So it took longer than I'd hoped, but I now have a working first-pass AMD64 ASM program that can decode an ASCII number in the .data segment and print it out again.
Build instructions in adjacent run.txt.
I have some questions.
(1 of 2). I think I don't like GNU/AT&T assembly format and would like to switch to Intel assembly format. Is Intel format… documented… somewhere? This is the closest I found. https://sourceware.org/binutils/docs/as/i386_002dVariations.html