Supported Formats

NovyWave supports the three waveform formats most commonly encountered in open hardware development.

VCD

Extension: .vcd

VCD is the most portable option and is supported by most Verilog and SystemVerilog toolchains.

Strengths

• human-readable text format
• easy to inspect or diff during debugging
• broad simulator compatibility

Tradeoffs

• large file sizes
• slower parsing than binary formats

Example Generation

initial begin
    $dumpfile("output.vcd");
    $dumpvars(0, top_module);
end

FST

Extension: .fst

FST is a compact binary format optimized for waveform viewing and random access.

Strengths

• much smaller than VCD
• faster to load for large simulations
• a good default for Verilator-based flows

Example Generation

Verilated::traceEverOn(true);
VerilatedFstC* tfp = new VerilatedFstC;
top->trace(tfp, 99);
tfp->open("output.fst");

GHW

Extension: .ghw

GHW is the native output format of the GHDL simulator. It preserves VHDL-specific type information — records, arrays, and enumerations — better than VCD, which flattens everything into bit vectors.

Strengths

• good fit for GHDL and VHDL projects
• preserves hierarchical and VHDL-oriented data cleanly
• retains VHDL-specific type information (records, arrays, enumerations) that other formats lose
• real-valued signals in GHW files are rendered as analog traces in NovyWave

NovyWave parses GHW files through the wellen library, which handles the binary format and exposes scopes, signals, and type metadata to the viewer.

Example Generation

ghdl -r testbench --wave=output.ghw

Comparison

Format	Best fit	File size	Parse speed
VCD	universal compatibility	large	slower
FST	Verilator and large traces	small	fast
GHW	GHDL and VHDL	medium	medium

Recommendation

• choose FST when your simulator supports it and performance matters
• choose GHW for GHDL-centered VHDL work
• choose VCD when portability matters more than file size