RTL (Register Transfer Level) in Modern C++: A Concise Guide

Register Transfer Level (RTL) is a fundamental abstraction used in hardware design and digital circuit modeling. Traditionally, RTL has been associated with hardware description languages (HDLs) like Verilog and VHDL. However, as C++ has evolved with more powerful abstractions and features in modern versions, particularly with the advent of tools like SystemC and High-Level Synthesis (HLS), RTL design using C++ has gained momentum. This article will explore how C++ has made inroads into RTL design and discuss the benefits, challenges, and practical applications of using C++ for hardware-level programming.

What is RTL?

At its core, RTL refers to a design abstraction where operations are described in terms of registers and the transfer of data between them. It specifies the behavior of a digital system at the register level and is a critical part of digital circuit design before going into physical implementation. RTL is commonly used in synthesizing hardware for Field Programmable Gate Arrays (FPGAs) or Application-Specific Integrated Circuits (ASICs).

The Role of C++ in RTL Design

Traditionally, HDLs like Verilog and VHDL have dominated RTL design. However, modern C++ brings a higher level of abstraction to this process, allowing developers to write RTL-level descriptions more intuitively and flexibly. Key developments, such as the introduction of HLS tools and the emergence of libraries like SystemC, have made it possible to simulate RTL designs directly in C++ and automatically generate synthesizable hardware designs from C++ code.

Key Technologies Driving RTL in C++

1. High-Level Synthesis (HLS) Tools: High-Level Synthesis is one of the most significant technological advancements that has made C++ a viable option for RTL design. HLS tools, such as Xilinx’s Vivado HLS and Intel’s HLS Compiler, allow C++ code to be compiled into synthesizable RTL code. This brings the advantage of writing hardware models at a higher level, using modern C++ constructs while leaving the generation of RTL code to the synthesis tools.

   Example: Consider a matrix multiplication algorithm written in C++. Using an HLS tool, this high-level description can be compiled into optimized RTL code, enabling its implementation on an FPGA.

   void matrix_mult(int A[4][4], int B[4][4], int C[4][4]) {
       for (int i = 0; i < 4; i++) {
           for (int j = 0; j < 4; j++) {
               C[i][j] = 0;
               for (int k = 0; k < 4; k++) {
                   C[i][j] += A[i][k] * B[k][j];
               }
           }
       }
   }
   

   The code above is typical C++ but can be synthesized into RTL using an HLS tool to implement it on FPGA hardware.

2. SystemC: SystemC is a C++ library used for hardware modeling and simulation. It allows designers to describe hardware behavior using C++ syntax, providing a higher level of abstraction than traditional HDLs. SystemC enables modeling of parallelism, signals, and clock cycles in a way that mimics real-world hardware designs.

   Example SystemC Code:

   #include <systemc.h>
   
   SC_MODULE(counter) {
       sc_in<bool> clk;
       sc_out<int> count;
   
       void increment() {
           count.write(count.read() + 1);
       }
   
       SC_CTOR(counter) {
           SC_METHOD(increment);
           sensitive << clk.pos();
       }
   };
   

   This simple counter module is written using SystemC primitives and can be used for RTL-level simulation of a counter circuit.

3. C++ Templates and Metaprogramming: One of the strengths of Modern C++ is its support for templates and metaprogramming, which can be leveraged to create highly reusable and parameterizable hardware modules. Templates enable the creation of generic hardware components that can be reused across different projects, enhancing code efficiency and modularity.

   Example of a FIFO Buffer Using Templates:

   template <typename T, int SIZE>
   class FIFO {
   public:
       FIFO() : head(0), tail(0) {}
       bool is_full() const { return (head + 1) % SIZE == tail; }
       bool is_empty() const { return head == tail; }
   
       void write(T value) {
           if (!is_full()) {
               buffer[head] = value;
               head = (head + 1) % SIZE;
           }
       }
   
       T read() {
           if (!is_empty()) {
               T value = buffer[tail];
               tail = (tail + 1) % SIZE;
               return value;
           }
           return T(); // Return default value if empty
       }
   
   private:
       T buffer[SIZE];
       int head, tail;
   };
   

   This C++ template can be used to describe a hardware FIFO buffer. Using HLS, this template code can be converted into synthesizable RTL code for implementation on an FPGA or ASIC.

Advantages of Using C++ for RTL Design

• Higher Abstraction: Modern C++ allows designers to describe hardware components at a higher level of abstraction than traditional HDLs, making the design process more intuitive and faster.

• Reuse of Code: C++ templates and libraries like SystemC enable the reuse of hardware components, improving productivity and reducing the need for writing new code for each design.

• Powerful Tools: The availability of advanced HLS tools allows designers to work in C++ while automatically generating RTL code, bridging the gap between software and hardware design.

Challenges

• Synthesis Limitations: Not all C++ features can be synthesized into hardware. For example, dynamic memory allocation and virtual functions are not supported in most hardware designs.

• Learning Curve: Hardware engineers unfamiliar with C++ need to learn modern C++ constructs and tools like SystemC and HLS to use them effectively in RTL design.

Modern C++ brings a wealth of powerful abstractions, tools, and libraries to the world of RTL design. With the aid of HLS tools and SystemC, designers can now describe RTL behavior at a higher level, leading to increased productivity and ease of development. While challenges remain, such as limitations in synthesizability and a steep learning curve for hardware engineers new to C++, the advantages of using C++ in RTL design make it a compelling choice for those looking to bridge the gap between software and hardware design.