In a multitasking OS like Win32, every time another DOS app is started, it gets its own little sandbox where it can muck around with what it thinks are the system internals without really changing anything. When control is passed to another app, the contents of the registers are dumped somewhere and then copied back when control is passed back to the DOS app. The app gets its own fake IVT and video memory among other things. That's why text and 13h apps get to display in a window.

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