recently i think about the concept of dual boot.
To boot up a Windows under legacy grub,

Question: Does this boot Windows directly or it chainloads
to the Windows's bootloader which then boots up Windows.

In reality Windows can be booted up, but I cannot tell
if it is the first interpretation or the second.

If the second, then how can the Windows bootloader
be found both in MBR (i.e., hd0) and in /dev/sda1 (i.e., (hd0,0) ).
Does it means in the old days, windows installation installs
the bootloader to two places: one to MBR and second
to the first sector of the first
partition?

This is an academic question that intrigues me.

grub doesn't know how to boot up windows, so it chains TO the Windows bootloader.
BTW: lilo does it the same way (with a bit different syntax);
which chains to the bootloader on partition sda1

Some of the windows boot code would go in the MBR and the majority of the boot code/files in the bootable partition. Same with Grub or Grub2 on Legacy systems.

Thank yancek:

It is remarkable that Linux bootloaders can work with the second majority part of Windows's bootloader
so to allow a proper chainloading to the "partial" Windows bootloader.
Amazing..

Thanks also to ehartman
for a useful statement:

grub doesn't know how to boot up windows


I mark this as solved.

Essential all other O/S'es but Unix/Linux are treated as "other" which have to be chainloaded to their own bootloader. grub wouldn't know what files to search for and how the file system those files are IN works.

Thanks for the information.