Robin2:
Just out of curiorsity does Windows run inside VirtualBox or does VirtualBox run inside Windows? Or, put another way, does one install VirtualBox on a blank hard disk and then install Windows and Linux etc?Also, out of curiosity, doesn't the use of VirtualBox (or equivalent) use up a significant amount of the PC's performance before any OS gets a look in?
I have been thinking about something like this but I have never tried it.
...R
The only thing that can be installed on a blank hard drive so that you can boot it is an operating system (because the definition of an operating system is that you can boot it), so your question about installing it on a blank hard drive is equivalent to asking "Is VirtualBox literally an installable operating system like Linux"? Which is an interesting question and the answer is "No". As far as I know neither virtualbox, nor VMWare, nor QEMU nor any number of other other options can literally be an operating system themselves that you can install on a blank hard drive. They are all applications, like Chrome, Firefox.
Chrome and Firefox can run Javascript, and VirtualBox and other VM's can run a virtual computer.
It's possible to emulate a complete PC in javascript, for example here is an Apple 2 emulator: Apple ][js - An Apple 2 Emulator in JavaScript (warning, it's a bit noisy.)
So, the Apple 2 operating system thinks it's running on Apple 2 hardware. But those calls are really being answered by Javascript. In this case we would say that the Apple 2 is the guest operating system. The host in the above link is actually a javascript program run inside a browser.
Obviously this is very slow, since a browser probably executes hundreds to thousands of instructions for every one it emulates. You wouldn't boot a Windows 7 computer in a Javascript emulator that emulates a machine, because that would be insane.
However, if you step outside of javascript and into a downloaded and installed application like VirtualBox, suddenly it's not insane. Because modern CPU's provide virtualization instructions, so it's easy for them to safely emulate another architecture.
So. If Firefox exists for a Mac, you can run Firefox on the mac and visit the above link to emulate an Apple 2.
If VirtualBox exists on a Mac, you can install virtualbox and run it and actually emulate a complete PC. "Inside" the VM you can install anything, just like "inside" the javascript VM the apple 2 can do anything a normal real apple 2 could do.
what you install inside the emulated computer is called the guest, as someone stated. The computer you're running the program on is called the host.
So just think of VM's like a Firefox browser, and the emulated computer like my Apple 2 emulated link - except a lot faster, because they're closer to the metal. They can also install drivers that let you copy and paste between the guest OS and the host OS, and so forth.
so the first thing you would do after installing VM software is create a blank emulated/fake hard drive which the guest will see. Then you mount a file into the emulated dvd-rom of the guest, such as for example windows installation media. then the guest "boots" (but it only thinks it's doing so on hardware, just like the apple 2 operating system isn't really booting anythign, javascript is answering its instructions). the guest operating system doesn't even know it's not booting on a real computer, and you let it install itself onto the emulated/fake computer inside. it's super-safe.
for example it's a great way to isolate viruses. they don't have any way to "break out" into the rest of your computer, just like the emulated Apple 2 inside Firefox can't write to your hard-drive. it doesn't even know about your hard drive and can't ask about it.