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Icarus Verilog
https://github.com/steveicarus/iverilog

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Icarus Verilog

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README 

        
# The ICARUS Verilog Compilation System



Copyright 2000-2019 Stephen Williams



Table of Contents



1. [What is ICARUS Verilog?](#what-is-icarus-verilog)

2. [Building/Installing Icarus Verilog From Source](#buildinginstalling-icarus-verilog-from-source)

	- [Compile Time Prerequisites](#compile-time-prerequisites)

	- [Compilation](#compilation)

	- [(Optional) Testing](#optional-testing)

	- [Installation](#installation)

3. [How Icarus Verilog Works](#how-icarus-verilog-works)

	- [Preprocessing](#preprocessing)

	- [Parse](#parse)

	- [Elaboration](#elaboration)

	- [Optimization](#optimization)

	- [Code Generation](#code-generation)

	- [Attributes](#attributes)

4.	[Running iverilog](#running-iverilog)

	- [Examples](#examples)

5. [Unsupported Constructs](#unsupported-constructs)

6. [Nonstandard Constructs or Behaviors](#nonstandard-constructs-or-behaviors)

	- [Builtin system functions](#builtin-system-functions)

	- [Preprocessing Library Modules](#preprocessing-library-modules)

	- [Width in %t Time Formats](#width-in-t-time-formats)

	- [vpiScope iterator on vpiScope objects](#vpiscope-iterator-on-vpiscope-objects)

	- [Time 0 Race Resolution](#time-0-race-resolution)

	- [Nets with Types](#nets-with-types)

7. [Credits](#credits)



## 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 behavioural

constructs. For a view of the current state of Icarus Verilog, see its

home page at https://steveicarus.github.io/iverilog/.



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.



## Building/Installing Icarus Verilog from Source



If you are starting from the 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 on Windows, see the mingw.txt file.



### Compile Time Prerequisites



You can use:

```bash

apt install -y autoconf gperf make gcc g++ bison flex

```



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

  OSX note: bison 2.3 shipped with MacOS including Catalina generates

  broken code, but bison 3+ works. We recommend using the Fink

  project version of bison and flex (finkproject.org), brew version

  works fine either.



- gperf 3.0 or later

  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 or later

  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 or later

  This generates configure scripts from configure.ac. The 2.53

  or later versions are known to work, autoconf 2.13 is

  reported to *not* work.



### Compilation



Compiling From Release



Unpack the tar-ball, `cd` into the `verilog-#########` directory,

and compile the source with the commands:



```bash

  ./configure

  make

```



Compiling From GitHub



If you are building from git, you have to run the command below before

compiling the source. This will generate the "configure" file, which is

automatically done when building from tarball.



```bash

  sh autoconf.sh

  ./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 behaviour:



```

	--prefix=

		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-suffix

	--enable-suffix=

	--disable-suffix

		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.



	--host=

		Compile iverilog for a different platform. You can use:

				x64_64-w64-mingw32 for building 64-bit Windows executables

				i686-w64-mingw32 for building 32-bit Windows executables

			Both options require installing the required mingw-w64 packages.

```



### (Optional) Testing



To run a simple test before installation, execute



```bash

  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.



### 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=` flag to the configure command.) You may need

to do this as root to gain access to installation directories.



```bash

	make install

```



### Uninstallation



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.



### 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.



### 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 ` flag to the `ivl` subcommand. This will cause ivl

to dump the pform into the file named ``. (Note that this is not

normally done, unless debugging the `ivl` subcommand.)



### 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 behavioural 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 ` 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 ``.



Elaboration is performed in two steps: scopes and parameters

first, followed by the structural and behavioural 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

`elab_scope.cc` file contains most of the code for handling this phase.



The tail of the elaborate_scope behaviour (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 behavioural 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.



### Optimization



This is 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 the need arises

to generate actual output.



The user selects the target code generator with the `-t` flag on the

command line.



### Attributes



> NOTE: The $attribute syntax will soon be deprecated in favour 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 (, , );

```



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 `` must be an identifier. This will be

the item to get an attribute. The `` and `` are strings, not

expressions, that give the key and the value of the attribute to be

attached to the identified object.



Attributes are `[ ]` 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 ``

names a primitive earlier in the compilation unit and the statement is

placed in the 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: Hello World



Example: Compiling `"hello.vl"`



```verilog

// ------------------------ hello.vl ----------------------------



module main();



initial

  begin

    $display("Hello World");

    $finish ;

  end



endmodule



// --------------------------------------------------------------

```



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

is available.



To compile the program:



```bash

	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 generated program:



```bash

	./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.



## 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.



* `$is_signed()`



	This system function returns 1 if the expression contained is

	signed, or 0 otherwise. This is mostly of use for compiler

	regression tests.



* `$sizeof()`, `$bits()`



	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 `$sizeof` function is deprecated in favour of `$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 the compilation of that file are those that it

defines itself (or includes) or that are defined in 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 modelled 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 supports an extended 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:



```verilog

	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.



## 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.