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 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++
C++ is integrable with VPI (PLI 2.0) and PLI 1.0, by using the "extern C/C++" keyword built into C++ compilers.
Example
As an example, consider the following Verilog code fragment:
<syntaxhighlight lang="text"> val = 41; $increment(val); $display("After $increment, val=%d", val); </syntaxhighlight>
Suppose the increment system task increments its first parameter by one. Using C and the VPI mechanism, the increment task can be implemented as follows:
<syntaxhighlight lang="c"> // Implements the increment system task static int increment(char *userdata) {
vpiHandle systfref, args_iter, argh; struct t_vpi_value argval; int value;
// Obtain a handle to the argument list systfref = vpi_handle(vpiSysTfCall, NULL); args_iter = vpi_iterate(vpiArgument, systfref);
// Grab the value of the first argument
argh = vpi_scan(args_iter);
argval.format = vpiIntVal;
vpi_get_value(argh, &argval);
value = argval.value.integer;
vpi_printf("VPI routine received %d\n", value);
// Increment the value and put it back as first argument argval.value.integer = value + 1; vpi_put_value(argh, &argval, NULL, vpiNoDelay);
// Cleanup and return vpi_free_object(args_iter); return 0;
} </syntaxhighlight>
Also, a function that registers this system task is necessary. This function is invoked prior to elaboration or resolution of references when it is placed in the externally visible vlog_startup_routines[] array.
<syntaxhighlight lang="c"> // Registers the increment system task void register_increment() {
s_vpi_systf_data data = {vpiSysTask, 0, "$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
}; </syntaxhighlight>
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
Sources
Sources for Verilog VPI interface
- Teal, for C++
- JOVE, for Java
- Ruby-VPI, for Ruby
- ScriptEDA, for Perl, Python, Tcl
- Cocotb [1], for Python
- OrigenSim, for Ruby