Heres a dumb one - where does intitrd `img' come from

[Harry Putnam <reader newsguy com>]

Where does the /boot/initrd*.img come from.

I never thought to much about that file other than seeing it in lilo
or grub.

rpm -qf doesn't know about it.  Its built during a kernel install I
think but just now having built a kernel (not yet installed) I don't
find that under the `build' directory.

There is 1012MB of data there though... yikes.  Is that normal?