/proc/cmdline

/proc/cmdline prints the parameters that were passed to the kernel at boot. It's useful for debugging (if you want to know how a machine was booted), and it's also useful during system initialization scripts to accept directives from the user in a non-interactive form.

rescue shell

When booting from a stock debian initial ramfs, You can have init drop you into a shell during the initial ramfs stage by supplying a "break=" boot parameter. The possible options are:

top
modules
premount
mount
bottom
init

other kernel parameters

vga=

what resolution/bitdepth should the framebuffer console run at?

rootdelay=

Delay (in seconds) to pause before attempting to mount the root filesystem

console=

you can ask the kernel to boot with an alternate console. the default is tty0, which is the first video console. even better is a serial line: console=ttyS0,115200n8 If you provide two (or more?) console= arguments, both consoles will get some limited kernel spew, but only the last one will get the full output from init, and accept input.