THE ICARUS VERILOG COMPILATION SYSTEM
    Copyright 2000-2004 Stephen Williams


1.0 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 <http://www.icarus.com/eda/verilog>.

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'' man page.


2.0 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.

2.1 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 CVS, you will also need software to generate
the configure scripts.

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

2.2 Compilation

Unpack the tar-ball and cd into the verilog-######### directory
(presumably that is how you got to this README) and compile the source
with the commands:

  ./configure
  make

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:

  --without-ipal
      This turns off support for Icarus PAL, whether ipal
      libraries are installed or not.

  --prefix=<root>
      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-vvp32 (experimental)
      If compiling on AMD64 systems, this enables the
      compilation of 32bit compatible vvp (vvp32) and the vpi
      modules that match.

2.2.1 Special AMD64 Instructions

(The Icarus Verilog RPM for x86_64 is build using these instructions.)

If you are building for Linux/AMD64 (a.k.a x86_64) then to get the
most out of your install, first make sure you have both 64bit and
32bit development libraries installed. Then configure with this
somewhat more complicated command:

  ./configure libdir64='$(prefix)/lib64' vpidir1=vpi64 vpidir2=. --enable-vvp32

This reflects the convention on AMD64 systems that 64bit libraries go
into lib64 directories. The "--enable-vvp32" also turns on 32bit
compatibility files. A 32bit version of vvp (vvp32) will be created,
as well as 32bit versions of the development libraries and bundled VPI
libraries.

2.3 (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.

2.4 Installation

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

2.5 Uninstallation

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

3.0 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.

3.1 Preprocessing

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.

3.2 Parse

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 compiler. This will cause iverilog to dump
the pform into the file named <path>.

3.3 Elaboration

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.

3.3.1 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
elab_scope.cc and elab_pexpr.cc files contain 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.

3.3.2 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.

3.4 Optimization

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).

3.5 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.

3.6 ATTRIBUTES

    NOTE: 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 then 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.

4.0 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.


4.1 EXAMPLES

Example: Compiling "hello.vl"

------------------------ hello.vl ----------------------------
module main();

initial
  begin
    $display("Hi there");
    $finish ;
  end

endmodule

--------------------------------------------------------------

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:

  ./a.out

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

5.0 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.

5.1 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.

    $is_signed(<expr>)
  This system function returns 1 if the expression contained is
  signed, or 0 otherwise. This is mostly of use for compiler
  regression tests.

    $sizeof(<expr>)
    $bits(<expr>)
  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.

    $simtime
  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.

    $mti_random()
    $mti_dist_uniform
  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:

    $bits
    $signed
    $sizeof
    $unsigned

  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.

6.0 CREDITS

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.