Booting

The kernel booting sequence is supervised by the Multiboot2 standard.

Command line arguments

Multiboot allows passing command line arguments to the kernel at boot. The following arguments are supported:

• -root <major> <minor> (required): Tells the major/minor version numbers of the VFS’s root device
• -init <path>: Tells the path of the binary to be run as the first process instead of the default path
• -silent: Tells the kernel not to show logs on screen while booting

Memory remapping

The kernel is divided into two parts:

• Booting stub, located at 0x100000 on virtual memory
• Main kernel code, located at different positions depending on the architecture in virtual memory:
  • x86: 0xc0200000
  • x86_64: 0xffff800000200000

Because GRUB loads the whole kernel at 0x100000, it is required to remap the memory to use the main code of the kernel. This is done through paging.

TODO: Add schematics of memory mapping

The mapping of memory at 0x100000 is removed later because it is not required anymore.

Init process

The init process is the first program to be run by the kernel, which is in charge of initializing the system.

The program must be located at /sbin/init, or another path if specified as a command line argument.

The init process has PID 1 and is running as the superuser (uid: 0, gid: 0). If this process is killed, the kernel panics.