This is an excerpt from the Chapter 1 of the book Principles of Verilog PLI by Swapnajit Mittra published from Kluwer Academic Publishers. ISBN: 0-7923-8477-6 [All copyright reserved. No portion of this text can be used for for commercial purpose without explicit permission from the author nor can it be used for re-print in hardcopy or electronic form.] Order the book here.


In this chapter we are going to take a quick tour to get ourselves acquainted with Programming Language Interface (PLI) of Verilog HDL.

We will go through the essentials of PLI without getting bogged down by too much of details. Our objective of this chapter would be to acquire a workable knowledge of PLI so that we can start writing basic programs. To achieve this, we will take an example of the very basic PLI activity - reading the value of a register from the design database. Once we have this we further modify it to do some other basic things using PLI. At the end, we will review what we have covered in this chapter and how it forms the basis for moving to the details of PLI.


Programming Language Interface (PLI) is a way to provide Application Program Interface (API) to Verilog HDL. Essentially it is a mechanism to invoke a C function from a Verilog code. The construct which invokes a PLI routine in a Verilog code is usually called a system call. The common system calls built-in to most Verilog simulators are $display, $monitor, $finish etc.


As the above examples of built-in PLI routines indicate, PLI is primarily used for doing such things which would not have been possible otherwise using Verilog syntax. For example, while Verilog has a way of doing file write ($fwrite, which is another built-in system call), it does not have any such construct for reading a register value directly from a file. There are other examples toowhere PLI is the only way of achieving the desired results. Some of them are:

  • Writing functional models
  • Delay calculations
  • Getting design information - for example, there is no Verilog construct which gives the name of the parent of the current module in the design hierarchy.



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