To enable the use of the initramfs you'll need to enable the relevant kernel option (e.g. General Setup --> Initial RAM filesystem and RAM disk (initramfs/initrd) support) and them supply the appropriate boot arguments to your boot loader (e.g. append initrd=my-initrd .....). At the very beginning of boot up you should see [on the console] the initrd being loaded (Loading initrd................), then further down it executes the ./init script within the initrd.
To build an initramfs from a suitably populated directory:
find . -print0 | cpio --null -ov --format=newc | gzip -9 > ../my-initrdTo check the contents of an initramfs:
gunzip -c my-initrd | cpio -tTo extract the contents of an initramfs into the current directory (you might want to create a temporary directory to unpack it into mkdir initrd.d ; cd initrd.d):
gunzip -c ../my-initrd | cpio -i -d -H newc --no-absolute-filenames
If you've been trying to use an initramfs and you're getting this error on boot up:
Failed to execute /init
Kernel panic - not syncing: No init found. Try passing init= option to kernel.Then maybe your ramfs image doesn't contain init, or it's not executable, or you're trying to use a shell within init that incompatible with your kernel's architecture (e.g. trying to use an amd64 arch shell with an i686 kernel).
There's a good Gentoo guide on building them.
On Ubuntu/debian there's a command to build an initrd called mkinitramfs. When running mkinitramfs whilst you can use an alternative config directory, it can only obtain kernel modules from /lib/modules/<kernel_version>. So you can run it using chroot, though its only a shell script so it can of course be tweaked....