How does grub2 know what to boot?

[Tom Horsley]

I just installed f19 beta, and it overwrote my MBR with
grub2ness (as expected).

But now I'm wondering - the actual installation of
f19 is entirely on /dev/sda2 (including the /boot
directory which is just a subdirectory of /, not
a separate partition).

My old f18 /dev/sda3 partition is the only one marked
as "boot".

So how the heck is grub2 locating and booting the
/dev/sda2 /boot stuff? Is there a pointer stashed in
the MBR telling it where to look?

Just curious :-).