16-bit Assembly on 64-bit Windows?

Question

I decided to start learning assembly a while ago, and so I started with 16-bit assembly, using FASMW. HOwever, I recently got a really new computer running Windows 7 64-bit, and

Answer 1

32-bit Windows 7 and older include / enable NTVDM by default. On 32-bit Win8+, you can enable it in Windows Features.

On 64-bit Windows (or any other 64-bit OS), you need an emulator or full virtualization.

A kernel in long mode can't use vm86 mode to provide a virtual 8086 real-mode environment. This is a limitation of the AMD64 / x86-64 architecture.

With a 64-bit kernel running, the only way for your CPU to natively run in 16-bit mode is 16-bit protected mode (yes this exists; no, nobody uses it, and AFAIK mainstream OSes don't provide a way to use it). Or for the kernel to switch the CPU out of long mode back to legacy mode, but 64-bit kernels don't do that.

But actually, with hardware virtualization (VirtualBox, Hyper-V or whatever using Intel VT-x or AMD SVM), a 64-bit kernel can be the hypervisor for an entire virtual machine, whether that VM is running in 16-bit real mode or running a 32-bit OS (like Windows 98 or 2000) which can in turn use vm86 mode to run 16-bit real-mode executables.

Especially on a 64-bit kernel, it's usually easier to just emulate a 16-bit PC entirely (like DOSBOX does), instead of using HW virtualization to running normal instructions natively but trap direct hardware access (in / out, loads/stores to VGA memory, etc.) and int instructions that make DOS system calls / BIOS calls / whatever.