In reply to @kryp7on "what I meant is:": It is not kernel but Nim can be used
for bare metal code for RP2040:
https://github.com/demotomohiro/MinimumNimPico
It still uses assembly code in pico-sdk, but replacing these assembly code with
C will be hard or result in larger code.
It seems C cannot read at address 0 because it is defined as "undefined
behavior" in standard.
Also, C doesn't seems to guarantee to generate code that doesn't use a stack
pointer when stack pointer is not properly set.