Before jumping to kernel entry point boot loader should do at least the following:
1. Setup and initialise the RAM.
2. Initialise one serial port.
3. Detect the machine type.
4. Setup the kernel tagged list.
5. Call the kernel image.
CPU register settings
r0 = 0,
r1 = machine type number discovered in (3) above.
r2 = physical address of tagged list in system RAM, or
physical address of device tree block (dtb) in system RAM