In Windows NT 3, the graphics calls were stubs that sent LPC messages to the CSRSS, where the graphics drivers lived. At the time it was a very microkernel-lik

In Windows NT 3, the graphics calls were stubs that sent LPC messages to the CSRSS, where the graphics drivers lived. At the time it was a very microkernel-like design, with message passing to server processes. And the criticisms were the contemporary 1990s criticisms of microkernel designs, namely that all of this message passing was slow. (In reality, this is a red herring in most critiques of microkernels. In systems ranging from AT&T STREAMS to Windows NT today, I/O subsystems are built anyway around the idea of passing request packets around, be they mbufs or IRPs.) So it all got moved into the kernel, and then some of it in later versions got moved back out again, and rearchitected when windowing became more about compositing multiple whole bitmaps that could be constructed locally rather than drawing into a shared area with lots of clip rectangles.