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As i know MBR is created when we partition the disk. According the the doc:

MBR... contains a boot loader for the installed operating system and information about the drive’s logical partitions

And here is my confusion. How can MBR contains a boot loader for the installed operating system while when we partition the disk there isn't any system installed yet?

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The MBR only has 512 bytes so it's not a lot of space to begin with. If you look at the Wikipedia article you will notice that it states:

Bootstrap code: Instructions to identify the configured bootable partition, then load and execute its volume boot record (VBR) as a chain loader.

It's not so much as a whole loader but more general code that helps to identify whatever partition should be used and where to find the next loader. Just by telling a disk it's a MBR disk you don't get a specific loader.

If you're interested in further technical details check this out and this.

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    So when we have a brand new disk and format it using for e.g. fdisk it creates MBR with general and universal bootstrap code not connected with any specific OS? And only when we install specific OS the OS-specific loader is installed on the installation partition boot sector (volume boot record)? – Mulligan Nov 24 '16 at 12:22
  • At least it's what those sources suggest. Most, if not all, OS installations are probably going to install some form of boot loader. – Seth Nov 24 '16 at 12:53

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