Bootloader - The bootloader, ARLO, does for EPOC what Loadlin does for Dos. It releases the memory from EPOC, places the kernel and initrd in the appropriate places in memory, and then boots the Linux kernel. The initrd is not necessary if you have a linux system installed on compactflash. All EPOC information is lost from memory as linux takes over. Thus, if you don't have a compactflash disk, then you have to reinstall linux (ARLO, kernel, initrd) to EPOC the next time you reboot - and you will have lost any information you created in EPOC or linux if you did not backup.
Kernel - The kernel has patches applied to it, which makes it appropriate for ARM based systems, and then it also has a few more patches for the Psion hardware. The kernel port to the Series 5MX is quite functional now, with ongoing development. Support for the various components of the 5MX is essentially complete.
Initrd - This is a virtual file system image, which contains a miniature installation of Linux. These can range in size, but the more RAM used for the ramdisk will leave less memory available for the operating system. The kernel loads the initrd image as the root partition. For fully functional installations, installing a system on compactflash disk is preferred.
Compactflash Disk - The linux installation can be set up on a compactflash disk, which behave exactly as IDE disks. The kernel will boot up, find the root partition on the disk and proceed as in any other linux system. The use of compactflash disk also means that all information is saved on disk, so that the system (including ARLO and kernel) is stored to disk for any reboots.
Dual Boot - Because the EPOC system is stored in ROM, it is always available. A 5MX with linux is a dual boot system by default, so you can go between EPOC or linux as you like.