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How many operating systems are allowed in a single computer. Is any extra RAM is need for each extra operating system?

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no extra RAM is needed because you execute only one SO each time you boot. –  logoff Nov 6 '12 at 7:57
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closed as not constructive by MaQleod, Bobby, Nifle, Diogo, techie007 Nov 6 '12 at 12:34


6 Answers

up vote 10 down vote accepted

No extra RAM needed, just enough hard drive space (or hard drives) to hold them all.

There is no limit... using boot loaders and similar technologies, you can simplyfy the process - When I was a younger geek, I used to think it was cool having everything from Windows 3.1 to XP and a few flavors of Linux on the same machine....

...As I grew older, I realised it was just a waste of time and a maintenance nightmare.

If you want multiple operating systems, I highly advise you stick with one and explore technologies such as virtualisation which will allow you to run multiple Operating systems from within one without all the headaches.

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As many as they can fit in your storage space. Your MBR can have up to 4 physical partitions, while each of them can have logical partitions in it. But you can actually boot some Linux distributions from an image files inside other partitions even with FAT or NTFS file formats, so as many as they fit. Note that not all operating systems supports the same boot methods, especially closed ones. RAM is only important for each individual OS, so it's depends of that specific OS minimum requirements.

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I would imagine that as long as you have the hard drive space for it, you can partition and install as many operating systems as you like. The RAM requirements will vary depending on the particular OS, but the number of systems won't be affected. Using a VM will make this whole process a lot easier.

You can even find out how to install 145 operating systems if you want.

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Infinite. Well at least by theory. You can have as many operating systems as your hard drive can handle. The smaller the size of the OS the more OS's you can have.

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The hard drive is not a boundary; you can safely boot without one via PXE. And on actual computers, the number is not infinite: on a 64 bit system, there can be at most 2^2^64 different OSs. –  phihag Nov 6 '12 at 9:46
@phihag What has the system bit-ness got to do with how many OSes one can install, and why 2^2^64? –  Michael Kjörling Nov 6 '12 at 10:20
@MichaelKjörling Oops, the boundary is wrong; it must be higher. If we define an OS on a single computer as a permutation of bytes in memory at "OS start" time, the number of OSs is finite on a physical computer. That's because there are only 256^n permutations of n bytes, and the highest possible n on a 64 bit system is 2^64, 256^2^64 is an upper boundary. This number is large (more than a hundred quadrillion times the number of atoms in the universe), but finite. –  phihag Nov 6 '12 at 15:34
@phihag 64-bit systems can support more than 2^64 bytes of memory, just like 16-bit systems can support more than 2^16 bytes of memory. It's more useful on 16- and 32-bit systems, but the same techniques (bank switching, paging, memory overlays, ...) can equally well be used on a 64-bit system. So all of an OS does not need to fit inside the CPU's data bus bitwidth or the architecture's address width (the latter of which may or may not have any relevance on the physical hardware's usable address space). Hence the factor 2^64 becomes limited by available total storage rather than address width. –  Michael Kjörling Nov 6 '12 at 19:29
@MichaelKjörling Oh, you're right, I just assumed that the bootloader(s) can't use any of these techniques, but that's of course not true. So the number of permutations of the total memory is the limiting factor here, and the size of the hard drive does not matter. –  phihag Nov 6 '12 at 21:07
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Older operating systems (such as MS-DOS) had certain requirements which could limit how many you could have on a single machine. I believe (correct me if I'm wrong) that the mentioned MS-DOS had to be installed on the first primary partition.

With modern operating systems there's no limit; you can have as many as you want provided you have the disk space to hold them all and a properly configured boot loader.

However, having worked with multiples OS's on a single machine I can say this is rarely worth the effort. A much better alternative is to have them running in a virtual environment; the requirements are slightly higher then (you need to have enough resources to run your regular OS and then enough resources to run the added OS within that).

Unless of course you're asking how many OS's can run at the same time on a single machine (all virtual)... RAM becomes the main limit then. But as CPU cores and disk space becomes shared, resources will get spread thinner and thinner and all VMs will be slower and slower. There's no theoretical limit on how many OS's you can have running as long as you have the RAM, but calculating it would require answering some questions like "what kind of OS". An old DOS requires far less resources to run than Windows Vista.

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With VM (VirtualBox, VMWare etc ...):

Extra RAM must be added for every guest OS that you need to run at the same time, plus the memory needed for the host OS

With boot manager:

No extra RAM; just size your memory for the biggest OS+application that you need

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