.. program:: ghdl
.. _USING:Simulation:

Simulation (runtime)
####################

.. _simulation_options:

Simulation options
==================

In most system environments, it is possible to pass CLI options while
invoking a program. Contrary to most programming languages, there is no
standard method in VHDL to obtain the arguments or to set the exit
status.
However, the GHDL runtime behaviour can be modified with some options:

- It is possible to pass parameters to your design through the generic
  interfaces of the top entity.
- It is also possible to stop simulation after a certain time. The exit
  status of the simulation is ``EXIT_SUCCESS`` (``0``) if the simulation
  completes, or ``EXIT_FAILURE`` (``1``) in case of error (assertion
  failure, overflow or any constraint error).

Here is the list of the most useful options. For further info, see :ref:`DEV:Debugging`.

.. HINT::
   Note that these arguments are represented as ``simulation_options...`` in this documentation.
   For analysis/elaboration options, see :ref:`USING:Invoking`.


.. option:: -gGENERIC=VALUE

  Set value `VALUE` to generic with name `GENERIC`.

  Example::

    $ ghdl -r --std=08 my_unit -gDEPTH=12

  .. NOTE::
    This is currently a run option; but in the (not near) future it might be deprecated to
    become an elaboration option only. As a result, now you can generate a single binary and
    execute it multiple times with different arguments. That might not be possible in the future.

    As explained in :option:`-e`, performing a complete elaboration in terms of the LRM requires
    to get rid of the compile and link model. This is mostly because delaying certain elaboration
    steps to the runtime prevents elaboration-time optimisions.

  .. HINT::
    Currently, GHDL has limited support for generic types in the CLI. It is suggested to use strings or integers. Nonetheless, project `JSON-for-VHDL <https://github.com/Paebbels/JSON-for-VHDL>`_ allows to encode a set of parameters as stringified JSON, and it provides VHDL functions to read specific values from it. It is valid for synthesis.

.. option:: --assert-level=<LEVEL>

  Select the assertion level at which an assertion violation stops the
  simulation. `LEVEL` is the name from the `severity_level`
  enumerated type defined in the `standard` package or the
  ``none`` name.

  By default, only assertion violation of severity level ``failure``
  stops the simulation.

  For example, if `LEVEL` was ``warning``, any assertion violation
  with severity level ``warning``, ``error`` or ``failure`` would
  stop simulation, but the assertion violation at the ``note`` severity
  level would only display a message.

  Option :option:`--assert-level=none <--assert-level>` prevents any
  assertion violation from stopping simulation.

.. option:: --backtrace-severity=<LEVEL>

  Select the assertion level at which an assertion violation display a
  backtrace (if available).

  This is useful when the assertion is generated by a function (like
  assertions in ``ieee.numeric_std``) whose location is not very
  useful.

.. option:: --ieee-asserts=<POLICY>
.. option:: --asserts=<POLICY>

  Select how assertions are handled.  `POLICY` can be ``enable`` (the
  default), ``disable`` which disables all assertions and
  ``disable-at-0`` which disables only at the start of simulation.

  The ``--ieee-asserts`` applies only to assertions from ``ieee``
  package.  This option can be useful to avoid assertion messages from
  ``ieee.numeric_std`` (and other ``ieee`` packages).

  The ``--asserts`` option applies to all assertions, including those
  from the ``ieee`` units.  The behaviour of the latter can be
  overridden by using the ``--ieee-asserts`` option after the
  ``--asserts`` option.

.. option:: --stop-time=<TIME>

  Stop the simulation after ``TIME``. ``TIME`` is expressed as a time
  value, *without* any space. The time is the simulation time, not
  the real clock time.

  For example::

    $ ./my_design --stop-time=10ns
    $ ./my_design --stop-time=ps

.. option:: --stop-delta=<N>

  Stop the simulation after `N` delta cycles in the same current
  time.  The default is 5000.

  .. index:: display time

.. option:: --disp-time

  Display the time and delta cycle number as simulation advances.

.. option:: --unbuffered

  Disable buffering on stdout, stderr and files opened in write or append mode (TEXTIO).

.. option:: --max-stack-alloc=<N>

  Emit an error message in case of allocation on the stack of an
  object larger than `N` KB.  Use 0 to disable these checks.

.. option:: --sdf=<PATH=FILENAME>

  Do VITAL annotation on `PATH` with SDF file :file:`FILENAME`.

  `PATH` is a path of instances, separated with ``.`` or ``/``.
  Any separator can be used. Instances are component instantiation labels,
  generate labels or block labels. Currently, you cannot use an indexed name.

  Specifying a delay::

   --sdf=min=PATH=FILENAME
   --sdf=typ=PATH=FILENAME
   --sdf=max=PATH=FILENAME

  If the option contains a type of delay, that is ``min=``,
  ``typ=`` or ``max=``, the annotator use respectively minimum,
  typical or maximum values. If the option does not contain a type of delay,
  the annotator uses the typical delay.

  See section :ref:`Backannotation`, for more details.

.. option:: --vpi=<FILENAME>

  Load VPI library. This option can be used multiple times to load different libraries.

  Any registration functions in the ``vlog_startup_routines`` array in the library will be called:

  .. code-block:: c

    void (*vlog_startup_routines[]) () = {
      my_handle_register,
      0
    };

.. option:: --vpi-trace[=<FILENAME>]

  Trace vpi calls. Trace is printed to :file:`FILENAME` if provided, otherwise to stdout.

.. option:: --vhpi=<FILENAME>[:<ENTRYPOINT>]

  Load VHPI library. This option can be used multiple times to load different libraries.

  If an ``ENTRYPOINT`` registration function is provided, it will be called.
  Otherwise, any registration functions in the ``vhpi_startup_routines`` array in the library will be called:

  .. code-block:: c

    void (*vhpi_startup_routines[])() = {
      register_foreign_app,
      register_foreign_func,
      0
    };

.. option:: --vhpi-trace[=<FILENAME>]

  Trace vhpi calls. Trace is printed to :file:`FILENAME` if provided, otherwise to stdout.

.. option:: --help

  Display a short description of the options accepted by the runtime library.

.. option:: --no-run

  Stop the simulation before the first cycle. This option actually elaborates the design, so it will catch any bound error in port maps. See also :option:`-e`.

  This may be used with :option:`--disp-tree` to display the tree without simulating the whole design.

.. _export_waves:

Export waveforms
================

.. option:: --read-wave-opt=<FILENAME>

  Filter signals to be dumped to the wave file according to the wave option
  file provided.

  Here is a description of the wave option file format currently supported ::

     $ version = 1.1  # Optional

     # Path format for signals in packages :
     my_pkg.global_signal_a

     # Path format for signals in entities :
     /top/sub/clk

     # Dump every signal named reset in first level sub entities of top
     /top/*/reset

     # Dump every signal named reset in recursive sub entities of top
     /top/**/reset

     # Dump every signal of sub2 which could be anywhere in the design except
     # on the top level
     /**/sub2/*

     # Dump every signal of sub3 which must be a first level sub entity of the
     # top level
     /*/sub3/*

     # Dump every signal of the first level sub entities of sub3 (but not
     # those of sub3)
     /**/sub3/*/*

.. option:: --write-wave-opt=<FILENAME>

  If the wave option file doesn't exist, creates it with all the signals of
  the design. Otherwise throws an error, because it won't erase an existing
  file.

.. option:: --vcd=<FILENAME>

.. option:: --vcdgz=<FILENAME>

  .. index:: vcd

  .. index:: value change dump

  .. index:: dump of signals

  Option :option:`--vcd` dumps into the VCD file `FILENAME` the signal
  values before each non-delta cycle. If `FILENAME` is ``-``,
  then the standard output is used, otherwise a file is created or
  overwritten.

  The :option:`--vcdgz` option is the same as the :option:`--vcd` option,
  but the output is compressed using the `zlib` (`gzip`
  compression). However, you can't use the ``-`` filename.
  Furthermore, only one VCD file can be written.

  :dfn:`VCD` (value change dump) is a file format defined
  by the `verilog` standard and used by virtually any wave viewer.

  Since it comes from `verilog`, only a few VHDL types can be dumped. GHDL
  dumps only signals whose base type is of the following:

  * types defined in the ``std.standard`` package:

    * ``bit``
    * ``bit_vector``

  * types defined in the ``ieee.std_logic_1164`` package:

    * ``std_ulogic``
    * ``std_logic`` (because it is a subtype of ``std_ulogic``)
    * ``std_ulogic_vector``
    * ``std_logic_vector``

  * any integer type

  I have successfully used `gtkwave` to view VCD files.

  Currently, there is no way to select signals to be dumped: all signals are
  dumped, which can generate big files.

  It is very unfortunate there is no standard or well-known wave file
  format supporting VHDL types. If you are aware of such a free format,
  please mail me (:ref:`Reporting_bugs`).

.. option:: --vcd-nodate

  Do not write date in the VCD file.

.. option:: --fst=<FILENAME>

  Write the waveforms into an `fst` file that can be displayed by
  `gtkwave`. The `fst` files are much smaller than VCD or
  `GHW` files, but it handles only the same signals as the VCD format.

.. option:: --wave=<FILENAME>

  Write the waveforms into a `ghw` (GHdl Waveform) file. Currently, all
  the signals are dumped into the waveform file, you cannot select a hierarchy
  of signals to be dumped.

  The format of this file was defined by myself and is not yet completely fixed.
  It may change slightly. The ``gtkwave`` tool can read the GHW files.

  Contrary to VCD files, any VHDL type can be dumped into a GHW file.

Export hierarchy and references
===============================

.. option:: --disp-tree=<KIND>

  .. index:: display design hierarchy

  Display the design hierarchy as a tree of instantiated design entities.
  This may be useful to understand the structure of a complex
  design. `KIND` is optional, but if set must be one of:

  * ``none`` Do not display hierarchy. Same as if the option was not present.

  * ``inst`` Display entities, architectures, instances, blocks and generates statements.

  * ``proc`` Like ``inst`` but also display processes.

  * ``port`` Like ``proc`` but display ports and signals too.
    If `KIND` is not specified, the hierarchy is displayed with the
    ``port`` mode.

.. option:: --xref-html [options] files...

  To easily navigate through your sources, you may generate cross-references. This command generates an html file for
  each ``file`` given in the command line, with syntax highlighting and full cross-reference: every identifier is a
  link to its declaration. An index of the files is created too.

  The set of ``files`` are analyzed, and then, if the analysis is successful, html files are generated in the directory
  specified by the ``-o <DIR>`` option, or :file:`html/` directory by default. The style of the html file can be
  modified with the :option:`--format` option.

.. option:: --psl-report=<FILENAME>

  Write a report for PSL at the end of simulation. For each PSL cover and assert statements, the name, source location and whether it passed or failed is reported. The file is written using the JSON format, but is still human readable.

.. option:: --psl-report-uncovered

  Reports warning for each uncovered PSL cover point when simulation ends.