What is VHDL?¶
VHDL is an acronym for Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (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 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 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 1076. Five revisions exist: 1987, 1993, 2002, 2008 and 2019. The standardization is handled by the VHDL Analysis and Standardization Group (VASG/P1076).
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.
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 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, LLVM or x86/i386 only, a built-in one named mcode) and runs on GNU/Linux, Windows ™ and macOS ™; 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, GHDL Waveform (GHW), VCD or FST files can be produced, which can be viewed with a waveform viewer, such as GtkWave.