Programming Language interface part 1

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Programming Language interface part 1

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[ Team LiB ] 13.1 Uses of PLI PLI provides a powerful capability to extend the Verilog language by allowing users to define their own utilities to access the internal design representation. PLI has various applications

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  1. [ Team LiB ] 13.1 Uses of PLI PLI provides a powerful capability to extend the Verilog language by allowing users to define their own utilities to access the internal design representation. PLI has various applications. • PLI can be used to define additional system tasks and functions. Typical examples are monitoring tasks, stimulus tasks, debugging tasks, and complex operations that cannot be implemented with standard Verilog constructs. • Application software like translators and delay calculators can be written with PLI. • PLI can be used to extract design information such as hierarchy, connectivity, fanout, and number of logic elements of a certain type. • PLI can be used to write special-purpose or customized output display routines. Waveform viewers can use this file to generate waveforms, logic connectivity, source level browsers, and hierarchy information. • Routines that provide stimulus to the simulation can be written with PLI. The stimulus could be automatically generated or translated from some other form of stimulus. • General Verilog-based application software can be written with PLI routines. This software will work with all Verilog simulators because of the uniform access provided by the PLI interface. [ Team LiB ] [ Team LiB ] 13.2 Linking and Invocation of PLI Tasks Designers can write their own user-defined system tasks by using PLI library routines. However, the Verilog simulator must know about the existence of the user-defined system task and its corresponding user-defined C function. This is done by linking the user-defined system task into the Verilog simulator. To understand the process, let us consider the example of a simple system task $hello_verilog. When invoked, the task simply prints out a message "Hello Verilog World". First, the C routine that implements the task must be defined with PLI library routines. The C routine hello_verilog in the file hello_verilog.c is shown below. #include "veriuser.h" /*include the file provided in release dir */ int hello_verilog()
  2. { io_printf("Hello Verilog World\n"); } The hello_verilog routine is fairly straightforward. The io_printf is a PLI library routine that works exactly like printf. The following sections show the steps involved in defining and using the new $hello_verilog system task. 13.2.1 Linking PLI Tasks Whenever the task $hello_verilog is invoked in the Verilog code, the C routine hello_verilog must be executed. The simulator needs to be aware that a new system task called $hello_verilog exists and is linked to the C routine hello_verilog. This process is called linking the PLI routines into the Verilog simulator. Different simulators provide different mechanisms to link PLI routines. Also, though the exact mechanics of the linking process might be different for simulators, the fundamentals of the linking process remain the same. For details, refer to the latest reference manuals available with your simulator. At the end of the linking step, a special binary executable containing the new $hello_verilog system task is created. For example, instead of the usual simulator binary executable, a new binary executable hverilog is produced. To simulate, run hverilog instead of your usual simulator executable file. 13.2.2 Invoking PLI Tasks Once the user-defined task has been linked into the Verilog simulator, it can be invoked like any Verilog system task by the keyword $hello_verilog. A Verilog module hello_top, which calls the task $hello_verilog, is defined in file hello.v as shown below. module hello_top; initial $hello_verilog; //Invoke the user-defined task $hello_verilog endmodule Output of the simulation is as follows: Hello Verilog World
  3. 13.2.3 General Flow of PLI Task Addition and Invocation We discussed a simple example to illustrate how a user-defined system task is named, implemented in terms of a user-defined C routine, linked into the simulator, and invoked in the Verilog code. More complex PLI tasks discussed in the following sections will follow the same process. Figure 13-2 summarizes the general process of adding and invoking a user-defined system task. Figure 13-2. General Flow of PLI Task Addition and Invocation [ Team LiB ] [ Team LiB ] 13.3 Internal Data Representation
  4. Before we understand how to use PLI library routines, it is first necessary to describe how a design is viewed internally in the simulator. Each module is viewed as a collection of object types. Object types are elements defined in Verilog, such as: • Module instances, module ports, module pin-to-pin paths, and intermodule paths • Top-level modules • Primitive instances, primitive terminals • Nets, registers, parameters, specparams • Integer, time, and real variables • Timing checks • Named events Each object type has a corresponding set that identifies all objects of that type in the module. Sets of all object types are interconnected. A conceptual internal representation of a module is shown in Figure 13-3. Figure 13-3. Conceptual Internal Representation a Module Each set contains all elements of that object type in the module. All sets are interconnected. The connections between the sets are bidirectional. The entire internal representation can be traversed by using PLI library routines to obtain information about the module. PLI library routines are discussed later in the chapter. To illustrate the internal data representation, consider the example of a simple 2-to-1 multiplexer whose gate level circuit is shown in Figure 13-4. Figure 13-4. 2-to-1 Multiplexer
  5. The Verilog description of the circuit is shown in Example 13-1. Example 13-1 Verilog Description of 2-to-1 Multiplexer module mux2_to_1(out, i0, i1, s); output out; //output port input i0, i1; //input ports input s; wire sbar, y1, y2; //internal nets //Gate Instantiations not n1(sbar, s); and a1(y1, i0, sbar); and a2(y2, i1, s); or o1(out, y1, y2); endmodule The internal data representation for the 2-to-1 multiplexer is shown in Figure 13-5. Sets are shown for primitive instances, primitive instance terminals, module ports, and nets. Other object types are not present in this module. Figure 13-5. Internal Data Representation of 2-to-1 Multiplexer
  6. The example shown above does not contain register, integers, module instances, and other object types. If they are present in a module definition, they are also represented in terms of sets. This description is a conceptual view of the internal structures. The exact implementation of data structures is simulator dependent. [ Team LiB ]
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