At one point when I was considerably younger I started learning 32-bit x86 assembly as my very naive career goal was to become the next Linux Torvalds. I managed to construct a multiboot-compliant 32-bit kernel that could load tasks from a ext2 ramdisk image passed to the bootloader and multitask up to around 64 processes. I figured out how to use the CR3 register and craft page tables, enter into kernel syscalls using software interrupts, handle CPU faults, page faults, double faults etc. It was quite the learning experience until it eventually crumbled under my lack of skill and foresight. In short, I got more or less about as far as most amateur, self-taught, lone OS developers get before losing interest and giving up.
Fast forward a couple of decades, and I found myself reverse engineering CP/M for the Z80 processor in order to create a virtual Z80-based system that ran inside Unreal Engine. I started with Udo Munk's wonderful Z80pack system, adapted a public domain Z80 CPU emulator which was written in C to C++, and did minimal reimplementation of the Z80pack terminal and disk I/O devices. Since the systems were implemented as "actors" in UE4 it's possible to spawn and run quite a few concurrently as long as you limit the CPU speed of each instance somewhat.
The resulting virtual system in UE4 is able to run original CP/M ports of Rogue and Zork (https://i.imgur.com/gnOCp3e.png), various Z80 instruction exercisers (https://i.imgur.com/kwNuq5X.png), a Z80 C compiler and and even Wordstar 4 (https://i.imgur.com/Q6307w3.jpg) and Microsoft BASIC.
Learning assembly can be a lot of fun - it can really teach you quite a bit about systems architecture that you otherwise might not get if you're always programming in high-level languages only.
