If you know C at the level of K&R, a good start would be learning from Bryant and O'Hallaron's book: http://csapp.cs.cmu.edu/ http://csapp.cs.cmu.edu/ From the

If you know C at the level of K&R, a good start would be learning from Bryant and O'Hallaron's book: http://csapp.cs.cmu.edu/ http://csapp.cs.cmu.edu/ From there, you would be well served by working through Hennessy and Patterson's Computer Architecture and Computer Organization and Design. In parallel, you could work through a book on Operating Systems, maybe even playing with the code for a toy system (e.g., http://pages.cs.wisc.edu/~remzi/OSTEP/ http://pages.cs.wisc.edu/~remzi/OSTEP/ or https://pdos.csail.mit.edu/6.828/2016/xv6.html https://pdos.csail.mit.edu/6.828/2016/xv6.html). An alternative or perhaps parallel approach would be to play the assembly games put out by Zachtronics.