NASM Linux general questions?

Question

linux

I decided to study programming under Linux, in assembler. And immediately a couple of questions
- If the function is from sys. vizov Linux must return the value as it returns it ( stack or what? )
- And how do I mean the values ​​of the flags for functions, for example, for Open, O_RDONLY how do I know what the value of this flag is?

Answer 1

www.stolyarov.info/books/programming_intro/vol2
But why do you need it? Not productive, Not in demand. It's better then to start learning C and Linux kernel programming.