Information on how bootable images work

Different machines have different methods and requirements for how they boot. It is often either impossible or impractical to change to bootloader that is installed on a platform. As a result the build system tries to accommodate different types of bootloaders and provides different bootable images as required.

There are essentially three different ways images are loaded currently. Firstly on system with bootloaders that can load an ELF file we simple provide a packed ELF to load applications. On systems that simply load a binary image and jump to the start we write our own ELF loader to the start of the image which then unpacks an ELF file, finally on systems using grub we take advntage of its ability to load multiple files.

Whether a specific machine uses an ELF loader is defined in the machine specification in machines.py. If the machine needs an ELF loader, specify "elfloader=True". You should also specify an "elf_entry", which is the address at which the ELF loader should be linked at. This differs on each machine as the physical memory layout differs.