Virtual DOS machine
On x86-64 CPUs, virtual 8086 mode is only available as a legacy sub-mode (if running a 32-bit operating system), but not in 64-bit long mode, which is why NTVDM is not included with 64-bit versions of Windows, so 16-bit applications cannot be run on 64-bit Windows. You'll need Windows XP Mode or similar virtualization software to run 16-bit programs within a 64-bit operating system.