BA 7

most if not all operating systems assume they are the most privileged piece of software on a computer. virtual machines break this assumption by placing a layer of software between the hardware and guess operating system. one early approach to this problem was to catch exceptions thrown by the hardware. once the exception was caught the VM would then emulate the call the guess OS was trying to make. there are problems with this approach. if the hardware doesn't raise an exception then there is nothing to catch and therefore nothing to emulate. the cost of avoiding this issue is performance. early incarnations of Xen suggested changing the guest OS to account for not being the lowest running piece of software on a machine. this solved a lot of issues that earlier Vm's had but it take a lot of force to get big proprietary software companies to change their software. if you're working with open source software or have the next "big idea" then this approach works fine but i don't think this is the common case yet.