Question: isn't printf a C standard library function? How can it be used in an OS with no cstdlib (yet)? Or, is the code in the article 'just a guideline'?

Yes, but it can also be done at kernel level if implemented as such.

https://github.com/shawnanastasio/ShawnOS/blob/4345c74e84c5d...

putchar() makes use of vga_textmode_putchar() which is a driver call:

https://github.com/shawnanastasio/ShawnOS/blob/54be43196e310...

Minor nitpick: `kernel_main' needs to be declared as cdecl for the assembly call to work.