In Windows, once you're in long mode, there's no 16-bit available to you. You can instead take the DOSBox or other VM route.

Linux isn't really relevant given the time frame.

I left it open as to if it was a hardware or OS level item that prevents 16bit. Because I don't know, and don't care to dig that rabbit hole.

Also- DOSBox is an emulator vs VMs are hardware, no? I suspect A VM won't fix the "no-op loop for timing" issue- with modern processors' lowest clock being 600-800Mhz before it gets C6/C7'd, 30 years of IPC improvement, and the possibility of the CPU itself optimizing such loops (I'm unsure for various reasons): I expect the UX of "just limit how many scheduler slices it gets" to be nasty.