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If a primary partition contains operating system then it is marked as active, so if we have more than one OS in different primary partition then more than one partition must be marked as active.

So is it possible to have more than one active partition?

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up vote 5 down vote accepted

Only one partition can be marked active. But to enable multiple OS on different partitions to work, the active partition will have a bootloader that allows you to select and boot operating systems from any partition.

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additionally, some boot loaders can look into and extended partition when they are finding operating systems (like GRUB.) but only the four total partitions (three primary and one extended that can contain multiple logical partitions) thats how you can have more than four OSes – PsychoData Sep 23 '13 at 2:02

A disk can have a maximum of four Primary Partitions, of which only one can be 'Active' at any one time. An operating system must be on a primary partition and will usually only be bootable.

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This is not a complete answer. You can also have a dynamic partition and GPT style partitions. The limits on these other types of partitions styles are different then your typical basic primary partition. – Ramhound Aug 6 '12 at 12:43

Once BIOS detects the bootable device then it executes the MBR (Master Boot Recorder). MBR is 1st sector (512 Bytes) of the 1st bootable device. In 512 bytes, 3 parts are there :

i. 1st 446 bytes has primary boot loader information.

ii. Next 64 bytes for Partitions (16+16+16+16) for this reason only we are able to make 4 Primary Partitions.

iii. Next 2 bytes for validation check of MBR. (Magic Number)

Only one of the four partitions can be marked as active. Second boot loader is then loaded from 1st sector of this active partition which gives you an option to chose your OS to boot.

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