Athlon 64: How AMD turned the tables on Intel

Nitpick: The author states that removal of 16-bit in Windows 64 was a design decision and not a technical one. That’s not quite true.

When AMD64 is in one of the 64-bit modes, long mode (true 64-bit) or compatibility mode (64-bit with 32-bit compatibility), you can not execute 16-bit code. There are tricks to make it happen, but they all require switching the CPU mode, which is insecure and can cause problems in complex execution environments (such as an OS).

If Microsoft (or Linux, Apple, etc) wanted to support 16-bit code in their 64-bit OSes, they would have had to create an emulator+VM (such as OTVDM/WineVDM) or make costly hacks to the OS.

Microsoft has just such an emulator. Via Windows source code leaks the NTVDM (Virtual DOS Machine) from 32-bit Windows versions has been built for 64-bit Windows targets[0].

I don't understand why Microsoft chose to kill it. That's not in their character re: backwards compatibility.

[0] https://github.com/leecher1337/ntvdmx64

Edit: Some nice discussion about the NTVDMx64 when it was released: https://www.vogons.org/viewtopic.php?t=48443

> I don't understand why Microsoft chose to kill it.

My personal suspicion: it's about handles.

Several kinds of objects in the Windows API are identified by global handles (for instance, HWND for a window), and on 16-bit Windows, these handles are limited to 16 bits (though I vaguely recall reading somewhere that they're actually limited to 15 bits). Not having the possibility of a 16-bit Windows process would allow them to increase the global limit on the number of handles (keeping in mind that controls like buttons are actually nested windows, so it's not just one window handle for each top-level window).