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).
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 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.
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