Open FST files in your browser
GTKWave's compressed waveform, far smaller than a VCD, as Verilator, Icarus Verilog, GHDL and NVC write it. Open one right here, every signal on one time ruler, with no GTKWave. Nothing leaves your device.
Or drop it anywhere on this page
What you'll see
Your waveform opens with its signals down one side, grouped by scope, and their values along one time ruler.
- Every signal by its scope • Buses in hex, decimal or binary
- Two cursors that snap to edges • The time between them and its frequency
- Find a signal by its name, or a value with =
- Export • PNG, and the values between the cursors as CSV
About FST
GTKWave made the Fast Signal Trace for dumps too large as VCD. Verilator writes it with --trace-fst, Icarus Verilog with -fst, GHDL with --fst and NVC by default, often at a tenth of the VCD's size or less.
An FST is a run of blocks: a header with the time scale and the simulator that wrote it, each signal's width, the design's scopes and names, and sections of value changes, each holding every signal's first values and its changes packed on their own with LZ4, FastLZ or zlib, so a viewer can read one signal without the others.
Filepeek reads it in your browser, every section unpacked by our own LZ4, FastLZ and zlib, and draws every signal on one time ruler: bits high and low, buses as their values in hex, decimal or binary, VHDL's enumerations by their names and reals as curves. Fold a scope, drag or pinch to move and zoom, put two cursors for the time between them, find a signal or a value, and save the view as PNG, the values between the cursors as CSV, or the whole waveform as a VCD for any other tool.
Fast Signal Trace
.fst
Format
- MIME type
- application/x-fst
- Made by
- GTKWave
Inside the file
- Header
- The time scale, the date and the simulator
- Hierarchy
- Scopes and signals, with VHDL's types
- Changes
- Each signal packed with LZ4, FastLZ or zlib
- Times
- Each moment of change, kept once
Opens in every browser
Filepeek opens FST files in Chrome, Edge, Firefox and Safari, from Verilator, Icarus Verilog, GHDL and NVC. On their own, browsers only download them.
- YouChromeOpens through Filepeek
- YouEdgeOpens through Filepeek
- YouFirefoxOpens through Filepeek
- YouSafariOpens through Filepeek
Apps that open FST
- WindowsGTKWave or Surfer, free
- MacGTKWave or Surfer, free
- iPhone and iPadNo app opens it
- AndroidNo app opens it
Questions
How do I open an FST file without GTKWave?
Drop it on this page or choose Peek an FST. Every signal opens on one time ruler in your browser, grouped by its scope, on a phone too.
Can I turn an FST into a VCD?
Yes. Choose Export, then Save as VCD, for a VCD of every signal and change, which any waveform viewer or script reads.
Why is an FST so much smaller than a VCD?
FST keeps each signal's changes together, packs them with LZ4, FastLZ or zlib and keeps each time once, where a VCD writes every change out as text.
Does it show VHDL's enumerations by name?
Yes. When GHDL or NVC saved an enumeration's names, like a state machine's states, its values show as those names.
How do I measure the time between two edges?
Click near the first edge to put cursor A, then shift-click near the second, or choose Measure and click, for cursor B. Each snaps to the nearest change, and the strip shows the time between them and its frequency.