.. only:: html

   .. exec::
      from helpers import createShields
      createShields('shieldswho')

About
#####

.. _INTRO:VHDL:

What is `VHDL`?
===============

:wikipedia:`VHDL <VHDL>` is an acronym for Very High Speed Integrated Circuit (:wikipedia:`VHSIC <VHSIC>`) Hardware Description
Language (:wikipedia:`HDL <HDL>`), which is a programming language used to describe a logic circuit by function, data flow
behavior, or structure.

Although VHDL was not designed for writing general purpose programs, VHDL *is* a programming language, and you can write any
algorithm with it. If you are able to write programs, you will find in VHDL features similar to those found in procedural
languages such as `C`, `Python`, or `Ada`. Indeed, VHDL derives most of its syntax and semantics from Ada. Knowing `Ada` is
an advantage for learning VHDL (it is an advantage in general as well).

However, VHDL was not designed as a general purpose language but as an `HDL`. As the name implies, VHDL aims at modeling or
documenting electronics systems. Due to the nature of hardware components which are always running, VHDL is a highly
concurrent language, built upon an event-based timing model.

Like a program written in any other language, a VHDL program can be executed. Since VHDL is used to model designs, the term
:dfn:`simulation` is often used instead of `execution`, with the same meaning. At the same time, like a design written in
another `HDL`, a set of VHDL sources can be transformed with a :dfn:`synthesis tool` into a netlist, that is, a detailed
gate-level implementation.

The development of VHDL started in 1983 and the standard is named `IEEE <https://www.ieee.org/>`__ `1076`. Five revisions
exist: `1987 <http://ieeexplore.ieee.org/document/26487/>`__, `1993 <http://ieeexplore.ieee.org/document/392561/>`__,
`2002 <http://ieeexplore.ieee.org/document/1003477/>`__, `2008 <http://ieeexplore.ieee.org/document/4772740/>`__ and
`2019 <https://ieeexplore.ieee.org/document/8938196>`__. The standardization is handled by the VHDL Analysis and
Standardization Group (`VASG/P1076 <http://www.eda-twiki.org/vasg/>`__).

.. _INTRO:GHDL:

What is GHDL?
=============

`GHDL` is a shorthand for `G Hardware Design Language` (currently, `G` has no meaning). It is a VHDL analyzer, compiler,
simulator and (experimental) synthesizer that can process (nearly) any VHDL design.

.. NOTE::
  For almost 20 years, GHDL was *not* a synthesis tool: you could not create a netlist. Hence, most of the content in this
  documentation corresponds to the usage of GHDL as a compiler/simulator. See :ref:`USING:Synthesis` for further details
  regarding synthesis.

Unlike some other simulators, GHDL is a compiler: it directly translates a VHDL file to machine code, without using an
intermediary language such as `C` or `C++`. Therefore, the compiled code should be faster and the analysis time should be
shorter than with a compiler using an intermediary language.

GHDL can use multiple back-ends, i.e. code generators, (`GCC <http://gcc.gnu.org/>`__, `LLVM <http://llvm.org/>`__ or
:wikipedia:`x86 <X86-64>`/:wikipedia:`i386 <Intel_80386>` only, a built-in one named *mcode*) and runs on :wikipedia:`GNU/Linux <Linux_distribution>`,
:wikipedia:`Windows <Microsoft_Windows>` |trade| and :wikipedia:`macOS <MacOS>` |trade|; on x86, x86_64, armv6/armv7/aarch32/aarch64,
ppc64, etc.

The current version of GHDL does not contain any built-in graphical viewer: you cannot see signal waves. You can still check
the behavior of your design with a test bench. Moreover, `GHW <http://ghdl.readthedocs.io/en/latest/using/Simulation.html?highlight=GHW#cmdoption-wave>`__,
:wikipedia:`VCD <Value_change_dump>` or `FST` files can be produced, which can be viewed with a :wikipedia:`waveform viewer <Waveform_viewer>`,
such as `GtkWave <http://gtkwave.sourceforge.net/>`__.

GHDL aims at implementing VHDL as defined by `IEEE 1076 <http://ieeexplore.ieee.org/document/4772740/>`__. It supports the
`1987 <http://ieeexplore.ieee.org/document/26487/>`__, `1993 <http://ieeexplore.ieee.org/document/392561/>`__ and
`2002 <http://ieeexplore.ieee.org/document/1003477/>`__ revisions and, partially, `2008 <http://ieeexplore.ieee.org/document/4772740/>`__.
:wikipedia:`Property Specification Language (PSL) <Property_Specification_Language>` is also partially supported.

Several third party projects are supported: `Yosys <https://github.com/YosysHQ/yosys>`__ (through the `ghdl-yosys-plugin <https://github.com/ghdl/ghdl-yosys-plugin>`__)
`cocotb <https://github.com/potentialventures/cocotb>`__ (through the `VPI interface <https://en.wikipedia.org/wiki/Verilog_Procedural_Interface>`__),
`VUnit <https://vunit.github.io/>`__, `OSVVM <http://osvvm.org/>`__, ...

.. _INTRO:WHO:

Who uses GHDL?
==============

.. container:: whouses


   .. only:: html

      +-------------------+--------------------+-----------------------------------------------------+----------------------------------------------------------------+
      | Project hub       | Documentation      | Name                                                | Brief description                                              |
      +===================+====================+=====================================================+================================================================+
      | |SHIELD:gh-poc|   | |SHIELD:rtd-poc|   | `PoC-Library <https://github.com/VLSI-EDA/PoC>`__   | A Vendor-Independent, Open-Source IP Core and Utility Library. |
      +-------------------+--------------------+-----------------------------------------------------+----------------------------------------------------------------+
      | |SHIELD:gh-vunit| | |SHIELD:doc-vunit| | `VUnit <https://vunit.github.io/>`__                | A unit testing framework for VHDL/SystemVerilog                |
      +-------------------+--------------------+-----------------------------------------------------+----------------------------------------------------------------+
      | |SHIELD:gl-p1076| | |SHIELD:tw-p1076|  | `IEEE P1076 WG <https://www.eda-twiki.org/vasg/>`__ | IEEE P1076 Working Group [VASG]                                |
      +-------------------+--------------------+-----------------------------------------------------+----------------------------------------------------------------+
      | |SHIELD:gh-tce|   | |SHIELD:doc-tce|   | `TCE <http://openasip.org/>`__                      | TTA-Based Co-Design Environment - an open-source ASIP toolset. |
      +-------------------+--------------------+-----------------------------------------------------+----------------------------------------------------------------+