Booting from Other DOS Partitions (31906)


The information in this article applies to:
    • Microsoft MS-DOS operating system 3.1
    • Microsoft MS-DOS operating system 3.2
    • Microsoft MS-DOS operating system 3.21
    • Microsoft MS-DOS operating system 3.3
    • Microsoft MS-DOS operating system 3.3a
    • Microsoft MS-DOS operating system 4.0
    • Microsoft MS-DOS operating system 4.01
    • Microsoft MS-DOS operating system 5.0
    • Microsoft MS-DOS operating system 6.0
    • Microsoft MS-DOS operating system 6.2
    • Microsoft MS-DOS operating system 6.21
    • Microsoft MS-DOS operating system 6.22
This article was previously published under Q31906

SUMMARY

Even though MS-DOS versions 3.x and later allow multiple logical drives on a single hard drive through the implementation of an extended DOS partition, they do not allow this extended partition to be bootable. In most hardware situations, any primary DOS partitions on other physical hard disks cannot be booted.

MORE INFORMATION

Each ROM BIOS that follows the IBM specifications for PC, XT, or AT ROM BIOS is programmed to look at bootup for a bootable partition (80H in the boot indicator) on the following physical drives:

    • The first physical floppy drive (logical Drive A)
    • The first physical hard drive (logical Drive C)
If the floppy partition (which is examined first) or one of the four fixed- disk partitions are marked bootable (also called active), the boot sector is loaded into memory and execution begins at that location in memory. If no bootable partitions are marked active, the computer boots into ROM BASIC. Since only IBM systems have ROM BASIC, what happens when no bootable partitions are marked active is an OEM-specific issue. The implementation of extended DOS partitions does not allow this partition type to be marked active by FDISK. The extended DOS partition does not contain a valid loader routine in its boot record even if the partition is marked active.

Any primary DOS partitions on other physical hard disks will not be encountered due to the limitations of the hardware boot routine. Your hardware manufacturer should be able to provide any additional information on your machine's hardware boot routine.
Modification Type: Major Last Reviewed: 5/12/2003
Keywords: KB31906
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