Given vmlinuz and initrd.gz, how do I find out, where the kernel is going to load / (root) file system from?

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http://unix.stackexchange.com – I'm trying various ways of Linux installation (from iso, flash, iso on flash, kernel on flash, root FS in iso-file on flash...) and want to understand what's going on. My question is: is it possible, given the built kernel and ramfs files from a distribution (vmlinuz and initrd), to find out, where they are going to look for the "/" file system? Is it possible to configure this without re-compiling the kernel? And one more: when kernel loads the root filesystem from loopback device, created from .iso-filesystem, how can I configure this process? Thanks! (HowTos)