# The vvp Code Generator (-tvvp)

The vvp target generates code for the “vvp” run time. This is the most commonly used target for Icarus Verilog, as it is the main simulation engine.

Example command:

    %  iverilog -o top.vvp -s top hello_world.v

Equivalent command:

    %  iverilog -o top.vvp -tvvp -s top hello_world.v

With this example code in hello_world.v:

    module top;
        initial $display("Hello World!");
    endmodule

The resulting top.vvp will contain something similar to:

    #! /usr/local/bin/vvp
    :ivl_version "13.0 (devel)" "(s20221226-119-g8cb2e1a05-dirty)";
    :ivl_delay_selection "TYPICAL";
    :vpi_time_precision + 0;
    :vpi_module "/usr/local/lib/ivl/system.vpi";
    :vpi_module "/usr/local/lib/ivl/vhdl_sys.vpi";
    :vpi_module "/usr/local/lib/ivl/vhdl_textio.vpi";
    :vpi_module "/usr/local/lib/ivl/v2005_math.vpi";
    :vpi_module "/usr/local/lib/ivl/va_math.vpi";
    S_0x563c3c5d1540 .scope module, "top" "top" 2 1;
     .timescale 0 0;
        .scope S_0x563c3c5d1540;
    T_0 ;
        %vpi_call 2 2 "$display", "Hello World!" {0 0 0};
        %end;
        .thread T_0;
    # The file index is used to find the file name in the following table.
    :file_names 3;
        "N/A";
        "<interactive>";
        "hello_world.v";

The first line contains the shebang. If this file is executed, the shebang tells the shell to use vvp for the execution of this file.

To run the simulation, execute:

    %  ./top.vvp

Or you can call vvp directly:

    %  vvp top.vvp

Next are some directives. The first one, :ivl_version specifies which version of iverilog this file was created with. Next is the delay selection with “min:typical:max” values and the time precision, which we did not set specifically, so the default value is used. The next lines tell vvp which VPI modules to load and in which order. The next lines tell vvp which VPI modules to load and in what order. Next, a new scope is created with the .scope directive and the timescale is set with .timescale. A thread T_0 is created that contains two instructions: %vpi_call executes the VPI function $display with the specified arguments, and %end terminates the simulation.

## Opcodes

The various available opcodes can be seen in Opcodes