Checking out systems programming, what should I learn, using what resources? [duplicate]

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• Compiler/OS Design - Where to start [closed] 3 answers

I have done some hobby application development, but now I'm interested in checking out systems programming (mainly operating systems, Linux kernel etc.). I know low-level languages like C, and I know minimal amounts of x86 Assembly (should I improve on it?).

What resources/books/websites/projects etc. do you recommend for one to get started with systems programming and what topics are important?

Note that I know close to nothing about the subject, so whatever resources you suggest should be introductory resources. I still know what the subject is and what it includes etc., but I have not done systems programming before (but some application development, as previously noted, and I'm familiar with a bunch of programming languages as well as software engineering in general and algorithms, data structures etc.).