Hardware Reference
In-Depth Information
The Raspberry Pi is a little different from your laptop, and even different from a lot of
traditional embedded computers. The heart of the Raspberry Pi is the Broadcom
BCM2835 system-on-chip, which is the CPU, GPU, and memory all combined in a
single component. This detail is important, because the Raspberry Pi actually boots
from the BCM2835 GPU. When you provide power to the Raspberry Pi, the CPU in the
BCM2835 system-on-chip is actually disabled!
The Raspberry Pi boots like this:
1.
First-stage bootloader:
A bootloader programmed into the BCM2835 system-
on-chip hardware mounts the FAT32 boot partition from the Linux distribution on
the SD card. Note that this first-stage bootloader is programmed at manufacture
time and is not modifiable or replaceable. A small, dedicated RISC core on the
Raspberry Pi GPU starts this process.
2.
Second-stage bootloader:
Read off the boot partition on the SD card, this firm-
ware (
bootcode.bin
) accesses the additional GPU firmware files, programs those
firmware files into the Raspberry Pi GPU, and then starts it.
3.
GPU firmware:
This firmware (
start.elf
) allows the GPU to enable the CPU. An
additional file,
fixup.dat
, configures the SDRAM partition between the GPU and
the CPU. At this point, the CPU is released, and execution is transferred to it from
the GPU.
4.
User code:
The CPU boots any supported binary, but the Linux kernel is the de-
fault. It assumes the filename is
kernel.img
, but you can be override the default
in
config.txt
.
Versions of the Raspberry Pi firmware prior to October 19, 2012 contained an ad-
ditional third-stage bootloader (
loader.bin
), but this is no longer required or used.
Previous builds also had different versions of the GPU firmware that had to be
swapped in and out to enable different memory splits between the ARM CPU and
GPU, but this is now configured in
config.txt
.
Because of how the Raspberry Pi boots, you must use an SD card to boot the Rasp-
berry Pi; you cannot boot it from any other device (such as network or USB storage)
alone. But this is a good thing. It prevents you from rendering the device unusable,
because you cannot override the first-stage bootloader. If you end up with damaged,
broken, or incomplete firmware, you can simply start over with a clean SD card.
The Raspberry Pi Foundation provides the firmware files that the GPU loads, which
then enable the Raspberry Pi to boot a specially formatted Linux kernel image. All the
Linux distribution images intended for use on the Raspberry Pi come with a copy of
this firmware, but it is constantly updated upstream. To enable new functionality (or
