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gEDA » Documentation » Icarus Verilog » The Icarus Verilog Compilation System

The Icarus Verilog Compilation System

Copyright 2000-2004 Stephen Williams

What is ICARUS Verilog?

Icarus Verilog is intended to compile ALL of the Verilog HDL as described in the IEEE-1364 standard. Of course, it's not quite there yet. It does currently handle a mix of structural and behavioral constructs. For a view of the current state of Icarus Verilog, see its home page at

Icarus Verilog is not aimed at being a simulator in the traditional sense, but a compiler that generates code employed by back-end tools.

For instructions on how to run Icarus Verilog, see the iverilog(1) man page.

Building/Installing Icarus Verilog From Source

If you are starting from source, the build process is designed to be as simple as practical. Someone basically familiar with the target system and C/C++ compilation should be able to build the source distribution with little effort. Some actual programming skills are not required, but helpful in case of problems.

If you are building for Windows, see the mingw.txt file.

Compile Time Prerequisites

You need the following software to compile Icarus Verilog from source on a UNIX-like system:

  • GNU Make
    The Makefiles use some GNU extensions, so a basic POSIX make will not work. Linux systems typically come with a satisfactory make. BSD based systems (i.e., NetBSD, FreeBSD) typically have GNU make as the gmake program.
  • ISO C++ Compiler
    The ivl and ivlpp programs are written in C++ and make use of templates and some of the standard C++ library. egcs and recent gcc compilers with the associated libstdc++ are known to work. MSVC++ 5 and 6 are known to definitely *not* work.
  • bison and flex
  • gperf 2.7
    The lexical analyzer doesn't recognize keywords directly, but instead matches symbols and looks them up in a hash table in order to get the proper lexical code. The gperf program generates the lookup table.
    A version problem with this program is the most common cause of difficulty. See the Icarus Verilog FAQ.
  • readline 4.2
    On Linux systems, this usually means the readline-devel rpm. In any case, it is the development headers of readline that are needed.
  • termcap
    The readline library in turn uses termcap.

If you are building from git, you will also need software to generate the configure scripts.

  • autoconf 2.53
    This generates configure scripts from The 2.53 or later versions are known to work, autoconf 2.13 is reported to *not* work.


Unpack the tar-ball and cd into the verilog-######### directory and compile the source with the commands:


Normally, this command automatically figures out everything it needs to know. It generally works pretty well. There are a few flags to the configure script that modify its behavior:

	    The default is /usr/local, which causes the tool suite to
	    be compiled for install in /usr/local/bin,
	    /usr/local/share/ivl, etc.

	    I recommend that if you are configuring for precompiled
	    binaries, use --prefix=/usr.  On Solaris systems, it is
	    common to use --prefix=/opt.  You can configure for a non-root
	    install with --prefix=$HOME.

	    Enable/disable changing the names of install files to use
	    a suffix string so that this version or install can co-
	    exist with other versions. This renames the installed
	    commands (iverilog, iverilog-vpi, vvp) and the installed
	    library files and include directory so that installations
	    with the same prefix but different suffix are guaranteed
	    to not interfere with each other.

(Optional) Testing

To run a simple test before installation, execute

make check

The commands printed by this run might help you in running Icarus Verilog on your own Verilog sources before the package is installed by root.


Now install the files in an appropriate place. (The makefiles by default install in /usr/local unless you specify a different prefix with the –prefix=<path> flag to the configure command.) You may need to do this as root to gain access to installation directories.

make install


The generated Makefiles also include the uninstall target. This should remove all the files that make install creates.

How Icarus Verilog Works

This tool includes a parser which reads in Verilog (plus extensions) and generates an internal netlist. The netlist is passed to various processing steps that transform the design to more optimal/practical forms, then is passed to a code generator for final output. The processing steps and the code generator are selected by command line switches.


There is a separate program, ivlpp, that does the preprocessing. This program implements the `include and `define directives producing output that is equivalent but without the directives. The output is a single file with line number directives, so that the actual compiler only sees a single input file. See ivlpp/ivlpp.txt for details.


The Verilog compiler starts by parsing the Verilog source file. The output of the parse is a list of Module objects in “pform”. The pform (see pform.h) is mostly a direct reflection of the compilation step. There may be dangling references, and it is not yet clear which module is the root.

One can see a human readable version of the final pform by using the -P <path> flag to the ivl subcommand. This will cause ivl to dump the pform into the file named <path>. (Note that this is not normally done, unless debugging the ivl subcommand.)


This phase takes the pform and generates a netlist. The driver selects (by user request or lucky guess) the root module to elaborate, resolves references and expands the instantiations to form the design netlist. (See netlist.txt.) Final semantic checks are performed during elaboration, and some simple optimizations are performed. The netlist includes all the behavioral descriptions, as well as gates and wires.

The elaborate() function performs the elaboration.

One can see a human readable version of the final, elaborated and optimized netlist by using the -N <path> flag to the compiler. If elaboration succeeds, the final netlist (i.e., after optimizations but before code generation) will be dumped into the file named <path>.

Elaboration is actually performed in two steps: scopes and parameters first, followed by the structural and behavioral elaboration.

Scope Elaboration

This pass scans through the pform looking for scopes and parameters. A tree of NetScope objects is built up and placed in the Design object, with the root module represented by the root NetScope object. The file contains most of the code for handling this phase.

The tail of the elaborate_scope behavior (after the pform is traversed) includes a scan of the NetScope tree to locate defparam assignments that were collected during scope elaboration. This is when the defparam overrides are applied to the parameters.

Netlist Elaboration

After the scopes and parameters are generated and the NetScope tree fully formed, the elaboration runs through the pform again, this time generating the structural and behavioral netlist. Parameters are elaborated and evaluated by now so all the constants of code generation are now known locally, so the netlist can be generated by simply passing through the pform.


This is actually a collection of processing steps that perform optimizations that do not depend on the target technology. Examples of some useful transformations are

  • eliminate null effect circuitry
  • combinational reduction
  • constant propagation

The actual functions performed are specified on the ivl command line by the -F flags (see below).

Code Generation

This step takes the design netlist and uses it to drive the code generator (see target.h). This may require transforming the design to suit the technology.

The emit() method of the Design class performs this step. It runs through the design elements, calling target functions as need arises to generate actual output.

The user selects the target code generator with the -t flag on the command line.


The $attribute syntax will soon be deprecated in favor of the Verilog-2001 attribute syntax, which is cleaner and standardized.

The parser accepts, as an extension to Verilog, the $attribute module item. The syntax of the $attribute item is:

$attribute (<identifier>, <key>, <value>);

The $attribute keyword looks like a system task invocation. The difference here is that the parameters are more restricted than those of a system task. The <identifier> must be an identifier. This will be the item to get an attribute. The <key> and <value> are strings, not expressions, that give the key and the value of the attribute to be attached to the identified object.

Attributes are [<key> <value>] pairs and are used to communicate with the various processing steps. See the documentation for the processing step for a list of the pertinent attributes.

Attributes can also be applied to gate types. When this is done, the attribute is given to every instantiation of the primitive. The syntax for the attribute statement is the same, except that the <identifier> names a primitive earlier in the compilation unit and the statement is placed in global scope, instead of within a module. The semicolon is not part of a type attribute.

Note that attributes are also occasionally used for communication between processing steps. Processing steps that are aware of others may place attributes on netlist objects to communicate information to later steps.

Icarus Verilog also accepts the Verilog 2001 syntax for attributes. They have the same general meaning as with the $attribute syntax, but they are attached to objects by position instead of by name. Also, the key is a Verilog identifier instead of a string.

Running iverilog

The preferred way to invoke the compiler is with the iverilog(1) command. This program invokes the preprocessor (ivlpp) and the compiler (ivl) with the proper command line options to get the job done in a friendly way. See the iverilog(1) man page for usage details.


Example: Compiling hello.vl

module main();
    $display("Hi there");
    $finish ;

Ensure that iverilog is on your search path, and the vpi library is available.

To compile the program:

iverilog hello.vl

(The above presumes that /usr/local/include and /usr/local/lib are part of the compiler search path, which is usually the case for gcc.)

To run the program:


You can use the -o switch to name the output command to be generated by the compiler. See the iverilog(1) man page.

Unsupported Constructs

Icarus Verilog is in development—as such it still only supports a (growing) subset of Verilog. Below is a description of some of the currently unsupported Verilog features. This list is not exhaustive, and does not account for errors in the compiler. See the Icarus Verilog web page for the current state of support for Verilog, and in particular, browse the bug report database for reported unsupported constructs.

  • System functions are supported, but the return value is a little tricky. See SYSTEM FUNCTION TABLE FILES in the iverilog man page.
  • Specify blocks are parsed but ignored in general.
  • trireg is not supported. tri0 and tri1 are supported.
  • tran primitives, i.e. tran, tranif1, tranif0, rtran, rtranif1 and rtranif0 are not supported.
  • Net delays, of the form wire #N foo; do not work. Delays in every other context do work properly, including the V2001 form wire #5 foo = bar;
  • Event controls inside non-blocking assignments are not supported. i.e.: a ⇐ @(posedge clk) b;
  • Macro arguments are not supported. `define macros are supported, but they cannot take arguments.

Nonstandard Constructs or Behaviors

Icarus Verilog includes some features that are not part of the IEEE1364 standard, but have well defined meaning, and also sometimes gives nonstandard (but extended) meanings to some features of the language that are defined. See the extensions.txt documentation for more details.

	This system function returns 1 if the expression contained is
	signed, or 0 otherwise. This is mostly of use for compiler
	regression tests.

	The $bits system function returns the size in bits of the
	expression that is its argument. The result of this
	function is undefined if the argument doesn't have a
	self-determined size.

	The $sizeof function is deprecated in favor of $bits, which is
	the same thing, but included in the SystemVerilog definition.

	The $simtime system function returns as a 64bit value the
	simulation time, unscaled by the time units of local
	scope. This is different from the $time and $stime functions
	which return the scaled times. This function is added for
	regression testing of the compiler and run time, but can be
	used by applications who really want the simulation time.

	Note that the simulation time can be confusing if there are
	lots of different `timescales within a design. It is not in
	general possible to predict what the simulation precision will
	turn out to be.

	These functions are similar to the IEEE1364 standard $random
	functions, but they use the Mersenne Twister (MT19937)
	algorithm. This is considered an excellent random number
	generator, but does not generate the same sequence as the
	standardized $random.

    Builtin system functions

	Certain of the system functions have well defined meanings, so
	can theoretically be evaluated at compile time, instead of
	using runtime VPI code. Doing so means that VPI cannot
	override the definitions of functions handled in this
	manner. On the other hand, this makes them synthesizable, and
	also allows for more aggressive constant propagation. The
	functions handled in this manner are:


	Implementations of these system functions in VPI modules will
	be ignored.

    Preprocessing Library Modules

	Icarus Verilog does preprocess modules that are loaded from
	libraries via the -y mechanism. However, the only macros
	defined during compilation of that file are those that it
	defines itself (or includes) or that are defined on the
	command line or command file.

	Specifically, macros defined in the non-library source files
	are not remembered when the library module is loaded. This is
	intentional. If it were otherwise, then compilation results
	might vary depending on the order that libraries are loaded,
	and that is too unpredictable.

	It is said that some commercial compilers do allow macro
	definitions to span library modules. That's just plain weird.

    Width in %t Time Formats

	Standard Verilog does not allow width fields in the %t formats
	of display strings. For example, this is illegal:

		$display("Time is %0t", %time);

	Standard Verilog instead relies on the $timeformat to
	completely specify the format.

	Icarus Verilog allows the programmer to specify the field
	width. The "%t" format in Icarus Verilog works exactly as it
	does in standard Verilog. However, if the programmer chooses
	to specify a minimum width (i.e., "%5t"), then for that display
	Icarus Verilog will override the $timeformat minimum width and
	use the explicit minimum width.

    vpiScope iterator on vpiScope objects.

	In the VPI, the normal way to iterate over vpiScope objects
	contained within a vpiScope object, is the vpiInternalScope
	iterator. Icarus Verilog adds support for the vpiScope
	iterator of a vpiScope object, that iterates over *everything*
	the is contained in the current scope. This is useful in cases
	where one wants to iterate over all the objects in a scope
	without iterating over all the contained types explicitly.

    time 0 race resolution.

	Combinational logic is routinely modeled using always
	blocks. However, this can lead to race conditions if the
	inputs to the combinational block are initialized in initial
	statements. Icarus Verilog slightly modifies time 0 scheduling
	by arranging for always statements with ANYEDGE sensitivity
	lists to be scheduled before any other threads. This causes
	combinational always blocks to be triggered when the values in
	the sensitivity list are initialized by initial threads.

    Nets with Types

	Icarus Verilog support an extension syntax that allows nets
	and regs to be explicitly typed. The currently supported types
	are logic, bool and real. This implies that "logic" and "bool"
	are new keywords. Typical syntax is:

	wire real foo = 1.0;
	reg logic bar, bat;

	... and so forth. The syntax can be turned off by using the
	-g2 flag to iverilog, and turned on explicitly with the -g2x
	flag to iverilog.


Except where otherwise noted, Icarus Verilog, ivl and ivlpp are Copyright Stephen Williams. The proper notices are in the head of each file. However, I have early on received aid in the form of fixes, Verilog guidance, and especially testing from many people. Testers in particular include a larger community of people interested in a GPL Verilog for Linux.

geda/icarus_readme.txt · Last modified: 2014/04/14 09:57 by rlutz