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在电子设计自动化（EDA）领域，Verilog 和 SystemVerilog 是两种广泛使用的硬件描述语言（HDL）。在软件与硬件交互的过程中，编程接口扮演着关键角色。本文将探讨Verilog的编程语言接口（PLI）与SystemVerilog的直接编程接口（DPI）之间的差异，以及它们各自的优势和适用场景。 1.0 引言随着SystemVerilog的出现，DPI被引入以替代或补充Verilog的PLI，这引发了一个问题：是否应当继续使用Verilog PLI，还是转向SystemVerilog DPI？本论文旨在分析这两种接口的特点，并帮助工程师做出明智的选择。 2.0 DPI概述 DPI是SystemVerilog引入的一种机制，旨在简化系统级验证中的C/C++代码与硬件模型间的交互。它允许直接调用C函数，并支持数据类型转换，使得混合语言编程更为便捷。 3.0 Verilog PLI能力 3.1 C函数调用 Verilog PLI通过预定义的库函数来调用C函数，如`task`和`function`，并且需要特殊的`begin_call`和`end_call`语句来启动和结束调用过程。 3.2 PLI的发展随着时间的推移，Verilog PLI经历了多次升级，包括添加了用户级任务（UVMT）、并发调用等特性，以增强其功能和灵活性。 3.3 Verilog PLI标准 Verilog PLI标准在不断演进，从最初的IEEE 1364-1995到后来的IEEE 1364-2001，再到IEEE 1800-2005（SystemVerilog），这些标准反映了对更强大接口的需求。 4.0 深入了解SystemVerilog DPI 4.1 DPI导入声明 SystemVerilog DPI通过`import`语句来声明可以访问的C函数，这使得接口定义更为清晰且易于管理。 4.2 函数形式参数 DPI支持多种数据类型之间的自动转换，包括固定长度和动态大小的数组，这在Verilog PLI中通常需要手动处理。 5.0 DPI的优势 DPI的主要优势在于其简洁性和易用性，它可以更自然地映射到C语言，提高了代码重用和可读性。DPI还提供了异步调用和线程支持，适合复杂的并发环境。 6.0 PLI的独特性尽管DPI有许多优点，但Verilog PLI仍有其独特之处，例如在某些低级别硬件操作中，PLI可能提供更直接的访问权限。此外，PLI在某些特定的工具和平台上的支持可能更为成熟。 7.0 结论选择Verilog PLI还是SystemVerilog DPI取决于项目需求、团队技能和工具支持。对于需要高效集成C代码，且不涉及复杂硬件操作的项目，DPI可能是更好的选择。然而，如果需要更底层的硬件控制或与现有PLI代码兼容，Verilog PLI可能仍然是必要的。虽然SystemVerilog DPI在许多方面简化了软件与硬件的交互，但Verilog PLI仍然在特定场景中占有重要地位。工程师应根据实际需求，权衡两者的利弊，以决定最合适的接口技术。

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SNUG San Jose 2004 1 The PLI Is Dead (maybe)—Long Live The DPI

The Verilog PLI Is Dead (maybe)

Long Live The SystemVerilog DPI!

Stuart Sutherland

Sutherland HDL, Inc.

stuart@sutherland-hdl.com

ABSTRACT

In old England, when one monarch died a successor immediately took the throne. Hence the

chant, “The king is dead—long live the king!”. The Verilog Programming Language Interface

(PLI) appears to be undergoing a similar succession, with the advent of the new SystemVerilog

Direct Programming Interface (DPI). Is the old Verilog PLI dead, and the SystemVerilog DPI the

new king? This paper addresses the question of whether engineers should continue to use the

Verilog PLI, or switch to the new SystemVerilog DPI. The paper will show that the DPI can

simplify interfacing to the C language, and has capabilities that are not possible with the PLI.

However, the Verilog PLI also has unique capabilities that cannot be done using the DPI.

Table of Contents

1.0 Introduction ............................................................................................................................2

2.0 An overview of the DPI .........................................................................................................2

3.0 Verilog PLI capabilities .........................................................................................................3

3.1 How the PLI calls C functions ..................................................................................... 3

3.2 The evolution of the Verilog PLI ................................................................................. 4

3.3 Verilog PLI standards — Where they’ve been and where they’re heading ................ 6

4.0 A more detailed look at the SystemVerilog DPI ...................................................................7

4.1 The DPI import declaration ......................................................................................... 8

4.2 Function formal arguments .......................................................................................... 8

4.3 Function return values ................................................................................................. 9

4.4 Data type restrictions ................................................................................................... 9

4.5 Pure, context and generic C functions ....................................................................... 10

4.6 Referencing PLI libraries from DPI functions ........................................................... 11

5.0 Exporting Verilog tasks and functions .................................................................................11

6.0 Using the DPI to interface to C++, SystemC, and other languages .....................................12

7.0 Does the SystemVerilog DPI replace the Verilog PLI? ......................................................13

8.0 Conclusions ..........................................................................................................................16

8.1 When to use the DPI .................................................................................................. 16

8.2 When to use the PLI ................................................................................................... 16

8.3 Can the PLI 1.0 standard be deprecated? ................................................................... 17

8.4 The correct chant ....................................................................................................... 17

9.0 References ............................................................................................................................17

10.0 Glossary of acronyms ..........................................................................................................17

11.0 About the author ..................................................................................................................18

SNUG San Jose 2004 2 The PLI Is Dead (maybe)—Long Live The DPI

1.0 Introduction

The Verilog Programming Language Interface (PLI) provides a mechanism for Verilog code to

call functions written in the C programming language. The venerable Verilog PLI is one of the

reasons the Verilog language has been so successful for hardware design. Using the PLI, third

party companies and end-users can extend the capabilities of commercial Verilog simulators.

Virtually every serious design project using Verilog has utilized the Verilog PLI.

Over its nearly 20 year history, the Verilog PLI has seen two major generations, often referred to

as “PLI 1.0” and “PLI 2.0” (officially called the PLI-VPI). Accellera has recently introduced the

“SystemVerilog” extensions to the IEEE Verilog standard, which includes a new way for Verilog

code to call C and C++ functions. This new interface is called the “Direct Programming

Interface” or “DPI”. Some engineers feel that the advent of the SystemVerilog DPI marks the

demise of the old Verilog PLI. That is, that the DPI replaces the complex Verilog PLI with a

simple and straight forward C language interface. Does SystemVerilog make the Verilog PLI

obsolete? Can we all chant “The Verilog PLI is dead—long live the SystemVerilog DPI!” ?

Before the celebration begins, however, there are some important questions to answer. Is the

SystemVerilog DPI really better than the Verilog PLI, or are there good reasons to continue to use

the PLI? Are there applications that cannot be done using the SystemVerilog DPI, that will

necessitate keeping the Verilog PLI alive?

2.0 An overview of the DPI

The SystemVerilog Direct Programming Interface (DPI) allows Verilog code to call the names of

C functions as if the function were a native Verilog task or function (a task or function defined in

the Verilog language). This is done by importing the C function name into the Verilog language,

using a simple import statement. The import statement defines that the function uses the DPI

interface, and contains a prototype of the function name and arguments. For example:

import "DPI" function real sin(real in); // sine function in C math library

This import statement defines the function name sin for use in Verilog code. The data type of the

function return is a

real value (double precision) and the function has one input, which is also a

real data type. Once this C function name has been imported into Verilog, it can be called the

same way as a native Verilog language function. For example:

always @(posedge clock) begin

slope <= sin(angle); // call the "sin" function from C

end

The imported C function must be compiled and linked into the Verilog simulator. This process

will vary with different simulators. With the Synopsys VCS simulator, the process is very

straightforward; the C source file name, or a pre-compiled object file name, is listed along with

Verilog source code files as part of the

vcs compile command.

With the SystemVerilog DPI, the Verilog code is unaware that it is calling C code, and the C

function is unaware that it is being called from Verilog.

SNUG San Jose 2004 3 The PLI Is Dead (maybe)—Long Live The DPI

DPI compared to PLI. The ability to import a C function and then directly call the function

using the DPI is much simpler than the Verilog PLI. With the PLI, users must define a user-

defined system task or user-defined system function. The user-defined system task/function name

must begin with a dollar sign ( $ ). For example, the sine function above might be represented in

Verilog with $sine. This function is then associated with a user-supplied C function known as a

calltf routine. The calltf routine can then invoke the

sin function from the C math library. The

calltf routine, and any functions it calls, must then be compiled and linked into the Verilog

simulator. Once these steps have been performed, the

$sine system function can then be called

from within Verilog in the same way as a regular function. When simulation executes the call to

$sine, it will invoke the C calltf function that can then call the C sin function.

always @(posedge clock) begin

slope <= $sine(angle); // call the sine PLI application

end

At first glance, it might seem that within the Verilog code, there is very little difference between

using the PLI function and an imported DPI function. Indeed, within the Verilog code, there is no

significant difference; in both cases, the Verilog code is calling a task or function. Where the

difference becomes apparent—and it is a significant difference—is in what it takes to define the

PLI user-defined system task or system function. Using the DPI, the Verilog code can directly call

the sin function from the C math library, directly pass inputs to the function, and directly receive

a return value from the C function. Using the PLI, several steps are required to create a user-

defined system task that indirectly calls and passes values to the sin function. This indirect

process is described in more detail in the following section.

3.0 Verilog PLI capabilities

In order to fully discuss the strengths and weaknesses of the SystemVerilog DPI, it is necessary to

understand the capabilities of the Verilog PLI, and the C libraries that are part of the PLI standard.

The Verilog PLI is a simulation interface. It provides run-time access to the simulation data

structure, and allows user-supplied C functions a way to access information within the simulation

data structure. This user-supplied C function can both read and modify certain aspects of the data

structure. The Verilog PLI does not work with Verilog source code. It only works with a

simulation data structure. The PLI is not designed for use with tools other than simulation, such as

synthesis compilers.

3.1 How the PLI calls C functions

The Verilog PLI provides a set of C language functions that allow programmers to access the data

structure of a Verilog simulation. These C functions are bundled into three PLI libraries, referred

to as the TF library, the ACC library and the VPI library. The PLI access to the simulation data

structure is dynamic, in that it occurs while simulation is running. The access is also bidirectional.

Through the PLI, a programmer can both read information from the simulator’s data structure,

and modify information in the data structure.

The Verilog PLI allows programmers to extend the Verilog language though the creation of

system tasks and system functions. The names of these user-defined system tasks and functions

SNUG San Jose 2004 4 The PLI Is Dead (maybe)—Long Live The DPI

must begin with a dollar sign ( $ ). A task in Verilog is analogous to a subroutine. When a task is

called, the simulator’s instruction flow branches to a subroutine. Upon completion of the task, the

instruction flow returns back to the instruction following the task call. Verilog tasks do not return

values, but can have input, output and inout (bidirectional) formal arguments. A function in

Verilog is analogous to functions in most languages. When a function is called, it executes a set of

instructions, and then returns a value back to the statement that called the function. Verilog tasks

and functions can be defined as part of the Verilog language.

Using the Verilog PLI, a programmer must first define a system task function name, such as

$sine. The programmer then creates a C function referred to as a calltf routine, which will be

associated with the $sine task/function name. When simulation executes the statement with the

$sine system function call, the simulator calls the calltf routine associated with $sine. This calltf

routine is a layer between $sine and the sin function in the C math library. The arguments to

$sine are not passed directly to the calltf routine. Instead, the calltf routine must call special PLI

functions from a PLI library to read the input argument of $sine. The calltf routine can then call

the sin function, passing the input to the function. The calltf routine will receive the return value

from the sin function, and then call another special PLI function to write this return value back to

the $sine function. A final step when using the Verilog PLI is to bind the user-defined system

task/function name with the user-defined calltf routine. This step is different for each simulator,

and can range from editing a special table file to editing and compiling a complex C language file.

The PLI libraries allow a calltf routine to do more than just work with the arguments of a system

task or function. The libraries also allow a calltf application to search for objects in the simulation

data structure, modify delays and logic values in the data structure, and synchronize to simulation

activity and simulation time.

3.2 The evolution of the Verilog PLI

The Verilog PLI was originally developed in the mid to late 1980s as a proprietary interface to the

Gateway Design Automation Verilog-XL simulator product (now owned by Cadence Design

Systems). Gateway developed the PLI to satisfy two fundamental needs: First was to allow

Verilog-XL users a mechanism to use the C language for tasks such as file I/O. Second was to

provide a mechanism for ASIC foundries to analyze the usage of their standard cell libraries in

order to calculate accurate propagation delays within each cell instance. From these two

fundamental intents, the usage of the Verilog PLI has expanded in many ways. Common

applications of the Verilog PLI in today’s engineering environments include: commercial and

proprietary waveform viewers, commercial and proprietary design debug utilities, RAM/ROM

program loaders, scan chain vector loaders, custom file readers and writers, power analysis,

circuit tracing, C model interfaces, co-simulation environments, parallel process distribution, and

much, much more. The ways in which the PLI can be used to extend Verilog simulators is limited

only by the imagination of programmers.

The following paragraphs introduce the several generations of the Verilog PLI.

1985 — the TF interface. The first generation of the Verilog PLI is referred to as the Task/

Function interface, or TF interface. The TF interface comprises a set of C language functions

defined in a library file called verisuer.h. These C functions are typically referred to as the TF

routines. The primary purpose of the TF routines is to allow task/function arguments to be passed

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