Microsoft MinWin project explained

You'd think that the MinWin hype would die down, that it would lose its appeal. However, it still seems to appeal to a lot of people as if it's some sort of gift from God. In a talk on Channel 9, NT kernel guru Mark Russinovich confirmed what Eric Traut had already said a year ago: MinWin is part of Windows 7. He also explained what MinWin is: a stripped down, bare metal NT kernel. Microsoft looked at calls made by the kernel towards higher items on the dependency stack, and re-arranged parts of the kernel to eliminate these calls, and make sure the small kernel could run on its own.

Look at it this way. On top of the inverted pyramid is Windows-proper. It has everything - all the bells and whistles. Then comes Server Core. This is what's left after Windows-proper had all the stuff cut out that was not needed for the various server roles that ship with Windows Server. The problem with Server Core, however, is that the cutting isn't exactly a very pretty process - it leaves all sorts of dependencies unanswered. If you were to take a regular Windows application, and tried to run in on Sever Core, and it would make a call to something that has been cut out, it would fail.

So, somewhere between Windows-proper and Server Core, there's a cutting line. In order to prevent this cutting line from moving around between Windows releases (due to APIs coming and going, for instance), Microsoft created MinWin. MinWin is the result of a project within Microsoft that mapped the dependencies in Windows in a layer graph. The goal is to make sure that calls happen only from layer x to layer x-1 - not from layer x to x+1. They achieved this by moving APIs around. Now, MinWin is the place where no calls are made upwards. It's an isolated, bootable kernel that can be used for testing purposes, and it can be built separately from the rest of Windows.