Ironically, the Atari ST's OS -- much maligned as 'primitive' by Amiga users -- was not like this. It had a proper syscall mechanism through TRAPs -- so proper

Ironically, the Atari ST's OS -- much maligned as 'primitive' by Amiga users -- was not like this. It had a proper syscall mechanism through TRAPs -- so proper 68000 architecture memory protection entirely possible with user/supervisor separation etc etc -- and an event loop with message passing (tho rarely used). Later extensions to add unix-like multitasking (MiNT -> FreeMiNT) actually ended up fairly elegant, and memory protection is a possibility for some things. My understanding is that AmigaOS syscalls were basically JSRs? The original shipped OS was basically a fork of CP/M and PC-DOS-ish but GEM overtop of it showed more attention to cleaner architectural concerns, though it was never really used to its full intent.