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).
Contents
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++
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:
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:
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.
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 41After $increment, val=42