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This is really cool, and there doesn't seem to be a lot of code in the repo. But before trying to read it… does anyone know how this works?

Edit to answer myself: found the answer in this comment from the last time this was discussed: https://news.ycombinator.com/item?id=28574202

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It uses the Virtual 8086 Mode of the 386. The Intel 80386 Reference Programmer's Manual explains that well [1], I think.

So, in this case, the first mega of RAM, containing MS-DOS and the BIOS is virtualized in a VM86 box while the CPU is running in protected mode, permitting the host OS to cohabit with a protected-mode OS.

It is in fact, a VM86 monitor. Another example of such a program is EMM386 or similar like Jemm386 whose source code is available [2].

[1] https://pdos.csail.mit.edu/6.828/2018/readings/i386/c15.htm

[2] https://github.com/Baron-von-Riedesel/Jemm

edit: the link [1] was wrong

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