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Comment by zabzonk | Hacker News Reader

> I had never run into compatibility issues with programs or games built for older Windows version

Try running a real-mode Windows program on a modern version of Windows.

Intel backwards compatibility has meant that real mode is still with us more of less, but to give context to people who aren't as familiar: real mode was superseded by protected mode with the introduction of the 80286, in 1982.

I believe Microsoft removed support for 16bit Windows applications some time ago.

The hardware may be willing but the software is now incapable.

Yes, that was removed with the 64-bit versions of Windows. My understanding is because they were using a compatibility layer to run 16-bit apps, and with 64-bit Windows they changed that compatibility layer to run 32-bit apps. But I'm not a Windows internals expert so I could be mistaken.

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

You just need to install 86Box as a compatibility layer. Everything runs perfectly.

You only have to copy-paste one .exe file and then you can launch your app from Windows, that’s it. Sounds perfectly reasonable.

86box is a PC emulator, not a compatibility layer.

The official compatibility layer of Microsoft works the same way as 86box: https://betawiki.net/wiki/NTVDM

There is no magic

That link says NTVDM uses the CPUs virtual-8086 mode, rather than emulating a CPU; NTVDM's approach can't be done on a CPU that's in 64bit mode (per Intel's docs), the CPU would have to be stepped back into protected mode first (which isn't the same as when you run a 32bit application in a 64bit OS, that's a separate compatibility state within 64bit mode).

It's not the same at all. 86box emulates the whole PC, every component and peripheral. NTVDM is just a compatibility layer.

For sure.

Not building NTVDM for 64-bit Windows was a major departure from previous strategy and marks a clear regression in Microsoft's attitudes toward backwards compatibility.

> and marks a clear regression in Microsoft's attitudes toward backwards compatibility.

Yeah... but for what purpose should it have been kept? Anyone with a legitimate need to run 16 bit software on a modern Windows machine can always go for virtualization or emulation. The effort required in supporting that technology is far from zero, and old code to work with legacy stuff - no matter in which project - is always a fruitful source of security exploits.

My observation is the "old Microsoft" would have kept it in and supported it because that's how they rolled. The lack of NTVDM in x64 Windows signaled a change that the commitment to compatibility is now on shaky ground.

Whether it should have been kept for a technical reason is secondary, in my mind, to eroding the confidence their Customers had that old software would continue to work.

The market doesn't seem to give a damn so I guess they made the right call.

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