Getting Started with EpicSim

EpicSim is a Verilog compiler. It is suitable for use as a simulator and recommended to run under Red Hat and CentOS. Instructions in this document are generally applicable to all environments.

The project is developed based on icarus iverilog under LGPL. Special thanks to Stephen Williams (steve@icarus.com).

1. Install From Source

If you are starting from the source, the build process is designed to be as simple as practical. Someone basically familiar with the target system and C/C++ compilation should be able to build the source distribution with little effort. Some actual programming skills are not required, but helpful in case of problems.

1.1 Compile Time Prerequisites

You need the following softwares to compile EpicSim from source on a UNIX-like system:

CMake
GNU Make
ISO C++ Compiler
bison and flex
gperf 3.0 or later
readline 4.2 or later
termcap
bash

1.2 Compilation

Unpack the tar-ball and compile the source with the commands:

  cd EpicSim
  mkdir build
  cd build
  cmake ..
  make install

After the installtion, set the environment variables with the commands:

  cd EpicSim
  #bash
  export PATH="target_installation_path/EpicSim/install/bin:$PATH"
  #cshell
  setenv PATH target_installation_path/EpicSim/install/bin:$PATH"

2. Hello, World!

As a user, the first thing you want to do is probably learning how to compile and execute even the most trivial design. For simulation, we use the following design as an example:

  module main;
  initial
    begin
      $display("Hello, World");
      $finish ;
    end
  endmodule

Arrange for the above program to be in a text file, hello.vl using a text editor or copy the hello.vl file from the EpicSim examples directory.
Compile the program with the epicsim command:
```
% epicsim hello.vl
```
The results of this compile are placed into the epicsim-run file, and it is excuted:
```
Hello, World
```
The compiled program can also be executed by two steps:
```
% epicsim-driver hello.vl -o epicsim-run 
% epicsim-vvp ./epicsim-run  
```
```
Hello, World
```
And there it is, the program has been executed. So what happened? The first step, the epicsim-driver command, read and interpreted the source file, then generated a compiled result. The compiled form may be selected by command line switches, but the default form is vvp, which is actually run by the epicsim-vvp command.

The epicsim-driver and epicsim-vvp commands are the only commands that users use to invoke EpicSim. What the compiler actually does is controlled by command line switches. In our little example, we asked the compiler to compile the source program to the default vvp form, which is in turn executed by the epicsim-vvp program.

3. How EpicSim Works

This tool includes a parser which reads in Verilog (plus extensions) and generates an internal netlist. The netlist is passed to various processing steps that transform the design to more optimal or practical forms, then is passed to a code generator for final output. The processing steps and the code generator are selected by command line switches.

3.1 Preprocessing

The preprocessing is done by a separate program, ivlpp. This program implements the include and define directives producing output that is equivalent but without the directives. The output is a single file with line number directives, so that the actual compiler only sees a single input file. See ivlpp/ivlpp.txt for details.

3.2 Parse

The Verilog compiler starts by parsing the Verilog source file. The output of the parse is a list of Module objects in “pform“. The pform is mostly a direct reflection of the compilation step (see pform.h). There may be dangling references, and it is not yet clear which module is the root.

One can see a human-readable version of the final pform by using the -P <path> flag to the ivl subcommand. This will cause ivl to dump the pform into the file named <path>. (Note that this is not normally done, unless debugging the ivl subcommand.)

3.3 Elaboration

The elaboration takes the pform and generates a netlist. The driver selects (by user request or lucky guess) the root module to elaborate, resolves references and expands the instantiations to form the design netlist (see netlist.txt). Final semantic checks and some simple optimizations are performed during elaboration. The netlist includes all the behavioural descriptions, as well as gates and wires.

The elaborate() function performs the elaboration.

One can see a human-readable version of the final, elaborated and optimized netlist by using the -N <path> flag to the compiler. If elaboration succeeds, the final netlist (i.e., after optimizations but before code generation) will be dumped into the file named <path>.

The elaboration is performed in two steps:

Scopes and parameters elaboration
Structural and behavioural elaboration.

3.3.1 Scope Elaboration

The scope elaboration traverses the pform looking for scopes and parameters. A NetScope object tree is built and placed in the Design object, with the root module represented by the root NetScope object. The elab_scope.cc file contains most of the code for handling this step.

The tail of the elaborate_scope behaviour (after the pform is traversed) includes a scan of the NetScope object tree to locate the defparam assignments that were collected during scope elaboration. This is when the defparam overrides are applied to the parameters.

3.3.2 Netlist Elaboration

After the scopes and parameters are generated and the NetScope project tree is fully formed, the elaboration traverses the pform again to generate the structural and behavioural netlist. The parameters are also elaborated and evaluated, so all the constants of code generation are now known locally and the netlist can be generated by simply passing through the pform.

3.4 Optimization

The optimization is a collection of processing steps that perform optimizations that do not depend on the target technology. Examples of some useful transformations are as follows:

Eliminate null effect circuitry
Combinational reduction
Constant propagation.

The actual functions performed are specified on the ivl command line by the -F flags.

3.5 Code Generation

The code generation takes the design netlist and uses it to drive the code generator (see target.h). This may require transforming the design to suit the technology.

The emit() method of the Design class performs this step. It runs through the design elements, calling target functions as the need arises to generate actual output.

The user selects the target code generator with the -t flag on the command line.

4 Unsupported Constructs

Specify blocks are parsed but ignored in general.
Trireg is not supported. tri0 and tri1 are supported.
Tran primitives, i.e. tran, tranif1, tranif0, rtran, rtranif1, and rtranif0 are not supported.
Event controls inside non-blocking assignments are not supported, for example:
```
a <= @(posedge clk) b;
```

5 Commonly Used Flags/Options

-o <output file>: Output the compilation result file.
-f <command file>: Read in files with options.
-D <macro=value>: Assign a value to the macro.
-s <root module>: Set the root module to elaborate.
T min|typ|max: Select the timings (minimum, typical, and maximum) to use for simulation. The default set is typical.