Windows NT Multi-Boot Support Limitations

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Microsoft only supports dual-booting between Windows NT and MS-DOS or Windows NT and OS/2 version 1.x. The mechanism used by NTLDR.EXE, the program that controls the Windows NT boot process, permits as many instances of Windows NT as desired to be included in the boot menu plus one other operating system. When the other operating system is selected, NTLDR.EXE loads the BOOTSECT.DOS file that was created during installation and transfers processor control to that image. This results in the other operating system booting as if NTLDR had not intervened. In the case of MS-DOS, the boot sector code looks for IO.SYS; in the case of OS/2 1.x, the boot sector looks for OS2LDR.EXE. In both cases, the BOOTSECT.DOS file is created when Windows NT is installed by copying the existing boot sector that was present before installation.

This scheme breaks down when OS/2 2.x or other operating systems are installed because these operating systems use a different mechanism for controlling their own multi-boot functionality or use a file system that is not supported by Windows NT. Briefly, in the case of OS/2 2.x, a small boot manager partition is installed, which is made the active partition. The boot code in this partition can then transfer control to the boot code in one of the inactive partitions on the disk, effectively booting the operating system installed to that partition. Because the boot code for OS/2 2.x is not located in the same partition as the boot code for Windows NT, the mechanism used by Windows NT is not able to correctly select OS/2 2.x as a boot option. In the case of other operating systems, the operating system in question often utilizes its own file system and must be installed from a different partition than Windows NT for that reason, thus also rendering the boot mechanism used by Windows NT ineffective.

For more information about the Windows NT boot sequence query on the following words in the Microsoft Knowledge Base:

   ntldr and boot and sequence