Comment by dwattttt

Yes, as sibling noted, 64bit Windows dropped support for running 16bit applications (which require real mode).

I don't recall seeing a technical justification, however I suspect it's related to what CPU state transitions are allowed; protected mode (32bit) is allowed to swap to "virtual-8086" mode, which I guess is how real mode applications were run (rather than the processor completely turning off virtual memory, or completely emulating a CPU a la qemu).

IA32e mode (Intel's name for X64) however is not allowed to transition into virtual-8086 mode[0]; it has to drop the processor into 32 bit mode first. That would chop off the top level of page tables, so short of something you do to affect the whole machine, that sounds like a non-starter for running an application.

[0] Intel software developer manual 3, 2.2