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A friend of mine asked a question that got me thinking. Is it possible (theoretically, I assume it hasn't been done) to run two operating systems simultaneously on the same computer? Not in the sense of multitasking in one window, but being able to quickly switch from one to the other without restarting. Could something low-level be created that would intercept system calls (or whatever needs to be done) to make each OS think it is running alone? Or hardware itself that would accomplish this?

I'm not talking about dual booting or using virtual machines, but actually running two OS's side-by-side.

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Virtual machines are multiple OSes running side-by-side. The software/hardware that you described as intercepting calls, etc. is a Hypervisor. –  Synetech May 25 '11 at 18:31
    
Furthermore it has been done extensively and is a big thing now with servers. Examples of well known hypervisors are VMWare ESXi, Microsoft's Hyper-V (see @typoknig's answer below), Xen, and others. –  ultrasawblade May 25 '11 at 18:55
    
Technically, you could run two OSes side-by-side by running both in VMs :-). Actually, I believe this is how some of the big iron from IBM does it - only the hypervisor runs "on the metal", and all the real workload is virtualized. –  sleske Jun 18 '12 at 7:49

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Access to the hardware must be serialized for various reasons, and it is the hypervisor's job to handle this. That is why virtualization is the only feasible mechanism for this.

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@jrdioko the key word to take out of Ignacio's answer is hypervisor. If you have a 64 bit processor that is hypervisor capable you can download a trial of Windows Server 2008 R2 and try hypervisor out with your favorite flavor of Linux for free. –  ubiquibacon May 25 '11 at 18:37

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