The OS/2 Museum has made available the first version of a display driver disk for Windows 9x running on VirtualBox. The driver uses a linear framebuffer and supports 8/16/24/32bpp modes with resolutions up to 1920×1200 pixels (see more below). The driver is not accelerated but tends to be very speedy on modern hardware.

I’d like to say that it was easy to adapt the existing Windows NT video miniport driver for Windows 9x… but of course it wasn’t. The Windows 9x display driver model is completely different and has nothing in common with NT. The Windows 9x display driver has much more in common with Windows 3.1 (and 3.0 and 2.x) drivers, and it has clearly directly evolved from those older drivers.

So what makes it a “minidriver”? A Windows 2.x/3.x display driver has to implement a very significant chunk of GDI. Bit blits, lines, text output. There is a lot of cases to handle and a great deal of complexity. To give some sense of the complexity, the Windows 3.1 DDK sample driver for Video 7 cards is about 1.6 MB (circa 60,000 lines) of assembler source code. And that’s just for 8bpp displays.

Windows 9x drivers still have to do all that. The difference is that Windows 95 introduced the “DIB Engine”, a large library that implements more or less all of GDI and that the display driver can “punt” to. The minidriver has to set modes and implement various housekeeping tasks, but all drawing can be handled by the DIB Engine. The minidriver can implement what it can do better than the DIB Engine, but doesn’t have to.

The DIB Engine in Windows 95 (DIBENG.DLL) is very similar to the DIB Engine that Microsoft released in September 1994 as part of WinG (WINGDE.DLL).

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