Virtual 8086 Mode: Difference between revisions

Virtual 8086 mode is a sub-mode of [[Protected Modemode]]. In short, virtual 8086 mode is whereby the CPU (in protected mode) is running a "Emulated" 16bit 'segmented' model (real mode) machine.

The most common problem with v86 mode is that you can't enter Protected Modemode from inside of a v86 task. In other words, if you are running Windows or have emm386 in memory, you can't do a "raw" switch into protected mode (it causes an exception). DOS extenders worked around that problem using either VCPI or DPMI interfaces to switch into pmode (actually, promoting their V86 task as a 'regular' user task). For an OS programmer such interfaces are simply useless as they're part of another OS.

VM86 can be of high interest if one needs access to BIOS functions while in Protected Modemode. This is essentially useful in order to set up video mode. As many modern card/chipsets lack support for VBE3 protected mode interface, setting up a VM86 task that will perform the proper 'set video mode' call remains the method.