Reconfigurable ComputingS. Reda, Brown University

Reconfigurable Computing(EN2911X, Fall07)

Lecture 13: SystemC (1/3)

Prof. Sherief RedaDivision of Engineering, Brown University

http://ic.engin.brown.edu

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Introduction to SystemC

• SystemC is not a language, but rather a class library within a well established language C++.

• The primary motivation for using SystemC is to attain the productivity increases required to design modern electronic systems with their ever increasing complexity.

[SystemC: From Ground Up]

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SystemC resources

• SystemC: from the ground up / by David C. Black and Jack Donovan

• SystemC: methodologies and applications / edited by Wolfgang Müller, Wolfgang Rosenstiel, and Jürgen Ruf

• System design with SystemC / by Thorsten Grotker, Stan Liao, Grant Martin, Stuart Swan

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C++ mini refresher• C++ is an object oriented language• A key concept in C++ is the class. A class is an expanded concept

of a data structure: instead of holding only data, it can hold both data and functions.

class CRectangle { private: int x, y; public: void set_values (int,int); int area (void); } rect;

rect.set_values (3,4);myarea = rect.area();

Examples from http://www.cplusplus.com/doc/tutorial/classes.html

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CRectangle example#include <iostream>

class CRectangle { private: int x, y; public: void set_values (int,int); int area () { return (x*y); } };

void CRectangle::set_values (int a, int b) { x = a; y = b; }

int main () { CRectangle rect; rect.set_values (3,4); cout << "area: " << rect.area(); return 0; }

declaration

declaration & definition

definition

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Constructors#include <iostream>

class CRectangle { int width, height; public: CRectangle (int,int); int area () { return (width*height); } };

CRectangle::CRectangle (int a, int b) { width = a; height = b; }

int main () { CRectangle rect (3,4); CRectangle rectb (5,6); return 0; }

A constructor is automatically called whenever a new object of this class is created. The constructor function must have the same name as the class, and cannot have any return type; not even void.

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(Destructors)class CRectangle { int *width, *height; public: CRectangle (int,int); ~CRectangle (); int area () { return (*width * *height); } }; CRectangle::CRectangle (int a, int b) { width = new int; height = new int; *width = a; *height = b; } CRectangle::~CRectangle () { delete width; delete height; }

destructor definition

allocating memory

freeing memory

destructor definition

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Pointer to classes

• It is perfectly valid to create pointers that point to classes. We simply have to consider that once declared, a class becomes a valid type, so we can use the class name as the type for the pointer. For example:

CRectangle *prect; prect = new CRectangle; prect->set_values(1, 2);

As with data structures, in order to refer directly to a member of an object pointed by a pointer we can use the arrow operator (->) of indirection.

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Inheritance between classes

polygon

rectangle triangle

• A key feature of C++ classes is inheritance. Inheritance allows to create classes which are derived from other classes, so that they automatically include some of its "parent's" members, plus its own.

• The class CPolygon contain members that are common for both types of polygon. In our case: width and height. And CRectangle and CTriangle would be its derived classes, with specific features that are different from one type of polygon to the other.

• Classes that are derived from others inherit all the accessible members of the base class.

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Inheritance exampleclass CPolygon { protected: int width, height; public: void set_values (int a, int b) { width=a; height=b; } };

class CRectangle: public CPolygon { public: int area () { return (width * height); } };

class CTriangle: public CPolygon { public: int area () { return (width * height / 2); } }; int main () { CRectangle rect; CTriangle trgl; rect.set_values (4,5); trgl.set_values (4,5);}

The protected access specifier is similar to private. The only difference occurs in fact with inheritance. When a class inherits from another one, the members of the derived class can access the protected members inherited from the base class, but not its private members.

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Class templates motivationclass mypairi { int a, b; public: mypair (int first, int second) { a=first; b=second; } int getmax () { int retval; retval = a>b? a : b; return retval;}; int main () { mypairi myobject (100, 75); cout << myobject.getmax(); return 0; }

class mypairf { float a, b; public: mypair (float first, float second) { a=first; b=second; } float getmax () { float retval; retval = a>b? a : b; return retval;}; int main () { mypairf myobject (100, 75); cout << myobject.getmax(); return 0; }

Can we have an automatic way to avoid writing multiple versions of the same class for different data types?

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Class templatestemplate <class T> class mypair { T a, b; public: mypair (T first, T second) { a=first; b=second; } T getmax () { T retval; retval = a>b? a : b; return retval;};

int main () { mypair <int> myobject (100, 75); cout << myobject.getmax(); return 0; }

C++ Class Templates are used where we have multiple copies of code for different data types with the same logic. If a set of functions or classes have the same functionality for different data types, they become good candidates for being written as Templates.

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What is wrong with plain C++?

• Concurrency: hardware systems are inherently parallel; SW are not.

• Time: C++ has no notion of time• Hardware style communication: signals, protocols• Reactivity: hardware is inherently reactive• Hardware data types: bit type, multi-valued logic• Some of the functionalities in C++ are not simply

applicable in hardware systems (e.g. destructors)

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Extending C++ with SystemC library

• SystemC library of C++ classes:– Processes (for concurrency)– Clocks (for time)– Hardware data types (bit vectors, 4-valued logic,

fixed-point types, arbitrary precision integers)– Waiting, watching, and sensitivity (for reactivity)– Modules, ports, signals (for hierarchy)

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Synthesizable SystemC

Our discussions will focus on SystemC synthesis as afforded by the Celoxica SystemC agility compiler

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SC_MODULE

SC_MODULE(module_name){... // port declarations to connect modules together... // variable declarations... // function declarations/definitions

SC_CTOR(module_name) { ... // body of constructor ... // process declarations, sensitivities }};

The arguments to SC_MODULE and SC_CTOR must be the same.

creates a class inherted from sc_module

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Module ports• Ports are used to communicate with the external modules or

channels. – Input ports (defined using sc_in and sc_in_clk)– Output ports (defined using sc_out and sc_out_clk)– Input/output ports (defined using sc_inout, sc_inout_clk and sc_inout_rv )

SC_MODULE (module_name) { //Module port declarations sc_in <port_data_type> port_name; sc_out <port_data_type> port_name; sc_inout <port_data_type> port_name; sc_in <port_data_type> port_name; …};

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Datatypes• Supported native C++ synthesizable data types

– long long– long – int– short– char– bool

• SystemC also allows further refined storage types– sc_bit– sc_bv <width>– sc_int <width>– sc_uint <width> – sc_bigint <width> – sc_biguint <width>

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Module process• Processes describe the parallel behavior of hardware systems.

Processes execute concurrently. The code within a process, however, executes sequentially.

• Defining a process is similar to defining a C++ function. A process is declared as a member function of a module class and registered as a process in the module’s constructor.

SC_MODULE (my_module) { sc_in <bool> clk; sc_in <bool> in1; sc_out <bool> out1; void my_method();

SC_CTOR(my_module) { SC_METHOD(my_method); sensitive << in1 << clk.pos(); }}

module

process

in1

clk

out

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Definition of process body

A thread process body within a module definition

SC_MODULE (my_module) {void my_method();...};

A thread process body outside a module definition

The process body contains the implementation of the process. Like C++ functions, it may be defined:

• within the module definition, typically in a .h header file• outside the module, typically in a .cpp file

SC_MODULE (my_module) {void my_method() { ...}...};

my_module.h

my_module.h

void my_module::my_thread(){...}

my_module.cpp

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SC_CTOR construct

• The SC_CTOR constructor is used to:– Initialize variables declared in the module.– Specify the functionality of the module in terms of SC_METHOD, SC_THREAD and SC_CTHREAD.

– The threads and methods must be defined using synthesizable code.

• The constructor should also contain:– The sensitivity lists describing the inputs that each process is

sensitive to.– Instantiation of sub-modules.– Port mapping code for hierarchical modules.

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OR gate example#include "systemc.h"

SC_MODULE(or_gate) { sc_in<sc_bit> a; sc_in<sc_bit> b; sc_out<sc_bit> c;

void prc_or_gate() { c=a | b; }

SC_CTOR(or_gate) { SC_METHOD(prc_or_gate); sensitive << a << b; }};

OR_GATEa

b

c

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Full adder exampleSC_MODULE( half_adder ){ sc_in<bool> a,b; sc_out<bool> sum, carry;

void half_adder_defn(); SC_CTOR( half_adder ) { SC_METHOD ( half_adder_defn ); sensitive << a << b; }};

void half_adder::half_adder_defn() { sum = a ^ b; carry = a & b;}

FAab

sumcarry