Content of this manual

This manual is the user and reference manual for GHDL. It does not contain an introduction to VHDL. Thus, the reader should have at least a basic knowledge of VHDL. A good knowledge of VHDL language reference manual (usually called LRM) is a plus.

What is VHDL?

VHDL is an acronym for Very High Speed Integrated Circuit Hardware Description Language which is a programming language used to describe a logic circuit by function, data flow behaviour, or structure.

VHDL is a programming language: although VHDL was not designed for writing general purpose programs, you can write any algorithm with the VHDL language. 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. 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 (hardware description language). 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.

Like a program written in another hardware description language, a VHDL program can be transformed with a synthesis tool into a netlist, that is, a detailed gate-level implementation.

What is GHDL?

GHDL is a shorthand for G Hardware Design Language. Currently, G has no meaning.

GHDL is a VHDL compiler that can execute (nearly) any VHDL program. GHDL is not a synthesis tool: you cannot create a netlist with GHDL.

Unlike some other simulators, GHDL is a compiler: it directly translates a VHDL file to machine code, using the GCC or LLVM back-end and 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.

The Windows(TM) version of GHDL is not based on GCC but on an internal code generator.

The current version of GHDL does not contain any graphical viewer: you cannot see signal waves. You can still check with a test bench. The current version can produce a VCD file which can be viewed with a wave viewer, as well as ghw files to be viewed by gtkwave.

GHDL aims at implementing VHDL as defined by IEEE 1076. It supports most of the 1987 standard and most features added by the 1993 standard.