# Supported formats

GTKWave supports various file formats:

- `VCD`: Value Change Dump \
  This is the industry standard file format
  generated by most Verilog simulators and is specified in IEEE-1364.
  This is the **slowest** of the formats for the viewer to process and
  requires the most memory. However, this format is ubiquitous, and
  almost all tools support it, which is why native support remains.
- `FST`: Fast Signal Trace \
  It is a trace file format developed by GTKWave
  which is designed for very fast sequential and random access.
- `LXT`: InterLaced eXtensible Trace \
  This is an optimized format
  utilizing interleaved back pointers and value changes. Processing
  LXT files is faster than VCD. It was created specifically for use
  with GTKWave, however, some other simulators (notably, Icarus
  Verilog) support it natively. 
- `LXT2`: InterLaced eXtensible Trace Version 2 \
  This is a block-based
  variant of LXT that allows for greater compression and access speeds
  than can be achieved with LXT. It allows random-access at the block
  level and also optionally allows partial loading of blocks for even
  faster operation. Icarus Verilog also supports LXT2 natively.
- `VZT`: Verilog Zipped Trace \
  This is an outgrowth of LXT2 as it is
  also block based, however it employs a different heuristic for
  compression that allows for file sizes much smaller than most other
  dumpfile formats including commercial ones. VZT file write
  performance is the slowest of all the formats, however reading them
  can be extremely fast on multiprocessor machines as the file format
  has been designed such that the reader was able to be parallelized.
- `IDX`: VCD Recoder Index File \
  This format used to be written by
  GTKWave when instructed to generate fastload files. 
- `GHW`: GHDL Wave file \
  This is a nine state ("01XZHUWL-") file format
  written by the VHDL simulator GHDL.
- `AET2`: All Events Trace Version 2 \
  This is a format used by various
  IBM EDA tools. File size is very small and access is extremely fast.
  Support for it is determined at compile time. If the AET2 reader API
  libraries are not found, it is disabled. Users of IBM tool sets can
  set the environment variable SIMARAMA_BASE to point to the
  *libae2rw.a* and/or *libae2rw.so* files in order to enable this
  feature.
- `VPD`: VCD Plus Dump \
  This is generated by Synopsys VCS. In order to
  read these files, the executable *vpd2vcd* must be in your \$PATH
  during *configure* and *gtkwave* must be invoked with the -o option.
- `WLF`: Wave Log File \
  This is generated by ModelSim. In order to read
  these files, the executable *wlf2vcd* must be in your \$PATH during
  *configure* and *gtkwave* must be invoked with the -o option.
- `FSDB`: Fast Signal Database \
  Reading these files generally requires
  that the executables *fsdb2vcd* and *fsdbdebug* are in your \$PATH
  during *configure* and *gtkwave* must be invoked with the -o option.
  FSDB files can also be read without conversion with a processing
  speed similar to FST if the FsdbReader libraries nffr and nsys are
  found during *configure*, pointed to by the environment variable
  `FSDBREADER_LIBS`. Headers are pointed to by `FSDBREADER_HDRS``.

Converter helper applications are packaged with the viewer in order to
convert VCD files into LXT, LXT2, VZT, or FST files. Conversion from
LXT2, VZT, and FST back into VCD is possible. Wholesale conversion from
LXT is not currently possible, however it is possible to save the traces
visible in the main GTKWave window as VCD so conversion to LXT is not
strictly irreversible.

:::{admonition} Format deprecations in GTKWave 4
:class: attention

It is planned to remove support for the following formats is in GTKWave 4:

* LXT
* LXT2
* VZT
* IDX
* AET2
* VPD
* WLF
* FSDB

If you need continued support for one of these formats please open a
[GitHub issue](https://github.com/gtkwave/gtkwave/issues).
:::