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I thought that the specific primitives needed to make NTVDM work were removed from AMD64, specifically that you cant go to Virtual 8086 mode, directly from long mode, without another reset inducing mode change someplace else? I read an article about this like 5 years ago.

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But you _can_ go into vm86 mode from long mode. Linux used to allow you to do that and there was even a patch floating around to restore this when it was removed from the kernel.

Mostly true, this would be protected mode 16bit code (286), not DOS code or real mode code. Protected mode 16bit code was rare and thus support was less needed.

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