We had to create a CPU and program it for my undergrad, assembly turned out to be really easy to learn. I also found it really easy to program in, and my first program had no errors on first run (how rare!), but I think that's because it forces you to break it down into really small chunks, plus it was just an RPN calculator, nothing fancy.

I did enjoy it, and even wrote a toy compiler for it.

I recently stepped through "Programming from the Ground Up", and found it very educational. It's aimed at the x86 processor on Linux, so it's still quite relevant.

Worth going through, if you have the time.

http://programminggroundup.blogspot.com/