Multibooting means installing two or more operating systems on one computer. At boot time you use a menu (or some other mechanism) to specify which one of the installed operating systems you want to use for this session.

First we explain how to set up a multiboot system. Then, we give you some background information. If you have a recent system, and are using recent versions of Linux, installing multiboot is almost trivial and you might not need to bother with the background details. But read through them first before you start, because …

• Your BIOS, disk size and disk partitioning as explained above.
• The type of disks you have (SCSI or IDE).
• The amount of memory in the machine.
• Where the boot information is stored – on the MBR (master boot record) of the first disk in the system, or elsewhere.
• Which operating systems you’re using.

Our example installs four operating systems on the machine:

If you don’t need all these, omit any you don’t want. We haven’t tested an installation with Windows-XP but would expect it to work equally well. (The Notes below give you pointers on how to install DOS or Windows-9x with Linux.)

• First, decide. how much space you’re going to allocate to each operating system. In addition to a partition for each operating system you may want:

• A swap partition for Linux.
• A shared data area that all the operating systems can access. Because in general each operating system can’t understand other operating system’s filesystems, this will probably have to be a DOS "FAT" () filesystem, which Linux and all versions of Windows can use.

• You don’t have an operating system on the machine yet, so how can you partition the disk? The best way we’ve found is to boot the Linux installation system and use its partition tools. (The Debian Linux install in particular makes it very easy to do this.)

This works so easily and so well because the Linux boot loaders neatly handle booting non-Linux operating systems by "chain loading" – calling the other operating system’s own boot loader in turn. (RedHat 8 uses the GRUB boot loader by default, and Debian 3.0 uses Lilo.)

On test machines we use multibooting extensively. Sometimes we need a particular version of Windows, e.g. when we’re checking out how a Windows-98 client machine interacts with a domain controller running Windows-2000. At other times we might want Linux, e.g. when trying out a new bind DNS server. By having all the different operating systems on each of our test machines, we cut down on the total number of machines we need, but can still build almost any network configuration we need.

On a production machine, if you use one operating system all the time and only need to use a different one occasionally, multibooting saves the cost of a separate machine. Typical examples are:

• You use Linux as your main system, but occasionally need Windows to read Microsoft Word or Excel documents.
• You use Windows as your main system but occasionally need Linux to use some of its network or diagnostic tools.
• You want more than one instance of the same operating system, but with different configuration options and settings.

• The newer the software and hardware you are working with, the easier it will be to set up a flexible multiboot system:

• Recent versions of Linux, and to some extent Windows-2000 and Windows-NT don’t assume that they are the only operating system present on the PC. Consequently …

• Old BIOSes are limited in the amount of disk they can access, to 2Gb or 8Gb (or as little as 528Mb on ancient systems). While recent operating systems (Windows-NT etc. and Linux) can bypass this limit once they’re up and running, the BIOS is used to access the disk in the early part of the booting process. In turn this means that on systems with these BIOS limitations, any operating system you are trying to boot must be on a disk partition that falls within the maximum accessible via the BIOS. (A partition is an area of disk that an operating system treats as a unit and that contains a single filesystem. E.g. "drive" C: or D: in Windows, or /dev/hda2 or /dev/dsb5 in Linux. See below for more on partitions.) In practice, this means that your bootable partitions must be below the BIOS maximum size, and forces you to partition your disk with booting in mind.
• Installing a single operating system is time consuming enough, and installing two or three takes a long time. The faster your machine is, the easier it will be to install, especially as we often find after installing that we need a different partition layout or something, and have to repeat the whole process again.

To complicate matters further, partitioning inherits a lot of weirdness from DOS. (DOS is prehistoric and was the first operating system for PCs.) If you want 1, 2, 3, or 4 partitions on your disk, you can use the simplest partition type – primary partitions: you divide the space in the ratio you want between the partitions you allocate. If you want more than four partitions, you can only allocate a max. of three primary partitions, and the next partition must be specified as an extended partition. Then, within this extended partition, you can define as many logical partitions as you want, each one using some of the total space you allocated to the extended partition in the first place. (Loosely, the extended partition is a bit like a container, which holds the logical partitions.) You can only have one extended partition, and you can’t use it directly for data – you have to allocate logical partitions to use the space.

That’s all we cover on disk partitions. For further information see the Notes.

• http://www.tldp.org/HOWTO/Large-Disk-HOWTO-4.html#ss4.2 Large Disk HOWTO; 4.2 History of BIOS and IDE limits.
• http://www.extremetech.com/print_article/0,3998,a=1528,00.asp The BIOS: Get Inside

On Linux there are five common partitioning tools:

There are other partitioning tools, especially for Windows. Some of the commercial tools are very flexible in what they let you do, e.g. resizing and moving partitions.

• /usr/doc/util-linux/README.fdisk.gz (supplied as documentation with the Linux version of the fdisk partitioning tool). This gives very readable background on disk partitioning.
• http://tldp.org/HOWTO/Multi-Disk-HOWTO.html Multi-Disk-HOWTO explains how you go about deciding on the number and size of partitions.