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The principle behind a virtual machine is very simple. Your "host" OS runs a program, which emulates all the hardware, including the BIOS where needed, for a "guest" OS. There are currently two commercial solutions available that do this (besides the partial solutions DOSemu and Wine on Linux):
Connectix VirtualPC [connectix.com]: Runs on Windows ME/NT/2000/XP and MacOS 9.x/X as host OS, and is capable of running virtual copies of Windows 3.1/95/98/ME/NT/XP.
Vmware [vmware.com]: Runs on Windows NT/2000/XP and Linux, and is capable of running virtual copies of DOS 6, Windows 3.1/95/98/ME/NT/2000/XP, and Linux 2.
I use Vmware on Linux myself, but both systems appear to be quite similar. VirtualPC has the advantage of cross-OS drag & drop and a more flexible clipboard, while Vmware supports more hardware and works slightly faster.
Why use a virtual system?
Flexibility: You can try out installation options, test software in different environments, and check the browser compatibility of your web pages all from one machine, without ever rebooting the hardware.
Security: This is the reason why I mentioned the topic in the other thread. You can restrict network access for the "guest" OS, so that it can only access the local host. This means, that you'll never have to install any of Microsoft's security patches ever again. If your sandbox web server doesn't reside on the local system, then a forwarding proxy will be trivial to install (and comes preconfigured with most Linux distributions).
Convenience: You will normally install your guest OS in a virtual disk. This is just a normal file in the host's file system. Make the virtual file system as large as you want, the "real" file will only get as big as the actual data stored within. Copy the complete installation to a different machine, and continue working there as if nothing happened. Send it to a partner on the other side of the globe, and they can work with exactly the same environment as you do. And best of all, configure your virtual disk to be "undoable", and you can run any potentially virus ridden software you like, and if you're fed up with it, simply discard the changes, and you're back to the clean system you started with.
Cost effective: Run several OSes next to each other, without the cost and hassle of buying seperate hardware for each of them. All you need is lots of RAM and a big disk, both of which are cheap nowadays.
Oh, and if all this is not enough for you... Vmware also has a server version, which allows to run several guest OSes in parallel without a host OS. Works basically like a Mainframe on standard PC hardware.
I can't believe I haven't crashed the system yet :)
VmWare has a thirty day trial. I thought I would just play with this, but it is making me think serious about it.
I'm running this on a 330 Mhz machine. Of course, this isn't blindingly fast, but I have no problem working with the combined system for software testing. The real bottleneck are my 256 MB or RAM, so I have to restart the leaking Netscape once in a while. I made the mistake back then of buying a machine that requires horribly expensive EDO dimms, so I'll rather eventually buy a new machine than to upgrade the old one.
is it possible to run Vmware on FreeBSD as well?
I don't think so. FreeBSD is primarily known for network speed and security, which is why many hosting companies use it. But it covers such a small market compared to Linux that it's probably not worth the effort for Vmware.
While working in Linux I couldn't feel any slow down. My mouse started to get a little jumpy in windows.
Linux is a genuine multitasking system, so it would take a lot more to make you feel anything there. The situation when you can see a jumpy cursor in the Windows box is most often during heavy disk access, possibly on a Samba mount, because this means that both worlds are busy at the same time (and have to wait for each other's results).