drop, performance notes) see the repo README.