It's 6502 not x64 but simpler is better here. It is self-contained but if you would like more here is a 5 page introduction to the 6502 from another author:
I don't think 6502 is a particularly good arch for someone wanting to learn x86 assembly to start with. Its simply to restrictive, the lack of registers, mul, div, the lack of a decent accumulator, all the zero page tricks, etc all are pointless "tricks" for learning a more modern architecture. If you want something simple, but more modern, ARMv4 (AKA arm7tdmi) is probably a good bet. Its only got a handful of instructions (unlike v8 which does away with the predication). Plus, its 32-bit, and the predication might be a little confusing but probably wont damage a beginner like the 6502. I say this as someone who learned 6502 assembly with an apple ][ and then moved to to a 286, and it felt like learning assembly all over again without all the restrictions.
http://skilldrick.github.io/easy6502/
It's 6502 not x64 but simpler is better here. It is self-contained but if you would like more here is a 5 page introduction to the 6502 from another author:
http://people.cs.umass.edu/~verts/cmpsci201/spr_2004/Lecture...