You might get intimidated when you hear OS or kernel development, the basic functions of an OS (especially the kernel) are surprisingly simple. Even Linux, which is often cited as a huge open-source software, was only 8,413 lines in version 0.01. Today’s Linux kernel is overwhelmingly large, but it started with a tiny codebase, just like your hobby project.

We’ll implement basic context switching, paging, user mode, a command-line shell, a disk device driver, and file read/write operations in C. Sounds like a lot, however, it’s only 1,000 lines of code!

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In this book, I chose RISC-V as the target CPU because:

• The specification is simple and suitable for beginners.
• It’s a trending ISA (Instruction Set Architecture) in recent years, along with x86 and Arm.
• The design decisions are well-documented throughout the spec and they are fun to read.

We will write an OS for 32-bit RISC-V. Of course you can write for 64-bit RISC-V with only a few changes. However, the wider bit width makes it slightly more complex, and the longer addresses can be tedious to read.