Verilog Procedural Interface

The Verilog Procedural Interface (VPI), originally known as PLI 2.0, is an interface primarily intended for the C programming language. It allows behavioral Verilog code to invoke C functions, and C functions to invoke standard Verilog system tasks. The Verilog Procedural Interface is part of the IEEE 1364 Programming Language Interface standard; the most recent edition of the standard is from 2005. VPI is sometimes also referred to as PLI 2, since it replaces the deprecated Program Language Interface (PLI).

It should be noted that while PLI 1 was depreciated in favor of VPI (aka. PLI 2). PLI 1 is still commonly used over VPI due to its much more widely documented tf_put, tf_get function interface that is described in many verilog reference books.

use of C++

It is widely recognized that C++ is easily integrable with VPI (PLI 2.0) and PLI 1.0, by using the "extern C/C++" keyword built into C++ compilers. While this code tends to be difficult to setup properly, this is code that only need to be setup once and is often made part of a company wide proprietary Verilog/C++ interface that is reusable thought out a company's verification environments.

Example

As an example, consider the following Verilog code fragment:

val = 41;
$increment(val);
$display("After $increment, val=%d", val);

Suppose the increment system task increments its first parameter by one. Using C and the VPI mechanism, the increment task can be implemented as follows:

// Implements the increment system task
static int increment(char *userdata) {
  vpiHandle systfref, args_iter, argh;
  struct t_vpi_value argval;
  int value;

  // Obtain a handle to the argument list
  systfref = vpi_handle(vpiSysTfCall, NULL);
  args_iter = vpi_iterate(vpiArgument, systfref);

  // Grab the value of the first argument
  argh = vpi_scan(args_iter);
  argval.format = vpiIntVal;
  vpi_get_value(argh, &argval);
  value = argval.value.integer;
  vpi_printf("VPI routine received %d\n", value);

  // Increment the value and put it back as first argument
  argval.value.integer = value + 1;
  vpi_put_value(argh, &argval, NULL, vpiNoDelay);

  // Cleanup and return
  vpi_free_object(args_iter);
  return 0;
}

Also, a function that registers this system task is necessary. This function is invoked prior to elaboration or resolution of references when it is placed in the externally visible vlog_startup_routines[] array.

// Registers the increment system task
void register_increment() {
  s_vpi_systf_data data = {vpiSysTask, 0, "$increment", increment, 0, 0, 0};
  vpi_register_systf(&data);
}

// Contains a zero-terminated list of functions that have to be called at startup
void (*vlog_startup_routines[])() = {
  register_increment,
  0
};

The C code is compiled into a shared object that will be used by the Verilog simulator. A simulation of the earlier mentioned Verilog fragment will now result in the following output:

VPI routine received 41
After $increment, val=42

See also

• Verilog
• SystemVerilog DPI

Sources

• IEEE Xplore

Sources for Verilog VPI interface

• Teal, for C++
• JOVE, for Java
• Ruby-VPI, for Ruby
• ScriptEDA, for Perl, Python, Tcl

External links

• Verilog PLI primer
• Verilog VPI tutorial