Once the kernel is loaded by either loader (as usual) or boot2 (bypassing the loader), it examines its boot flags, if any, and adjusts its behavior as necessary.
Here are the more common boot flags:
-aduring kernel initialization, ask for the device to mount as the root file system.
-Cboot from CDROM.
-crun UserConfig, the boot-time kernel configurator
-sboot into single-user mode
-vbe more verbose during kernel startup
There are other boot flags, read boot(8) for more information on them.
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