Boot

What happens between pressing the power button and seeing a prompt.

1. The CPU runs the EEPROM

A CPU only runs what the EEPROM holds. The EEPROM is a fixed slot of the board, like the reset vector of a real chip. If the EEPROM has nothing the CPU can run, the CPU aborts, which is a panic.

A new EEPROM comes with a small boot program, like a BIOS. It looks through the bus in order for the first storage device that has a file /init.lua. It reads the file, and runs it, giving it the disk’s address. With no such disk it writes boot: no bootable disk to the console and halts. If the file doesn’t compile, it writes why and halts.

So the order of the bus decides which disk starts the computer. A computer with several disks boots the first one that has /init.lua.

2. The kernel starts

/init.lua is the kernel. It is told its boot disk’s address and, optionally, a command line of words. These words are console=<address> (the input device of its console, the board’s console by default), init=<path> (the program to start on the console, /bin/sh by default) and rc=<path> (the script to run first, /etc/rc by default). The EEPROM that comes with the mod gives none.

The kernel:

1. loads its own parts from /lib/modules/ on the boot disk,
2. mounts the boot disk on /, the device files on /dev and a filesystem in memory on /tmp,
3. clears the screen and prints mcos and its version,
4. mounts every other disk on the bus (see Filesystem).

3. /etc/rc

The kernel runs /etc/rc with sh on the console, to its end. This is the boot script, as BSD’s /etc/rc is. By default it:

1. mounts a filesystem in memory on /run,
2. sets the host’s name from /etc/hostname, if there is one,
3. runs rc boot, which starts every service enabled in /etc/rc.conf.

See Services.

4. The shell

When /etc/rc is done, the kernel starts /bin/sh -l on the console, a login shell. It runs /etc/profile, goes to $HOME (/home) and runs $HOME/.shrc if there is one, then shows the prompt: see The shell.

The kernel is init: when the shell ends (you pressed Ctrl+D on an empty line), the kernel starts it again, as init respawns a getty. If it can’t start it (the file is missing or broken), it says why on the console and tries again after five seconds.

Stopping

halt, poweroff and reboot end every process first, closing their files so the disks get what was written, and then halt the CPU, turn the computer off, or restart it.

Watching a boot

dmesg shows the kernel’s log: the devices that joined the bus, mounts, and what the kernel saw. See dmesg.

The computer after a save

A saved, running computer does not boot again when its chunk loads: it goes on where it was. See Persistence.