Orange Pi 4 (Rockchip 3399) Bare metal coding

On the other hand, you can write a really tiny "bare metal" demo by depending on hardware initialization done by U-Boot. You almost always depend on the clocks for RAM being initialized. What I am thinking about is UART initialization. U-Boot is already sending messages to the console serial port, so you know the clocks, power, baud rate and pinmux settings have been set up properly. All you need to do is to write characters to a data register and poll until they get sent.

In the general case though, bare metal code needs at least the following 4 files.

• Makefile -- required for all of my projects. You should be able to "cd" to the directory holding the code and type "make clean; make". Using an IDE indicates a lack of skill and style.
• loader.lds -- a script for the linker that specifies base addresses for different segments.
• startup.s -- a minimal amount of startup code written in assembler. Typically setting up the stack, clearing BSS, and whatever else is absolutely essential.
• demo.c -- the actual code to run the project.

A quick comment is appropriate. Some people write (or borrow) ridiculously complicated makefiles. The makefile for a bare metal project should be simple, almost trivial -- and you should clearly understand every line in it. This isn't rocket science. If it seems like it, invest the time to learn the basics of using "make".