stimc — a Lightweight Verilog-VPI Wrapper for Stimuli Generation

stimc Usage

Verilog Shell

The first thing you need for a stimc controlled module is a Verilog shell. This is a Verilog module with inputs/outputs and optionally parameters. The module should have no actual content except for an initial block calling the system task $stimc_<modulename>_init(); (with <modulename> replaced by the name of the Verilog module name. The system task will be defined by the module's stimc implementation and initialize the stimc data of the module.

An example could look like this:

module dummy (

    parameter DATA_W = 32;

    input               clk_i;
    input               reset_n_i;
    input  [DATA_W-1:0] data_in_i;
    output [DATA_W-1:0] data_out_o;

    initial begin


stimc Module

The stimc core is written in C with a thin C++ wrapper, so the stimc part of the module can be implemented in C or C++, but C++ will produce more readable code and here the C++ version will be explained. The C++ code is wrapped in a stimcxx namespace.

Module Class

First a C++ class is needed that inherits stimcxx::module (include stimc++.h). For every (used) port of the Verilog shell the class should have a port member and for every (used) parameter a parameter member of the same name as the Verilog counterpart. Additionally, it can have stimcxx::event members in case waiting on events triggered by individual stimc threads or value-change events on ports is needed (similar to SystemC events).

The class needs a constructor without arguments and can have (non-static) void functions that can be run as initially started threads (comparable to a Verilog initial block or a SystemC thread) or port event based in which case they are limited to triggering events or immediately reacting (similar to SystemC methods).

An excerpt from the project's examples looks like this:

class dummy : public stimcxx::module {
        parameter DATA_W;

        port clk_i;
        port reset_n_i;
        port data_in_i;
        port data_out_o;

        stimcxx::event clk_event;

        dummy ();
        virtual ~dummy ();

        void testcontrol ();
        void clock ();
        void dinchange ();


The constructor needs to initialize ports, parameters, event-triggered methods and threads started at simulation start. For ports and parameters preprocessor defines are prepared to use in the member initializer list. For ports this is STIMCXX_PORT (<port-name>), for parameters STIMCXX_PARAMETER (<parameter-name>). Inside the constructor threads and event-triggered methods can be setup with the defines STIMCXX_REGISTER_STARTUP_THREAD (<function-name>); and STIMCXX_REGISTER_METHOD (<event-type>, <port-name>, <function-name>);, where <event-type> is one of posedge, negedge or change (corresponding to the Verilog posedge, negedge or just plain @ events).

The main part of the example constructor looks like this:

dummy::dummy () :
    STIMCXX_PORT (clk_i),
    STIMCXX_PORT (reset_n_i),
    STIMCXX_PORT (data_in_i),
    STIMCXX_PORT (data_out_o)
    STIMCXX_REGISTER_METHOD (posedge, clk_i, clock);
    STIMCXX_REGISTER_METHOD (change, data_in_i, dinchange);

On every posedge event on the clk_i input the member function clock will be called, on every value change on input data_in_i the function dinchange will be called and a stimc thread will be created and run at simulation start calling testcontrol.


To provide the Verilog initializer system task and functionality the preprocessor define STIMCXX_EXPORT (<module-name>) (without semicolon) should be put at some place in the code.

This will provide the system tasks for the given module on loading the VPI library.


To interact with simulation, parameters can be read, ports can be read and assigned. For reading it is possible to directly assign a port or parameter to a variable. For writing to a port you can assign an integer type to it directly (corresponding to Verilog blocking assignment) or via the overloaded <<= operator (corresponding to Verilog non-blocking assignment, optical similarity to Verilog is intended, by accepting the risk of misinterpretation as shift operation).

In case only a bit-range of a port should be accessed or an integer type is not sufficiently wide it is possible to access the port's operator() (<msb>,<lsb>) and similarly read from or write to the bit-range. Ports can also be assigned to Verilog x or z values using the similarly named constants X and Z or checked to contain unknown values via comparison against X.

Threads and Events

Threads can be suspended to wait for a specified amount of simulation time and/or an event to be triggered.

There are some helper functions in the stimcxx namespace:

Events can be triggered by calling their trigger() member function. When triggering an event, all threads currently waiting for it will be resumed (in any order). It is not possible for a thread to trigger itself as it would either need to first wait (not being able to trigger) or first trigger (and not yet waiting on the event, so not being resumed on this trigger).

Apart from a module class method it is also possible to spawn a standalone function taking a single pointer argument as a thread via STIMCXX_SPAWN_THREAD (<function>, <data>);


Resource cleanup is mainly useful in cases where simulation can be reset. Threads will be recreated and run after a reset and might cause conflicts with remaining resources from a previous run.

While modules are cleaned up by their destructor being called at end of simulation, threads could be suspended in a waiting state, having their resources still acquired.

Stack unwinding

Since version 1.2 a stimc++ thread's stack will by default be unwound on termination at and of simulation or simulator reset. If programmed accordingly by writing the associated thread code exception safe it is possible to have resources freed this way.

Stack unwinding is achieved by letting wait functions throw a stimc++ specific exception at end of simulation which therefore should either not be caught or otherwise rethrown if necessary. It is important to consider that all thread's stacks can be unwound in any order.

If stack unwinding causes problems it can be disabled by defining STIMCXX_DISABLE_STACK_UNWIND. Alternatively it can be selectively disabled for individual threads by calling stimc_thread_resume_on_finish (false).

Explicit cleanup

In order to explicitly control freeing of memory or cleanup of resources like open files when a thread is terminated it is possible to register a cleanup callback.

In stimc++ it is possible to wrap this into one or more objects of custom cleanup classes inheriting stimcxx::thread_cleanup which is created within the thread via new and performs cleanup tasks in its destructor. The creation via new is important, as the object must also remain in case the thread function returns and will be destroyed via delete in the cleanup process. The object must be created within the thread it should cleanup as it will be called when the creating thread is deleted. Furthermore it is necessary to cover all possible scenarios where the cleanup could happen, which is typically whenever the thread is destroyed while suspended (waiting, which might also happen in a function calling a wait function) or when the thread function returns. In all of these cases the cleanup in the destructor should do the right thing (e.g. free allocated resources and ignore not yet allocated or already freed resources).

Here an example with a snippet from a thread and a cleanup class for 2 variables a and b being objects of two classes A and B shows the general idea:

/* custom cleanup class */
class cleanup_ab : protected stimcxx::thread_cleanup {
        A *a;
        B *b;

        cleanup_ab () : a(nullptr), b(nullptr) {}
        ~cleanup_ab () {
            if (a) delete a;
            if (b) delete b;

/* simulation thread of custom module dummy */
void dummy::testcontrol ()
    cleanup_ab *cleanup_data = new cleanup_ab ();

    /* ... */

    A *a = new A ();
    /* from here on, if thread is deleted, a will be deleted */
    cleanup_data->a = a;

    /* ... */

    B *b = new B ();
    /* from here on, if thread is deleted, a and b will be deleted */
    cleanup_data->b = b;

    /* ... */

    delete a;
    /* a already freed -> do not delete when thread is deleted */
    cleanup_data->a = nullptr;

    /* ... */

In case cleanup functionality causes unwanted problems (use after free or similar), it is possible to disable it via the STIMC_DISABLE_CLEANUP preprocessor define when compiling stimc. In this case besides memory being leaked, resetting simulation will likely cause problems.