Nim OS porting

[RodSteward]

I would like to port Nim to a custom OS, that isn't POSIX compatible. Is there 
any documentation how Nim is supposed to interface with an OS? How much does 
Nim rely on the C library for OS functionality? For example for memory 
allocation, does Nim use malloc/free from the C library or does it use the OS 
page mapping API directly? Does Nim provide a generic interface that is 
supposed to be implemented for OS functionality or is everything just selected 
in the sources file with "when thisOS" etc.