Verilog Procedural Interface

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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).

While PLI 1 was deprecated 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++[edit]

It is widely recognized[citation needed] 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 set up properly, this is code that only needs to be set up once and is often made part of a company-wide proprietary Verilog/C++ interface that is reusable throughout a company's verification environments.


As an example, consider the following Verilog code fragment:

val = 41;
$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
  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};

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

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[edit]


Sources for Verilog VPI interface[edit]

External links[edit]