You can't run a COM program today. Not without emulation. Recent PCs can't even run DOS EXE because they're missing the BIOS interrupts most DOS programs use.
Nothing more privileged than ring 0 is required for that.
"v86" is what allowed real mode to be virtualized under a 32-bit OS. This is no longer available in 64-bit mode, but the CPU still includes it (as well as newer virtualization features which could be used to do the same thing).
https://man7.org/linux/man-pages/man2/modify_ldt.2.html
Set seg_32bit=0 and you can create 16-bit code and data segments. Still works on 64 bit. What's missing is V86 mode, which emulates the real mode segmentation model.
I'm pretty sure that if I make a dual-kernel 9front (9pc and 9pc64 available at boot) in a 64 bit machine and I compile emu2 for it, DOS COM binaries might be trapped enough to run simple text mode tools under the 386 port.
