Having started my career at Bell Labs in 1984, the fact that you included AT&T X86 syntax cracked me up.

If you are still interested in learning more, ARM and PowerPC are also both interesting assembly targets.

Prior to Bell Labs (college) my assembly experience was 8085 bare metal (no OS) or Z80 under CP/M and MP/M. I wrote a couple console management applications for MP/M that had to dig into BIOS/BDOS routines. It was fun, and that was the thing that made my EE self realize that I wanted to work on software in embedded systems.