Boot process (300 series)
U-boot reads the u-boot environment from it's flash block, to know what to do. Then it loads a Linux kernel (uImage) from either flash block kernel_1 or kernel_2 in memory, and executes it. It passes a commandline, which it read from the environment.
The kernel initializes the hardware, extracts an embedded ramdrive an initrd, containing a complete Linux root file system. According to the commandline it has to start /init from this ramdrive.
The script /init does some network initialisations. (It actually renames /eth0 to /egiga0, probably to keep the firmware more or less compatibel with the ancient NSA-220. The last action of /init is to start /linuxrc
/linuxrc is just a symlink to busybox, and this will basically execute the script /etc/inittab. (OK, it doesn't *look* like a script, but that's only because of it's strange syntax). According to /etc/inittab the script /etc/init.d/rcS will be called.
This script is mainly responsible for bringing up other filesystems.
/dev/sda1 is mounted on /zyxel/mnt/sysdisk. This partition contains a file sysdisk.img, which is actually an ext2 filesystem. The file will be loopmounted on /ram_bin/
This filesystem contains mainly the contents of /usr, which is the webinterface, and (almost) all daemons. The directories from this filesystem are bindmounted on several directories of the rootfs. Further is contains a script etc/init.d/rcS2, and calling this is the last action of /etc/init.d/rcS
When no (prepared) harddisk is available, this system has to completely work from RAM.
A ramdrive is created to which the content of either flashblock rootfs1 or rootfs2 is extracted. This becomes a file sysdisk.img, which is indeed the same one which is available on a prepared harddisk.
The script rcS2 mounts the raid array (s), starts all daemons which bear the firmware. Samba, ntdpd, apache, dropbox, ..., it is also responsible for initiating and starting the package system. The last action of rcS2 is to turn the sysled to green. (Blinking blue is the power-on default)