Implementation of VHDL

VHDL standards

Unfortunately, there are many versions of the VHDL language, and they aren’t backward compatible.

The VHDL language was first standardized in 1987 by IEEE as IEEE 1076-1987, and is commonly referred as VHDL-87. This is certainly the most important version, since most of the VHDL tools are still based on this standard.

Various problems of this first standard have been analyzed by experts groups to give reasonable ways of interpreting the unclear portions of the standard.

VHDL was revised in 1993 by IEEE as IEEE 1076-1993. This revision is still well-known.

Unfortunately, VHDL-93 is not fully compatible with VHDL-87, i.e. some perfectly valid VHDL-87 programs are invalid VHDL-93 programs. Here are some of the reasons:

  • the syntax of file declaration has changed (this is the most visible source of incompatibility),
  • new keywords were introduced (group, impure, inertial, literal, postponed, pure, reject, rol, ror, shared, sla, sll, sra, srl, unaffected, xnor),
  • some dynamic behaviours have changed (the concatenation is one of them),
  • rules have been added.

Shared variables were replaced by protected types in the 2000 revision of the VHDL standard. This modification is also known as 1076a. Note that this standard is not fully backward compatible with VHDL-93, since the type of a shared variable must now be a protected type (there was no such restriction before).

Minor corrections were added by the 2002 revision of the VHDL standard. This revision is not fully backward compatible with VHDL-00 since, for example, the value of the ‘instance_name attribute has slightly changed.

The latest version is 2008. Many features have been added, and GHDL doesn’t implement all of them.

You can select the VHDL standard expected by GHDL with the --std=STANDARD option, where STANDARD is one of the list below:

Select VHDL-87 standard as defined by IEEE 1076-1987. LRM bugs corrected by later revisions are taken into account.
Select VHDL-93; VHDL-87 file declarations are not accepted.

Select VHDL-93 standard with relaxed rules:

  • VHDL-87 file declarations are accepted;
  • default binding indication rules of VHDL-02 are used. Default binding rules are often used, but they are particularly obscure before VHDL-02.
Select VHDL-2000 standard, which adds protected types.
Select VHDL-2002 standard.
Select VHDL-2008 standard (partially implemented).

Multiple standards can be used in a design:

87 87
93 93, 93c, 00, 02
08 08


The standards in each group are considered compatible: you can elaborate a design mixing these standards. However, standards of different groups are not compatible.

PSL support

GHDL implements a subset of PSL.

PSL implementation

A PSL statement is considered a process, so it’s not allowed within a process.

All PSL assertions must be clocked (GHDL doesn’t support unclocked assertion). Furthermore only one clock per assertion is allowed.

You can either use a default clock like this:

default clock is rising_edge (CLK);
assert always
  a -> eventually! b;

or use a clocked expression (note the use of parentheses):

assert (always a -> next[3](b)) @rising_edge(clk);

Of course only the simple subset of PSL is allowed.

Currently the built-in functions are not implemented, see issue #662.

PSL usage

PSL annotations embedded in comments

GHDL understands embedded PSL annotations in VHDL files:

  -- psl default clock is rising_edge (CLK);
  -- psl assert always
  --   a -> eventually! b;
end architecture rtl;
  • A PSL assertion statement must appear within a comment that starts with the psl keyword. The keyword must be followed (on the same line) by a PSL keyword such as assert or default. To continue a PSL statement on the next line, just start a new comment.


As PSL annotations are embedded within comments, you must analyze your design with option -fpsl to enable PSL annotations:

ghdl -a -fpsl vhdl_design.vhdl
ghdl -e vhdl_design

PSL annotations (VHDL-2008 only)

Since VHDL-2008 PSL is integrated in the VHDL language. You can use PSL in a VHDL(-2008) design without embedding it in comments.

  default clock is rising_edge (CLK);
  assert always
    a -> eventually! b;
end architecture rtl;


You have to use the --std=08 option:

ghdl -a --std=08 vhdl_design.vhdl
ghdl -e --std=08 vhdl_design

PSL vunit files

GHDL supports vunit (Verification Unit) files.

vunit vunit_name (design_name)
  default clock is rising_edge(clk);
  assert always cnt /= 5 abort rst;
  • A vunit can contain PSL and VHDL code.
  • It is bound to a VHDL entity or an instance of it.
  • The PSL vunit is in the same scope as the VHDL design it is bound to. You have access to all objects (ports, types, signals) of the VHDL design.


The PSL vunit file has to be analyzed/elaborated together with the VHDL design file, for example:

ghdl -a --std=08 vhdl_design.vhdl vunit.psl
ghdl -e --std=08 vhdl_design

Source representation

According to the VHDL standard, design units (i.e. entities, architectures, packages, package bodies, and configurations) may be independently analyzed.

Several design units may be grouped into a design file.

In GHDL, a system file represents a design file. That is, a file compiled by GHDL may contain one or more design units.

It is common to have several design units in a design file.

GHDL does not impose any restriction on the name of a design file (except that the filename may not contain any control character or spaces).

GHDL does not keep a binary representation of the design units analyzed like other VHDL analyzers. The sources of the design units are re-read when needed (for example, an entity is re-read when one of its architectures is analyzed). Therefore, if you delete or modify a source file of a unit analyzed, GHDL will refuse to use it.

Library database

Each design unit analyzed is placed into a design library. By default, the name of this design library is work; however, this can be changed with the --work option of GHDL.

To keep the list of design units in a design library, GHDL creates library files. The name of these files is <LIB_NAME>-obj<GROUP>.cf, where <LIB_NAME> is the name of the library, and <GROUP> the VHDL version (87, 93 or 08) used to analyze the design units.

For details on GROUP values see section VHDL standards.

You don’t have to know how to read a library file. You can display it using the -d of ghdl. The file contains the name of the design units, as well as the location and the dependencies.

The format may change with the next version of GHDL.

Top entity

There are some restrictions on the entity being at the apex of a design hierarchy:

  • The generic must have a default value, and the value of a generic is its default value.
  • The ports type must be constrained.

Using vendor libraries

Many vendors libraries have been analyzed with GHDL. There are usually no problems. Be sure to use the --work option. However, some problems have been encountered. GHDL follows the VHDL LRM (the manual which defines VHDL) more strictly than other VHDL tools. You could try to relax the restrictions by using the --std=93c, -fexplicit, -frelaxed-rules and --warn-no-vital-generic.