General Information on Starting Multiple Operating Systems (101787)

This article was previously published under Q101787

When an Intel x86-based computer starts, sector 0, or the master boot record (MBR), is loaded from the first hard disk and executed. Sector 0 contains the partition table and some code, or the master boot code (MBC). The MBC scans the partition table for the single active partition and loads sector 0 from this partition into memory and executes it. This sector may be a utility or diagnostic program or a boot sector containing boot code for an operating system. The boot code starts the operating system in a manner defined by the operating system.

If a hard disk contains, for example, an MS-DOS, a Unix, and an IBM MOST partition, a user can change which of these systems will be started by changing the active partition. (If you are running MS-DOS, you can use FDISK to do this.)

Windows NT is started when a partition containing Windows NT boot code is active. The boot code loads the Windows NT Boot Loader. The Boot Loader enables you to choose which Windows NT installation is to be started, or to start the previous root-based operating system on the C drive.

The Boot Loader starts the root-based operating system by executing BOOTSECT.DOS, thus simulating the root-based operating system's ordinary boot sequence. BOOTSECT.DOS is usually the MS-DOS (or OS/2) boot sector, but this is not necessary. BOOTSECT.DOS could be another multi-boot program.

Each operating system provides one or more means of organizing data within partitions it recognizes. Some operating systems recognize and use the same file systems and some do not. For example, MS-DOS and Windows NT both recognize and use FAT partitions; Windows NT and Unix each use file systems that are unrecognizable by each other. Operating systems that recognize and use the same file systems can share partitions, meaning that a user can see files on such partitions from whichever of the operating systems is currently running.

If an operating system (like Unix) does not recognize the file system on the C drive, then it follows that the operating system cannot reside on the C drive and therefore the Windows NT Boot Loader cannot provide that operating system as a selection on its boot menu.

Also note that MS-DOS, OS/2, and Windows NT could all reside on separate partitions, in which case the user could select among them by changing the active partition. Or, MS-DOS and Windows NT could be on one partition and OS/2 on another. Or, as long as you keep the first partition FAT, you can have Windows NT and OS/2 on separate partitions, keeping only a bootable MS-DOS floppy for the times when you need to use MS-DOS.