I've been doing real-time embedded programming since the 1980s. I have worked on everything from PDP-8, PDP-11, VAX11, 6800, 6805, 6809, 6502, 68k, Coldfire, Powerpc. For hobby work I wrote a DOS for the 6502 using an AIM65 and some wire wrap to talk to an 8" floppy.
I worked at Dynalogic on the Hyperion. I worked at BSDi doing more embedded work on their BSD router program and then moved to more general work on their 386 platform.
I have written (simple) linking/loaders (wrote one for the 6800 asm and 68k) I hate doing these Bios.