MBR and boot loader code

[MulliGan81]

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?

 

[TheELF]

What is the DOC from?
When you partition a disk and use the bootable/active flag it only writes information to the mbr that is important for the bios to be able to start from this disk,the disk will just boot into a flashing cursor or some other error message.
If you then install a OS it will write additional info to the mbr telling it where to find the OS loader.
If you partition a disk from a OS installation media than it will write all the info including the OS specific parts.

 

[VirtualLarry]

For DOS/MBR systems, the BIOS reads sector 0 off of the HDD into a certain memory address, and executes it. That sector, contains additional-stage bootloader info, to read the partition table (already loaded into RAM, as part of sector 0), finds a primary partition marked bootable, and then loads the OS bootloader / partition boot sector, and kind of chain-loads it, I think. Then, that sector, loads whatever it has to from that partition to boot (NTLOADER).

For UEFI, it's a bit different.