Loadable Target API (ivl_target)

In addition to the standard VPI API, Icarus Verilog supports a non-standard loadable target module API. This API helps C programmers write modules that Icarus Verilog can use to generate code. These modules are used at compile time to write the elaborated design to the simulation or netlist files. For example, the vvp code generator is a loadable target module that writes vvp code into the specified file.

Loadable target modules gain access to the ‘elaborated’ design. That means, the source files have been checked for syntax and correctness, any synthesis and general optimization steps have been performed, and what is left is a design that reflects but is not exactly the same as the input Verilog source code. This relieves the modules of the burden of supporting all the odd corners and complexities of the Verilog language.

The Target Module API

The API is defined in the header file “ivl_target.h” which is installed with Icarus Verilog. The header defines the functions that the module writer can use to get at the elaborated design during the course of writing the output format.

The target module API function “target_design” is special in that the API does not provide this function: The target module itself provides it. When the compiler loads the target module, it invokes the “target_design” function with a handle to the design. This is the point where the target module takes over to process the design.

Compiling Target Modules

Compiling loadable target modules is similar to compiling VPI modules, in that the module must be compiled with the “-fPIC” flag to gcc, and linked with the “-shared” flag. The module that you compile is then installed in a place where the “iverilog” command can find it, and configuration files are adjusted to account for the new module.

This code:

# include  <ivl_target.h>

int target_design(ivl_design_t des)
     return 0;

is an example module that we can write into the file “empty.c”; and let us compile it into the module file “empty.tgt” like so:

% gcc -o empty.tgt -fpic -shared empty.c

This makes the “empty.tgt” file an a dynamically loaded shared object.

Creating the Target Config File

The target config file tells the Icarus Verilog core how to process your new code generator. The ivl core expects two configuration files: the name.conf and the name-s.config files. The “-s” version is what is used if the user gives the “-S” (synthesis) flag on the command line.

The stub target, included in most distributions, demonstrates the config files. The “stub.conf” file is:


and the “stub-s.conf” file is:


Note that the “stub-s.conf” file contains more lines to invoke internal synthesis functions, whereas the “stub.conf” invokes only the basic optimization steps.

In general, only the last line (The “flag:DLL=<name>.tgt” record) varies for each target. For your target, replace the <name> with the name of your target and you have a configuration file ready to install. Note that this is the name of your target module. This is in fact how the config file tells the compiler the name of your module.

The rest of the config file is best taken as boiler plate and installed as is, with one difference. If your target is a synthesis target (for example a mosis code generator or a pld code generator) that expects synthesis to happen, then it makes the most sense to create both your config file like the “stub-s.conf” config file. This causes the compiler to do synthesis for your target whether the user gives the “-S” flag or not.

Installing the Target Module

Finally, the “empty.conf”, the “empty-s.conf” and the “empty.tgt” files need to be installed. Where they go depends on your system, but in Linux they are normally installed in “/usr/lib/ivl”.

LPM Devices

All LPM devices support a small set of common LPM functions, as described in the ivl_target header file. The ivl_lpm_t object has a type enumerated by ivl_lpm_type_t, and that type is accessible via the ivl_lpm_type function.

The following are type specific aspects of LPM devices.


This LPM represents a user defined function. It is a way to connect behavioral code into a structural network. The UFUNC device has a vector output and a set of inputs. The ivl_lpm_define function returns the definition as an ivl_scope_t object.

The output vector is accessible through the ivl_lpm_q, and the output has the width defined by ivl_lpm_width. This similar to most every other LPM device with outputs.

There are ivl_lpm_size() input ports, each with the width ivl_lpm_data2_width(). The actual nexus is indexed by ivl_lpm_data2().