Using VPI

Icarus Verilog implements a portion of the PLI 2.0 API to Verilog. This allows programmers to write C code that interfaces with Verilog simulations to perform tasks otherwise impractical with straight Verilog. Many Verilog designers, especially those who only use Verilog as a synthesis tool, can safely ignore the entire matter of the PLI (and this chapter) but the designer who wishes to interface a simulation with the outside world cannot escape VPI.

The rest of this article assumes some knowledge of C programming, Verilog PLI, and of the compiler on your system. In most cases, Icarus Verilog assumes the GNU Compilation System is the compiler you are using, so tips and instructions that follow reflect that. If you are not a C programmer, or are not planning any VPI modules, you can skip this entire article. There are references at the bottom for information about more general topics.

How It Works

The VPI modules are compiled loadable object code that the runtime loads at the user’s request. The user commands vvp to locate and load modules with the “-m” switch. For example, to load the “sample.vpi” module:

% vvp -msample foo.vvp

The vvp run-time loads the modules first, before executing any of the simulation, or even before compiling the vvp code. Part of the loading includes invoking initialization routines. These routines register with the run-time all the system tasks and functions that the module implements. Once this is done, the run time loader can match names of the called system tasks of the design with the implementations in the VPI modules.

(There is a special module, the system.vpi module, that is always loaded to provide the core system tasks.)

The simulator run time (The “vvp” program) gets a handle on a freshly loaded module by looking for the symbol “vlog_startup_routines” in the loaded module. This table, provided by the module author and compiled into the module, is a null terminated table of function pointers. The simulator calls each of the functions in the table in order. The following simple C definition defines a sample table:

void (*vlog_startup_routines[])() = {

Note that the “vlog_startup_routines” table is an array of function pointers, with the last pointer a 0 to mark the end. The programmer can organize the module to include many startup functions in this table, if desired.

The job of the startup functions that are collected in the startup table is to declare the system tasks and functions that the module provides. A module may implement as many tasks/functions as desired, so a module can legitimately be called a library of system tasks and functions.

Compiling VPI Modules

To compile and link a VPI module for use with Icarus Verilog, you must compile all the source files of a module as if you were compiling for a DLL or shared object. With gcc under Linux, this means compiling with the “-fpic” flag. The module is then linked together with the vpi library like so:

% gcc -c -fpic hello.c
% gcc -shared -o hello.vpi hello.o -lvpi

This assumes that the “vpi_user.h header file and the libvpi.a library file are installed on your system so that gcc may find them. This is normally the case under Linux and UNIX systems. An easier, the preferred method that works on all supported systems is to use the single command:

% iverilog-vpi hello.c

The “iverilog-vpi” command takes as command arguments the source files for your VPI module, compiles them with proper compiler flags, and links them into a vpi module with any system specific libraries and linker flags that are required. This simple command makes the “hello.vpi” module with minimum fuss.

A Worked Example

Let us try a complete, working example. Place the C code that follows into the file hello.c:

# include  <vpi_user.h>

static int hello_compiletf(char*user_data)
      return 0;

static int hello_calltf(char*user_data)
      vpi_printf("Hello, World!\n");
      return 0;

void hello_register()
      s_vpi_systf_data tf_data;

      tf_data.type      = vpiSysTask;
      tf_data.tfname    = "$hello";
      tf_data.calltf    = hello_calltf;
      tf_data.compiletf = hello_compiletf;
      tf_data.sizetf    = 0;
      tf_data.user_data = 0;

void (*vlog_startup_routines[])() = {

and place the Verilog code that follows into hello.v:

module main;
  initial $hello;

Next, compile and execute the code with these steps:

% iverilog-vpi hello.c
% iverilog -ohello.vvp hello.v
% vvp -M. -mhello hello.vvp
Hello, World!

The compile and link in this example are conveniently combined into the “iverilog-vpi” command. The “iverilog” command then compiles the “hello.v” Verilog source file to the “hello.vvp” program. Next, the “vvp” command demonstrates the use of the “-M” and “-m” flags to specify a vpi module search directory and vpi module name. Specifically, they tell the “vvp” command where to find the module we just compiled.

The “vvp” command, when executed as above, loads the “hello.vpi” module that it finds in the current working directory. When the module is loaded, the vlog_startup_routines table is scanned, and the “hello_register” function is executed. The “hello_register” function in turn tells “vvp” about the system tasks that are included in this module.

After the modules are all loaded, the “hello.vvp” design file is loaded and its call to the “$hello” system task is matched up to the version declared by the module. While “vvp” compiles the “hello.vvp” source, any calls to “$hello” are referred to the “compiletf” function. This function is called at compile time and can be used to check parameters to system tasks or function. It can be left empty like this, or left out completely. The “compiletf” function can help performance by collecting parameter checks in compile time, so they do not need to be done each time the system task is run, thus potentially saving execution time overall.

When the run-time executes the call to the hello system task, the “hello_calltf” function is invoked in the loaded module, and thus the output is generated. The “calltf” function is called at run time when the Verilog code actually executes the system task. This is where the active code of the task belongs.

System Function Return Types

Icarus Verilog supports system functions as well as system tasks, but there is a complication. Notice how the module that you compile is only loaded by the “vvp” program. This is mostly not an issue, but elaboration of expressions needs to keep track of types, so the main compiler needs to know the return type of functions.

Starting with Icarus Verilog v11, the solution is quite simple. The names and locations of the user’s VPI modules can be passed to the compiler via the “iverilog” -m and -L flags and the IVERILOG_VPI_MODULE_PATH environment variable. The compiler will load and analyse the specified modules to automatically determine any function return types. The compiler will also automatically pass the names and locations of the specified modules to the “vvp” program, so that they don’t need to be specified again on the “vvp” command line.

For Icarus Verilog versions prior to v11, the solution requires that the developer of a module include the table in a form that the compiler can read. The System Function Table file carries this information. A simple example looks like this:

# Example sft declarations of some common functions
$random      vpiSysFuncInt
$bitstoreal  vpiSysFuncReal
$realtobits  vpiSysFuncSized 64 unsigned

This demonstrates the format of the file and support types. Each line contains a comment (starts with “#”) or a type declaration for a single function. The declaration starts with the name of the system function (including the leading “$”) and ends with the type. The supported types are:

  • vpiSysFuncInt

  • vpiSysFuncReal

  • vpiSysFuncSized <wid> <signed|unsigned>

Any functions that do not have an explicit type declaration in an SFT file are implicitly taken to be “vpiSysFuncSized 32 unsigned”.

The module author provides, along with the “.vpi” file that is the module, a “.sft” that declares all the function return types. For example, if the file is named “example.sft”, pass it to the “iverilog” command line or in the command file exactly as if it were an ordinary source file.

Cadence PLI Modules

With the cadpli module, Icarus Verilog is able to load PLI1 applications that were compiled and linked to be dynamic loaded by Verilog-XL or NC-Verilog. This allows Icarus Verilog users to run third-party modules that were compiled to interface with XL or NC. Obviously, this only works on the operating system that the PLI application was compiled to run on. For example, a Linux module can only be loaded and run under Linux. In addition, a 64-bit version of vvp can only load 64-bit PLI1 applications, etc.

Icarus Verilog uses an interface module, the “cadpli” module, to connect the worlds. This module is installed with Icarus Verilog, and is invoked by the usual -m flag to iverilog or vvp. This module in turn scans the extended arguments, looking for -cadpli= arguments. The latter specify the share object and bootstrap function for running the module. For example, to run the module, that has the bootstrap function “my_boot”:

% vvp -mcadpli a.out -cadpli=./

The “-mcadpli” argument causes vvp to load the cadpli.vpl library module. This activates the -cadpli= argument interpreter. The -cadpli=<module>:<boot_func> argument, then, causes vvp, through the cadpli module, to load the loadable PLI application, invoke the my_boot function to get a veriusertfs table, and scan that table to register the system tasks and functions exported by that object. The format of the -cadpli= extended argument is essentially the same as the +loadpli1= argument to Verilog-XL.

The integration from this point is seamless. The PLI application hardly knows that it is being invoked by Icarus Verilog instead of Verilog-XL, so operates as it would otherwise.

Other References

Since the above only explains how to get PLI/VPI working with Icarus Verilog, here are some references to material to help with the common aspects of PLI/VPI.

  • Principles of Verilog PLI by Swapnajit Mittra. ISBN 0-7923-8477-6

  • The Verilog PLI Handbook by Stuart Sutherland. ISBN 0-7923-8489-X