Verilog Procedural Interface

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Software interface between C and Verilog

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.

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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:

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 PLI_INT32 increment(PLI_BYTE8 *userdata) { vpiHandle systfref, args_iter, argh; s_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(void) { s_vpi_systf_data data = { /* type */ vpiSysTask, /* sysfunctype */ vpiSysTask, /* tfname */ "$increment", /* calltf */ increment, /* compiletf */ NULL, /* sizetf */ NULL, /* user_data */ NULL }; vpi_register_systf(&data); } // Contains a zero-terminated list of functions that have to be called at startup void (*vlog_startup_routines[])(void) = { register_increment, NULL };

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

Sources

IEEE Xplore

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

Use of C++

Example

See also

Sources

Sources for Verilog VPI interface

External links