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  <section id="vvp-interactive-mode">
<h1>VVP Interactive Mode<a class="headerlink" href="#vvp-interactive-mode" title="Link to this heading">¶</a></h1>
<p>The vvp command has an interactive debug mode, where you can stop the
simulation and browse the current state of the simulation. There are
a couple ways to enter the debug mode, but once in interactive debug
mode, the usage is the same. Consider the example below:</p>
<div class="highlight-verilog notranslate"><div class="highlight"><pre><span></span><span class="k">module</span><span class="w"> </span><span class="n">clock</span><span class="p">(</span><span class="k">output</span><span class="w"> </span><span class="kt">reg</span><span class="w"> </span><span class="n">clock</span><span class="p">);</span>
<span class="w">  </span><span class="k">initial</span><span class="w"> </span><span class="n">clock</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">1</span><span class="mb">&#39;b1</span><span class="p">;</span>
<span class="w">  </span><span class="k">always</span><span class="w"> </span><span class="p">#</span><span class="mh">100</span><span class="w"> </span><span class="n">clock</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">!</span><span class="n">clock</span><span class="p">;</span>
<span class="k">endmodule</span><span class="w"> </span><span class="c1">// clock</span>

<span class="k">module</span><span class="w"> </span><span class="n">main</span><span class="p">;</span>

<span class="w">  </span><span class="kt">reg</span><span class="w"> </span><span class="p">[</span><span class="mh">2</span><span class="o">:</span><span class="mh">0</span><span class="p">]</span><span class="w"> </span><span class="n">foo</span><span class="p">;</span>
<span class="w">  </span><span class="kt">wire</span><span class="w">             </span><span class="n">clk</span><span class="p">;</span>

<span class="w">  </span><span class="n">clock</span><span class="w"> </span><span class="n">foo_clock</span><span class="p">(</span><span class="n">clk</span><span class="p">);</span>

<span class="w">  </span><span class="k">always</span><span class="w"> </span><span class="p">@(</span><span class="k">posedge</span><span class="w"> </span><span class="n">clk</span><span class="p">)</span>
<span class="w">    </span><span class="n">foo</span><span class="w"> </span><span class="o">&lt;=</span><span class="w"> </span><span class="n">foo</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mh">1</span><span class="p">;</span>

<span class="w">  </span><span class="k">initial</span><span class="w"> </span><span class="k">begin</span>
<span class="w">     </span><span class="n">foo</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">3</span><span class="mb">&#39;b000</span><span class="p">;</span>
<span class="w">    </span><span class="p">#</span><span class="mh">250</span><span class="w"> </span><span class="nb">$stop</span><span class="p">;</span>
<span class="w">  </span><span class="k">end</span>

<span class="k">endmodule</span>
</pre></div>
</div>
<p>In examples that follow, we will use the above sample program.</p>
<section id="enter-interactive-mode">
<h2>Enter Interactive Mode<a class="headerlink" href="#enter-interactive-mode" title="Link to this heading">¶</a></h2>
<p>The first and most common method is to put “$stop” system task
calls in the simulation at the times where you want to simulation
to break and enter interactive mode. The example above has a $stop,
so the output looks like this:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>../foo.vl:25: $stop called at 250 (1s)
** VVP Stop(0) **
** Flushing output streams.
** Current simulation time is 250 ticks.
&gt;
</pre></div>
</div>
<p>You can get some interactive help by using the “help” command:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt; help
Commands can be from the following table of base commands,
or can be invocations of system tasks/functions.

cd       - Synonym for push.
cont     - Resume (continue) the simulation
finish   - Finish the simulation.
help     - Get help.
list     - List items in the current scope.
load     - Load a VPI module, a la vvp -m.
ls       - Shorthand for &quot;list&quot;.
pop      - Pop one scope from the scope stack.
push     - Descend into the named scope.
step     - Single-step the scheduler for 1 event.
time     - Print the current simulation time.
trace    - Control statement tracing (on/off) when the code is instrumented.
where    - Show current scope, and scope hierarchy stack.

If the command name starts with a &#39;$&#39; character, it
is taken to be the name of a system task, and a call is
built up and executed. For example, &quot;$display foo&quot; will
call the function as $display(foo).
</pre></div>
</div>
<p>You can also enter interactive mode at the terminal by interrupting the
execution with a “^C” (Control-C) character. The vvp engine catches the
terminal interrupt and drops you into the interactive prompt:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>^C** VVP Stop(0) **
** Flushing output streams.
** Current simulation time is 533928600 ticks.
&gt;
</pre></div>
</div>
<p>This could be useful if you suspect that your simulation is stuck in
an infinite loop and you want to rummage around and see what’s going on.</p>
<p>And finally, you can pass the “-s” command line flag to vvp to tell it
to execute “$stop” at the beginning of the simulation, before any other
events are executed. This may be useful as a way to manually set up some
details about the simulation.</p>
</section>
<section id="browsing-the-design">
<h2>Browsing the Design<a class="headerlink" href="#browsing-the-design" title="Link to this heading">¶</a></h2>
<p>Now that you are in the interactive prompt, you can browse
around the design:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt; ls
2 items in this scope:
package : $unit
module  : main
&gt; cd main
&gt; ls
3 items in this scope:
reg     : foo[2:0]
module  : foo_clock
net     : clk
&gt; where
module main
&gt; $display foo
1
&gt; cd foo_clock
&gt; where
module foo_clock
module main
&gt; ls
2 items in this scope:
port    : clock -- output
reg     : clock
</pre></div>
</div>
<p>In the above example, the ‘cd’ and ‘pop’ commands descend into a scope
or pop back up a scope level. The ‘where’ command shows the scope stack,
and the ‘ls’ command lists the items present in the scope. With these
commands, one can browse freely throughout the design scope hierarchy.</p>
<p>It is also possible to call system tasks within the debug mode. The call
to the “$display” function is an example of this. In general, any system
task can be invoked, in the current context, with the objects that are
included on the command line passed as arguments. The arguments can be
variables or nets, and various kinds of literals:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>&gt; ls
2 items in this scope:
port    : clock -- output
reg     : clock
&gt; $display &quot;Hello, World! &quot; 10 &quot; &quot; clock
Hello, World!          10 1
</pre></div>
</div>
<p>This is a great way to call custom system tasks as well. And system task
that vvp knows about can be invoked this way.</p>
</section>
<section id="leave-interactive-mode">
<h2>Leave Interactive Mode<a class="headerlink" href="#leave-interactive-mode" title="Link to this heading">¶</a></h2>
<p>After you are done probing around in the interactive mode, you can
resume the simulation, or termimate execution. Resume the simulation
with the “cont” command, and terminate the simulation with the
“finish” command. The latter is the same as executing the
“$finish” system task.</p>
</section>
</section>


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