Provided by: iverilog_10.2-1.1build1_amd64 bug


       iverilog - Icarus Verilog compiler


       iverilog   [-ESVv]  [-Bpath]  [-ccmdfile|-fcmdfile]  [-Dmacro[=defn]]  [-Pparameter=value]
       [-pflag=value]     [-dname]     [-g1995|-g2001|-g2005|-g2005-sv|-g2009|-g2012|-g<feature>]
       [-Iincludedir]   [-mmodule]   [-M[mode=]file]  [-Nfile]  [-ooutputfilename]  [-stopmodule]
       [-ttype] [-Tmin/typ/max] [-Wclass] [-ypath] [-lfile] sourcefile


       iverilog is a compiler that translates Verilog source code into  executable  programs  for
       simulation,  or  other  netlist  formats  for  further processing. The currently supported
       targets are vvp for simulation, and fpga for synthesis. Other target types  are  added  as
       code generators are implemented.


       iverilog accepts the following options:

       -Bbase  The  iverilog program uses external programs and configuration files to preprocess
               and compile the Verilog source. Normally, the path used to locate these  tools  is
               built  into the iverilog program. However, the -B switch allows the user to select
               a different set of programs. The path given is used to  locate  ivlpp,  ivl,  code
               generators and the VPI modules.

       -cfile -ffile
               These  flags  specify  an input file that contains a list of Verilog source files.
               This is similar to the command file of other Verilog simulators, in that it  is  a
               file  that contains the file names instead of taking them on the command line. See
               Command Files below.

       -Dmacro Defines macro macro with the string `1' as its definition. This form  is  normally
               only used to trigger ifdef conditionals in the Verilog source.

               Defines macro macro as defn.

               Override  (i.e.  defparam)  a  parameter in a root module. This allows the user to
               override at compile time (defparam) a parameter in a  root  module  instance.  For
               example,  overrides the parameter foo in the root instance main with
               the value 2.

       -dname  Activate a class of compiler debugging messages. The -d  switch  may  be  used  as
               often  as  necessary  to  activate  all the desired messages.  Supported names are
               scopes, eval_tree, elaborate, and synth2; any other names are ignored.

       -E      Preprocess the Verilog source, but do not compile  it.  The  output  file  is  the
               Verilog input, but with file inclusions and macro references expanded and removed.
               This is useful, for example,  to  preprocess  Verilog  source  for  use  by  other

               Select  the  Verilog language generation to support in the compiler.  This selects
               between IEEE1364-1995, IEEE1364-2001, IEEE1364-2005, IEEE1800-2005, IEEE1800-2009,
               or   IEEE1800-2012.   Icarus  Verilog  currently  defaults  to  the  IEEE1364-2005
               generation of the language. This flag is used to restrict the language to a set of
               keywords/features, this allows simulation of older Verilog code that may use newer
               keywords and for compatibility with other tools. Much of the IEEE1800  generations
               functionality  is  not currently supported.  The IEEE1800 generations do parse all
               the keywords, so they can be used to verify that IEEE1364 compliant  Verilog  code
               does not use any of the new IEEE1800 keywords.

               Enable  or  disable  (default)  support  for Verilog-AMS.  Very little Verilog-AMS
               specific functionality is currently supported.

               Enable or disable (default) specify block support.  When  enabled,  specify  block
               code  is elaborated. When disabled, specify blocks are parsed but ignored. Specify
               blocks are  commonly  not  needed  for  RTL  simulation,  and  in  fact  can  hurt
               performance  of the simulation. However, disabling specify blocks reduces accuracy
               of full-timing simulations.

               Enable (default)  or  disable  the  search  of  a  standard  installation  include
               directory  after  all  other  explicit  include directories. This standard include
               directory is a convenient place to install standard header files  that  a  Verilog
               program may include.

               Enable  or  disable (default) adding the local files directory to the beginning of
               the include file search path. This allows files to be  included  relative  to  the
               current  file not the more common files are only found in the working directory or
               in the specified include file search path.

               Enable (default) or disable support for extended types.  Enabling  extended  types
               allows for new types that are supported by Icarus Verilog as extensions beyond the
               baseline Verilog. It may be necessary to disable extended types if compiling  code
               that clashes with the few new keywords used to implement the type system.

               The  standards  requires  that  a  vectored port have matching ranges for its port
               declaration as well as any net/register declaration. It was common practice in the
               past  to  only  specify  the range for the net/register declaration and some tools
               still allow  this.  By  default  any  mismatch  is  reported  as  a  error.  Using
               -gno-io-range-error  will  produce a warning instead of a fatal error for the case
               of a vectored net/register and a scalar port declaration.

               The standard requires that if any input  to  a  continuous  assignment  expression
               changes  value,  the  entire  expression is re-evaluated. By default, parts of the
               expression that do not depend on the changed input value(s) are not  re-evaluated.
               If  an  expression contains a call to a function that doesn't depend solely on its
               input values or that has side effects, the resulting  behavior  will  differ  from
               that  required  by  the  standard.  Using  -gstrict-ca-eval  will  force  standard
               compliant behavior (with some loss in performance).

               Enable or  disable  (default)  strict  compliance  with  the  standard  rules  for
               determining  expression  bit  lengths.  When  disabled,  the  RHS  of  a parameter
               assignment is evaluated as a lossless expression, as is any expression  containing
               an  unsized  constant  number,  and  unsized constant numbers are not truncated to
               integer width.

               Enable or disable (default) the  exclusion  of  for-loop  control  variables  from
               implicit  event_expression  lists.  When  enabled,  if a for-loop control variable
               (loop index) is only used inside the for-loop statement,  the  compiler  will  not
               include  it  in an implicit event_expression list it calculates for that statement
               or any enclosing statement. This allows the same control variable to  be  used  in
               multiple  processes  without  risk  of  entering  an  infinite loop caused by each
               process triggering all other processes that use  the  same  varaible.  For  strict
               compliance with the standards, this behaviour should be disabled.

               Append  directory  includedir  to list of directories searched for Verilog include
               files. The -I switch may be used many times  to  specify  several  directories  to
               search, the directories are searched in the order they appear on the command line.

       -lfile  Add  the specified file to the list of source files to be compiled, but mark it as
               a library file. All modules contained within that file will be treated as  library
               modules,  and  only  elaborated  if  they are instantiated by other modules in the

       -Mpath  This is equivalent to -Mall=path. Preserved for backwards compatibility.

               Write into the file specified by path a list  of  files  that  contribute  to  the
               compilation  of the design. If mode is all or prefix, this includes files that are
               included by include directives and files that are automatically loaded by  library
               support as well as the files explicitly specified by the user. If mode is include,
               only files that are included by include directives are listed. If mode is  module,
               only  files  that  are  specified  by the user or that are automatically loaded by
               library support are listed. The output is one file name per line, with no  leading
               or  trailing  space.  If  mode  is  prefix,  files  that  are  included by include
               directives are prefixed by "I " and other files are prefixed by "M ".

               Add this module to the list of VPI modules to be loaded by  the  simulation.  Many
               modules  can  be  specified,  and  all will be loaded, in the order specified. The
               system module is implicit and always included.  If a System  Function  Table  file
               (<module>.sft) exists for the module it will be loaded automatically.

       -Npath  This is used for debugging the compiler proper. Dump the final netlist form of the
               design to the specified file. It  otherwise  does  not  affect  operation  of  the
               compiler. The dump happens after the design is elaborated and optimized.

       -o filename
               Place  output  in the file filename. If no output file name is specified, iverilog
               uses the default name a.out.

               Assign a value to a target specific flag. The -p switch may be used  as  often  as
               necessary  to specify all the desired flags. The flags that are used depend on the
               target that is selected, and are described in target specific documentation. Flags
               that are not used are ignored.

       -S      Synthesize.  Normally,  if  the  target  can  accept  behavioral  descriptions the
               compiler will leave processes  in  behavioral  form.  The  -S  switch  causes  the
               compiler  to  perform synthesis even if it is not necessary for the target. If the
               target type is a netlist format, the -S switch is unnecessary and has no effect.

       -s topmodule
               Specify the top level module to elaborate. Icarus Verilog will by  default  choose
               modules  that are not instantiated in any other modules, but sometimes that is not
               sufficient, or instantiates too many modules. If the user specifies  one  or  more
               root modules with -s flags, then they will be used as root modules instead.

               Use  this  switch  to  select  min, typ or max times from min:typ:max expressions.
               Normally, the compiler will simply  use  the  typ  value  from  these  expressions
               (printing  a  warning  for  the  first ten it finds) but this switch will tell the
               compiler explicitly which value to use. This will suppress the  warning  that  the
               compiler is making a choice.

               Use this switch to specify the target output format. See the TARGETS section below
               for a list of valid output formats.

       -v      Turn on verbose messages. This will print the command lines that are  executed  to
               perform  the  actual  compilation, along with version information from the various
               components, as well as the version of the product as a  whole.   You  will  notice
               that  the  command  lines  include a reference to a key temporary file that passes
               information to the compiler proper.  To keep that file from being deleted  at  the
               end  of  the  process, provide a file name of your own in the environment variable

               If the selected target is vvp, the -v switch is appended to the  shebang  line  in
               the compiler output file, so directly executing the compiler output file will turn
               on verbose messages in vvp.  This extra verbosity can be avoided by using the  vvp
               command to indirectly execute the compiler output file.

       -V      Print the version of the compiler, and exit.

       -Wclass Turn  on  different  classes  of warnings. See the WARNING TYPES section below for
               descriptions of the different warning groups. If multiple -W  switches  are  used,
               the warning set is the union of all the requested classes.

               Append the directory to the library module search path. When the compiler finds an
               undefined module, it looks in these directories for files with the right name.

               Add suffix to the list of accepted  file  name  suffixes  used  when  searching  a
               library for cells. The list defaults to the single entry .v.


       The  Icarus Verilog compiler supports module libraries as directories that contain Verilog
       source files.  During elaboration, the compiler notices  the  instantiation  of  undefined
       module  types.  If the user specifies library search directories, the compiler will search
       the directory for files with the name of the missing module type. If it finds such a file,
       it loads it as a Verilog source file, they tries again to elaborate the module.

       Library  module  files should contain only a single module, but this is not a requirement.
       Library modules may reference other modules in the library or in the main design.


       The Icarus Verilog compiler supports a variety of targets, for different purposes, and the
       -t switch is used to select the desired target.

       null    The  null  target  causes  no  code to be generated. It is useful for checking the
               syntax of the Verilog source.

       vvp     This is the default. The vvp target generates code for the vvp runtime. The output
               is  a  complete  program  that  simulates  the  design  but must be run by the vvp
               command. The -pfileline=1 option can be used to add procedural statement debugging
               opcodes to the generated code.

       fpga    This is a synthesis target that supports a variety of fpga devices, mostly by EDIF
               format output. The Icarus  Verilog  fpga  code  generator  can  generate  complete
               designs  or  EDIF macros that can in turn be imported into larger designs by other
               tools. The fpga target implies the synthesis -S flag.

       vhdl    This target produces a VHDL translation of the Verilog netlist. The  output  is  a
               single  file  containing VHDL entities corresponding to the modules in the Verilog
               source code. Note that only a subset of the Verilog language  is  supported.   See
               the wiki for more information.


       These  are  the  types  of warnings that can be selected by the -W switch. All the warning
       types (other than all) can also be prefixed with no- to turn off  that  warning.  This  is
       most useful after a -Wall argument to suppress isolated warning types.

       all     This  enables  the  anachronisms, implicit, portbind, select-range, timescale, and
               sensitivity-entire-array warning categories.

               This enables warnings for use of features that have been deprecated or removed  in
               the selected generation of the Verilog language.

               This  enables  warnings  for  creation of implicit declarations. For example, if a
               scalar wire X is used but not declared in the Verilog source, this  will  print  a
               warning at its first use.

               This  enables  warnings  for ports of module instantiations that are not connected
               but probably should be.  Dangling  input  ports,  for  example,  will  generate  a

               This  enables warnings for constant out of bound selects. This includes partial or
               fully out of bound selects as well as a select containing a  'bx  or  'bz  in  the

               This  enables warnings for inconsistent use of the timescale directive. It detects
               if some modules have no timescale, or if modules inherit  timescale  from  another
               file.  Both  probably mean that timescales are inconsistent, and simulation timing
               can be confusing and dependent on compilation order.

       infloop This enables warnings for always statements that may have runtime  infinite  loops
               (has paths with no or zero delay). This class of warnings is not included in -Wall
               and hence does not have a no- variant.  A  fatal  error  message  will  always  be
               printed  when the compiler can determine that there will definitely be an infinite
               loop (all paths have no or zero delay).

               When you suspect an always statement is producing a runtime infinite loop use this
               flag  to  find the always statements that need to have their logic verified. It is
               expected that many of the warnings will be false positives, since the code  treats
               the value of all variables and signals as indeterminate.

               This  enables  warnings  for  when  a  part select within an "always @*" statement
               results in the entire  vector  being  added  to  the  implicit  sensitivity  list.
               Although  this  behaviour is prescribed by the IEEE standard, it is not what might
               be expected and can have performance implications if the vector is large.

               This enables warnings for when a word  select  within  an  "always  @*"  statement
               results in the entire array being added to the implicit sensitivity list. Although
               this behaviour is prescribed by the  IEEE  standard,  it  is  not  what  might  be
               expected and can have performance implications if the array is large.


       If  the  source file name as a .sft suffix, then it is taken to be a system function table
       file. A System function table file is used to describe to the compiler  the  return  types
       for  system  functions.  This  is necessary because the compiler needs this information to
       elaborate expressions that contain these system  functions,  but  cannot  run  the  sizetf
       functions since it has no run-time.

       The  format  of  the  table  is ASCII, one function per line. Empty lines are ignored, and
       lines that start with the '#' character are comment lines. Each  non-comment  line  starts
       with  the  function name, then the vpi type (i.e. vpiSysFuncReal). The following types are

               The function returns a real/realtime value.

               The function returns an integer.

       vpiSysFuncSized <wid> <signed|unsigned>
               The function returns a vector with the given width,  and  is  signed  or  unsigned
               according to the flag.


       The  command  file  allows  the  user  to place source file names and certain command line
       switches into a text file instead of on a long command line. Command files can  include  C
       or C++ style comments, as well as # comments, if the # starts the line.

       file name
               A  simple file name or file path is taken to be the name of a Verilog source file.
               The  path  starts  with  the  first  non-white-space  character.   Variables   are
               substituted in file names.

       -c cmdfile -f cmdfile
               A  -c  or  -f  token  prefixes a command file, exactly like it does on the command
               line. The cmdfile may be on the same line or the next non-comment line.

       -l file -v file
               A -l token prefixes a library file in the command file, exactly like  it  does  on
               the command line. The parameter to the -l flag may be on the same line or the next
               non-comment line. -v is an alias for -l, provided  for  compatibility  with  other

               Variables in the file are substituted.

       -y libdir
               A  -y token prefixes a library directory in the command file, exactly like it does
               on the command line. The parameter to the -y flag may be on the same line  or  the
               next non-comment line.

               Variables in the libdir are substituted.

               The  +incdir+ token in command files gives directories to search for include files
               in much the same way that -I flags work on the command  line.  The  difference  is
               that  multiple  +includedir  directories are valid parameters to a single +incdir+
               token, although you may also have multiple +incdir+ lines.

               Variables in the includedir are substituted.

               The +libext token in command files fives file extensions to try when looking for a
               library  file. This is useful in conjunction with -y flags to list suffixes to try
               in each directory before moving on to the next library directory.

               This is another way to specify library directories. See the -y flag.

               This is like the +libdir statement, but file names inside the directories declared
               here  are case insensitive. The missing module name in a lookup need not match the
               file name case, as long as the letters are correct.  For  example,  "foo"  matches
               "Foo.v" but not "bar.v".

               The  +define+  token  is  the same as the -D option on the command line. The value
               part of the token is optional.

               The +parameter+ token is the same as the -P option on the command line.

               The +timescale+ token is used to set the default  timescale  for  the  simulation.
               This  is  the  time units and precision before any `timescale directive or after a
               `resetall directive. The default is 1s/1s.

               This token causes file names after this in the command file to  be  translated  to
               uppercase.  This  helps with situations where a directory has passed through a DOS
               machine, and in the process the file names become munged.

               This is similar to the +toupper-filename hack described above.

               This allows the programmer to select  the  width  for  integer  variables  in  the
               Verilog source. The default is 32, the value can be any desired integer value.

               This  allows  the  programmer to select the width cap for unsized expressions.  If
               the calculated width for an unsized expression exceeds this  value,  the  compiler
               will issue a warning and limit the expression width to this value.


       In  certain  cases, iverilog supports variables in command files. These are strings of the
       form "$(varname)" or "${varname}", where varname is the name of the  environment  variable
       to  read.  The entire string is replaced with the contents of that variable. Variables are
       only substituted in contexts that explicitly support them, including  file  and  directory

       Variable  values  come  from  the  operating system environment, and not from preprocessor
       defines elsewhere in the file or the command line.


       The following macros are predefined by the compiler:

       __ICARUS__ = 1
               This is always defined when compiling with Icarus Verilog.

       __VAMS_ENABLE__ = 1
               This is defined if Verilog-AMS is enabled.


       These examples assume that you have a Verilog source file called hello.v  in  the  current

       To compile hello.v to an executable file called a.out:

            iverilog hello.v

       To compile hello.v to an executable file called hello:

            iverilog -o hello hello.v

       To compile and run explicitly using the vvp runtime:

            iverilog -ohello.vvp -tvvp hello.v


       Steve Williams (


       vvp(1), <>

       Tips   on   using,   debugging,   and   developing   the   compiler   can   be   found  at


       Copyright ©  2002-2016 Stephen Williams

       This document can be freely redistributed according to the terms of the
       GNU General Public License version 2.0

                                          Oct 2nd, 2016                               iverilog(1)