Bitwise is an educational project where we create the software/hardware stack for a computer from scratch.

Announcing Bitwise

Feynman’s blackboard quote “What I cannot create, I do not understand” never fails to give me goosebumps.

I’ve always been obsessed with how things work under the hood–physical contraptions as well as abstract concepts. Most engineers and programmers I’ve met are motivated by building things, and their knowledge and skills are first and foremost a tool for achieving that. But I’m motivated by learning how things work, and the process of solving problems and designing and implementing systems has been a tool for increasing my understanding. And of course there’s the unrivaled thrill of seeing what you’ve built actually work and come alive!

After working as a game developer and systems programmer for over 15 years at places like Epic Games, NVIDIA, RAD Game Tools and most recently Oculus, I decided it was time to take a break from professional programming and spend a few years pursuing a long-time dream of mine, a project I’ve dubbed Bitwise, where I want to share my passion and try to demonstrate by example how to build systems from scratch, with a low-level computing focus.

Here are some examples of what you will learn to build:

• Hardware description language compilers and simulators
• FPGA-based hardware, including CPU, GPU, HDMI controller, Ethernet MAC, DDR3 PHY, etc
• Kernels, including drivers, hardware abstraction layer, scheduler, virtual memory manager, file systems, TCP/IP stack, etc
• Systems libraries, including GUIs, memory allocators, etc.
• Systems applications, including compilers, assemblers, disassemblers, profilers, debuggers, etc
• Test infrastructure, including property-based testing, directed randomized testing, fuzz testing, etc.

My goal with Bitwise is to show that these things can be done much more simply and quickly than people realize if we strongly favor simplicity over marginal gains in feature completeness or performance. The goal is not to outdo or compete with any existing product; the goal is to show how things work with real hardware and software.

Bitwise will consist of educational videos, articles, and software/hardware source code. The code will be released into the public domain for everyone to study and extend as they see fit. Everything produced as part of the project is free of charge. I’m doing this project as a service to the community and neither ask nor accept financial donations.

Despite the hardware focus, I want to make it clear almost nothing we do will require any special hardware or FPGA development boards for you to participate. All our development will be done with free tools that run on normal Windows, Linux or Mac computers. I will periodically demonstrate code running on real FPGA boards, but it will mostly be for debugging purposes. In fact, we want to minimize the time we spend dealing with real hardware since it’s usually painful and time intensive compared to running in simulation.

Structure

NOTE: This is highly subject to change based on feedback from the community.

My plan is to treat Bitwise as a full-time commitment for several years. Avoiding burnout is a major priority.

I will be streaming daily or semidaily on Twitch, one or two hours per session. Videos will be posted later on YouTube.

When I’m not streaming, I will be working full-time on the project: programming, writing and other related activities.

I will be writing weekly or semiweekly blog posts summarizing the progress since the last update.

Code streams will begin with a code review and walk-through of the diffs that were committed since last stream. Thus, even though a lot of the code will necessarily be written off stream, you won’t miss out on a single line of code. After each review, I will push the latest changes to the GitHub repository, where everyone can download them.

To keep things fun for myself and viewers, the streams will frequently alternate between different development tracks. In a given week, streams might alternate between the primary software and hardware development tasks, but you can expect streams on random side topics to be commonplace. If you’re not interested in some topics, there will likely be others to your liking.

Most streams will be focused on live coding, while others will be in a more conventional presentation format.

Aside from the progress summary blog posts, I will try to distill what I cover on stream into standalone articles. Writing articles is a lot of work, but my hope is that by using my overviews from the stream as a template, I will be able to write them more quickly than if I had to start with a blank page. The audience for articles is much larger, so this will extend the project’s reach, and it helps prospective or lapsed viewers to catch up to the streams; it’s not practical for most people to catch up by watching archived streams.

Prerequisites

The intended audience are fluent C programmers. Python is used as a secondary language, but knowing Python in advance is not a prerequisite. The pace will be quick; while I will give overviews of many different topics, not everything can be covered in depth, so pointers to additional reading material will be provided whenever possible.

Schedule

The expected stream schedule is Monday through Friday, 6 PM PDT/9 PM EDT/2 AM CET.

The kick-off stream is planned for March 12. I expect it will mainly be an overview and Q&A.

The first major project will be the C-like systems language compiler. I expect the initial work on this to be a few weeks, after which we will start working on the hardware track as well.

I’m located in Thailand, so the schedule is my attempt at juggling time zone differences. I may balance it out by moving some streams to Saturdays or Sundays during morning or noon hours for Europeans. I will be spending a month in Europe each year in August, which will force a rotation of the schedule.

Roadmap

NOTE: This is subject to change, but it gives you an idea of what I have planned. The outline corresponds to an estimated six months of work, depending on my productivity and how much we decide to focus on the critical path versus side topics.

We will be building not only software but hardware.

On the hardware side of things, we will be designing a computer from scratch that can be synthesized and deployed on a real FPGA. This will include a RISC-V CPU, GPU, IO interface controllers for DDR3 memory, HDMI video/audio, Ethernet networking, and more.

But before getting there, we will need to learn about digital logic and how to design hardware with an HDL. In fact, we will designing our own HDL and the associated toolchain, including software-based simulators and debugging tools. Along the way we’ll be doing lots of fun mini-projects, like designing a version of Pong entirely using logic gates, and implementing a variant of the toy parallel computer used in the Zachtronics game TIS-100.

In tandem, we will also be building the software stack. This includes both the host-side tooling as well as the software that runs on the Bitwise computer. Eventually most of the host tooling will be portable to the Bitwise computer itself! This toolchain will start with a simple but powerful C-like systems programming language that will initially be bootstrapped to run on the host and which we will use to construct all the other software. As we first bring up the CPU, we will be writing the emulator, assembler, disassembler and debugger in this language.

From there we’ll be writing test programs in the assembly language to validate the CPU. Once the CPU is in a usable state, we will code a backend for our systems language compiler that targets our CPU and so start running simple compiled programs on it. At that point we will build a simple microcontroller-class task-switching operating system and some simple applications on top of it.

At first, the CPU will be a simple RV32I microcontroller-class core with limited features and performance, and the only IO peripheral will be a UART for communicating with the host. Over time we will make this more capable. We will extend the CPU to eventually support RISC-V’s RV32G profile, which includes support for integer multiply/divide and floating point instructions. To improve performance we will add instruction pipelining and branch prediction, and caches once we have DRAM support. We will also be building out our peripherals to support audio/video output and networking, and the software to support this will be constructed in parallel as the hardware comes online.

Eventually we will end up with a simple desktop-class operating system with a graphical user interface that can run games and other applications we’ve written. We might eventually try to bring up Linux on the computer once we’ve implemented MMU support, but that is not a primary goal.

And this is only the beginning!

Links

Please follow on Twitch, YouTube and Twitter to see when streams go live and when new videos are uploaded.

There is a Discord chat server if you have questions or just want to hang out and chat.

• GitHub: https://github.com/pervognsen/bitwise
• Forums: https://bitwise.handmade.network/forums
• Discord chat server: https://discord.gg/7TSA6ZF
• Twitch channel: https://twitch.tv/pervognsen
• YouTube channel: https://youtube.com/pervognsen
• Twitter: https://twitter.com/pervognsen