[copper_think]

The author mentions the screen being corrupted when a DOS prompt is opened in windowed mode. This can happen because the DOS prompt runs in a separate VM (in V86 mode), and makes calls into the VGA ROM BIOS via INT 10h. The VGA ROM BIOS on this machine is probably a wrapper over VBE; that is, it probably contains IN and OUT instructions that talk to the VBE I/O ports, 0x1CE and 0x1CF. These reads and writes from the DOS VM will, by default, be allowed to reach the physical hardware if they are not virtualized by the VMM.

This is a common problem that authors of Windows 3.x/9x display drivers had to handle, although the specific I/O port numbers to be virtualized vary by graphics adapter. There are samples in the Win95 DDK that show how to use the VMM services Install_IO_Handler and Enable/Disable_Global_Trapping to set up I/O port traps, and VDD_Get_VM_Info from within the trap handler to determine the VM that currently owns the CRTC. This allows the trap handler to reach a decision about whether to allow an I/O to reach the hardware, or to virtualize the access in some way. A good virtualization policy to start with is probably just to drop any writes from non-CRTC-owner VMs. Any additional needed complexity can be added from there.