Bare metal programming with RISC-V guide (2023)

Summary

Today we’re going to explore how to write a bare metal program for a RISC-V machine. For reproducibility, the target is a QEMU riscv64 virt machine. We will briefly cover the initial stages of the RISC-V machine bootup and where you can plug in your custom software to program the bare metal machine! At the end of this article, we will write a bare metal program for our RISC-V machine and send a string ‘hello’ to the user, without depending on any supporting software on the running machine whatsoever (OS kernel, libraries, anything). Table of contents Open Table of contents Machine bootup and running the initial software General concepts Feel free to skip the section on general concepts if you are familiar with how computers boot When a real machine is powered on, the hardware first runs the health checks and then loads the first instructions to run into its memory. Once the instructions are loaded, the processor core intializes its registers and the program counter points to the first instruction. From that point on, the software can run. In simpler setups like small microcontrollers, this is all the software there is, just a single binary blob of instructions. The processor will execute just that going forward. In a more complex setup like a laptop or a phone, there are more stages to the startup. In those more complex setups, traditionally, the first instructions are the BIOS, whose task is to subsequently load the bootloader into the memory and hand-off the control to it. The bootloader is usually small and easy to load into the running memory and the processor can easily start running its code. It proceeds to load the operating system kernel into the memory (implementing the bootloader though is a science of its own). Each machine loads the initial software in its own way. For example, the BIOS can be stored on a separate storage chip and upon powerup, the contents of the storage are simply filled into the memory at a fixed address and the processor just execute...