Building a GPU from Scratch: Exploring C++ and FPGA

Building a GPU from Scratch: Exploring C++ and FPGA

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Table of Contents

1. Introduction
2. Understanding the GPU and VGA Signal
   • 2.1 The Cathode Ray TVs and Electron Beam
   • 2.2 The VGA Port and Analog Data
   • 2.3 Horizontal and Vertical Sync
3. Creating a GPU Using C++ and FPGA
   • 3.1 Introduction to RTA7 FPGA Development Board
   • 3.2 The VGA Adapter and Digital-to-Analog Converters
4. Setting Up the C++ Code for VGA Signal Generation
   • 4.1 Using Xilinx's High-Level Synthesis Tool
   • 4.2 Bit Banging GPIOs and Analogous Variables
   • 4.3 Looping and Pipelining for Clock Cycle Synchronization
   • 4.4 Writing Boolean and Logical Expressions
5. Generating the VGA Signal and Displaying Psychedelic Patterns
6. Drawing Textures Using RGB Data
   • 6.1 Understanding the Color Struct and RGB Components
   • 6.2 Fetching the Corresponding Pixel Color
7. Drawing Triangles with Barycentric Coordinates
   • 7.1 Defining Points and Triangles
   • 7.2 Implementing Stack Overflow Code for Triangle Evaluation
   • 7.3 Updating the Main Loop for Triangle Rendering
8. Conclusion

Introduction

Welcome, readers, to this article that delves into the fascinating world of creating a GPU using C++ and an FPGA. In this guide, we will explore the concepts behind the VGA signal, understand the functionalities of a GPU, and learn how to generate a VGA signal using an FPGA development board and a VGA adapter. Whether you are a technology enthusiast or a curious programmer, this article will provide you with valuable insights into the process of building a GPU from scratch.

Understanding the GPU and VGA Signal

2.1 The Cathode Ray TVs and Electron Beam

Before we dive into the details of creating a GPU, let's take a moment to understand the basics of the VGA signal. Think back to the old cathode ray TVs that had a physical electron gun at the back. This electron beam would scan from left to right across the display, reading analog data from the red, green, and blue pins on the VGA port. This scanning process, combined with the coordination of the horizontal and vertical sync, created the images on the screen.

2.2 The VGA Port and Analog Data

The VGA port, also known as the Video Graphics Array port, is a standardized connector found on most computers and monitors. It consists of several pins, including those dedicated to the red, green, and blue analog signals. These analog signals carry information about the intensity and color of each pixel on the screen, allowing the GPU to create vibrant and detailed images.

2.3 Horizontal and Vertical Sync

The VGA signal relies on two crucial synchronization signals: horizontal sync and vertical sync. The horizontal sync signal, which operates at a high voltage, drops to zero and toggles back to high voltage when the electron beam reaches the end of each line. This toggle indicates that the beam should move back to the start and move down one row. The vertical sync signal toggles from high to low and back again to indicate that we are going back to the start of the display.

Creating a GPU Using C++ and FPGA

3.1 Introduction to RTA7 FPGA Development Board

To create our GPU, we will be using the RTA7 FPGA development board. FPGA stands for Field-Programmable Gate Array, which is a type of integrated circuit that can be customized using a hardware description language. The powerful capabilities of an FPGA allow us to implement complex digital circuits that can perform the tasks of a GPU.

3.2 The VGA Adapter and Digital-to-Analog Converters

To interface the FPGA with the VGA display, we need a VGA adapter. The VGA adapter contains three digital-to-analog converters that take binary inputs and use a resistor ladder to convert them into an analog signal. This simple device acts as a bridge between the digital circuitry of the FPGA and the analog requirements of the VGA display.

Setting Up the C++ Code for VGA Signal Generation

4.1 Using Xilinx's High-Level Synthesis Tool

Now that we have a basic understanding of the hardware setup, it's time to dive into the software side of things. We will be using Xilinx's high-level synthesis (HLS) tool, which allows us to convert our C++ code into a digital circuit. The HLS tool analyzes each expression, line, and function of our code and translates them into a digital representation that can be implemented on the FPGA.

4.2 Bit Banging GPIOs and Analogous Variables

To interface with the FPGA, we will treat our variables as analogous to bit-banging GPIOs in a microcontroller. This means that we will be directly manipulating the variables to control the behavior of our GPU. The HLS tool will identify if the variables are being written to or read from and deduce whether they are inputs or outputs.

4.3 Looping and Pipelining for Clock Cycle Synchronization

In our C++ code, we will have loops that iterate over each row and column of the display. To ensure synchronization with the clock cycles, we will use a pragma directive to pipeline the innermost loop. This pipelining allows us to start a new iteration of the loop on every clock cycle, ensuring that the VGA signal is generated at the correct frequency.

4.4 Writing Boolean and Logical Expressions

To set the values of horizontal sync, vertical sync, and the red, green, and blue components of the VGA signal, we will use boolean and logical expressions. These expressions will determine whether the GPU is inside the display area and whether to set the colors based on mathematical equations or to set them to zero when outside the display area.

Generating the VGA Signal and Displaying Psychedelic Patterns

By combining all the components and steps outlined above, we can now generate the VGA signal and display our first patterns on the monitor. Once we have run our code through the HLS tool and converted it to Verilog using Xilinx's Vivado Package, we can hook up the clock to the right frequency and point the outputs to the correct connectors. As a result, we will witness a psychedelic pattern appearing on our monitor, created by our GPU.

Drawing Textures Using RGB Data

Now that we have successfully generated the VGA signal and displayed patterns, let's take things further by adding textures to our graphics. We can define a struct called "color" with red, green, and blue components. We will create a function called "get sky pixel" that will return a color with specified coordinates. This function will look up the corresponding pixel in massive arrays of RGB data and allow us to add textured elements to our graphics.

Drawing Triangles with Barycentric Coordinates

To enhance the capabilities of our GPU, we can extend our code to draw triangles on the screen. We can define a point with x and y coordinates and a triangle as three points. To determine if a given point is inside a triangle, we can implement a well-known algorithm that utilizes barycentric coordinates. By modifying the main loop and evaluating whether a point lies within a triangle or not, we can render triangles with different colors on our display.

Conclusion

In this article, we have explored the fascinating process of creating a GPU using C++ and an FPGA. Starting from understanding the VGA signal and the basic concepts behind GPU functionality, we proceeded to set up the necessary hardware and software components. By utilizing Xilinx's high-level synthesis tool and implementing the correct logic and expression in our C++ code, we were able to generate a VGA signal and display patterns, textures, and even triangles on our monitor. This journey showcases the power and versatility of FPGA-based GPU development and opens doors to endless possibilities in the field of graphics processing.

Highlights

• Dive into the world of GPU creation using C++ and an FPGA
• Understand the intricate details of the VGA signal and its synchronization
• Build a GPU using the RTA7 FPGA development board and a VGA adapter
• Set up the C++ code for VGA signal generation using Xilinx's HLS tool
• Explore techniques for rendering patterns, textures, and triangles on the screen
• Witness the power and versatility of FPGA-based GPU development

FAQ

Q: What is the purpose of the VGA adapter? A: The VGA adapter acts as a bridge between the FPGA's digital circuitry and the analog requirements of the VGA display. It contains digital-to-analog converters that convert binary signals into analog signals for the VGA display.

Q: Can I use a different FPGA development board for this project? A: While the examples in this article specifically use the RTA7 FPGA development board, you can adapt the concepts and code to work with other FPGA development boards. Just ensure that the board has the necessary capabilities to generate a VGA signal.

Q: Can I create more complex graphics using this GPU? A: Absolutely! Once you understand the basic principles and techniques outlined in this article, you can expand your GPU's capabilities to create even more complex graphics. The sky's the limit!

Resources

• Xilinx's High-Level Synthesis Tool: link
• RTA7 FPGA Development Board: link