[CentOS] question on initrd.img file from images/pxeboot

What format is the images/pxeboot/initrd.img file on the DVD ?

Its not gzip
its not straight cpio

What is it?
I wish to change it.

I'm finding information on changing the gzip version of initrd
but not the PXE one.

If I do "file initrd.img" it just says data.

Thanks,

Jerry