Many thanks to asdfghkl from #mer @ irc.freenode.net for the overview. I've written it down here along with background info from the wikipedia EFI article to give the poor guys in #mer a break from answering the same questions over and over again from people like me! leftcase @ irc.freenode.net

The Joggler uses the Extensible Firmware Interface (EFI) for communication between the operating system and the firmware of the device. Put simply, EFI is like a more complicated version of the BIOS found in IBM compatible PCs. On the Joggler, the operating system bootloader is an EFI application, and we can control which .efi bootloader is invoked by using the boot.nsh and startup.nsh files.

You can use this functionality to boot from a USB stick. This USB stick needs formatting with 2 partitions. The 1st needs to be FAT16 (about 64MB) and the rest need to be an EXT3 partition. The FAT16 partition contains everything the Joggler needs to know to boot the OS and the EXT3 partition contains the operating system itself.

Here's what happens when you boot from USB.

EFI reads startup.nsh

startup.nsh in the example below looks to fs1 (the USB stick) and executes the instructions in boot.nsh on the same USB stick.

You can configure boot.nsh to call whichever bootloader you intend to use. The supplied tar.gz file contains bootloaders compiled as .efi applications.

startup.nsh

fs1:boot2
fs0:boot

boot.nsh Tells the joggler to boot the selected bootloader .efi file

fs1:
cd \efi\screen
screen