I wanted to get my feet wet with regards to writing operating systems from the ground up. I came across this awesome tutorial by James Molloy. In it, he walks through the creation of an ultra minimal unix-like operating system.

— Setup —

Setting up my environment took quite a bit of time. The setup instructions are aimed at a Linux/macOS environment whereas I am running Windows. Luckily, Windows Subsystem for Linux (WSL) is a thing and I was able to continue along. However, there was a catch - WSL does not have a graphical user interface (GUI).¹ This meant I could not use Bochs (the emulator used in the setup instructions).²

I started down this journey a bit randomly. I was researching how the Game Boy uses memory bank switching when I came across this video¹ in which the author attempts to emulate the Game Boy. Since the Game Boy’s CPU is a hybrid of the Intel 8080 and Zilog Z80 processors, they start out by implementing the 8080’s instructions. Though my initial intent was to learn about memory bank switching, seeing this inspired me to set off and create an 8080 emulator.

I lucked out in my choice to emulate the 8080. It turns out that retro CPUs are orders of magnitude simpler to understand than modern ones and that the 8080 is one of the earliest. As a byproduct of this simplicity, their documentation are detailed while remaining reasonable in size (i.e. readable). (Contrast the less than 300 pages of the 8080 User’s Manual with the almost 5000 pages of Intel’s current 64 and IA-32 Developer’s Manual!)

I finished the first half of the course.

And here’s a video of what my implementation looks like in action.

It can run programs! Albeit very slowly. The demo above is vastly sped up. In realtime, it took about 37 minutes to run.

The code for the emulator is on Github.