This web content serves as a reference for University level assembly language and systems programming courses. It focuses on the x86-64 instruction set used by the popular x86-64 class of processors, and specifically applies to the Ubuntu 64-bit Operating System. The provided code and examples are designed to be compatible with any Linux-based 64-bit OS, although they have only been tested under Ubuntu 14/16/18 LTS (64-bit). The content is available in the form of a x86-64 Assembly Language Programming with Ubuntu text in PDF format.
http://www.egr.unlv.edu/~ed/x86.html