To reduce cost, the Raspberry Pi (Model A & B) omits any on-board non-volatile memory used to store the boot loaders, Linux Kernels and file systems as seen in more traditional embedded systems. Rather, a SD/MMC card slot is provided for this purpose. (The Raspberry PI Compute Module has 4Gbyte eMMC Flash on-board)
The Raspberry Pi’s Broadcom BCM2835 system on a chip (SoC) powers up with its ARM1176JZF-S 700 MHz processor held in reset. The VideoCore IV GPU core is responsible for booting the system. It loads the first stage bootloader from a ROM embedded within the SoC. The first stage bootloader is designed to load the second stage bootloader (bootcode.bin) from a FAT32 or FAT16 filesystem on the SD Card.
The second stage bootloader - bootcode.bin - is executed on the VideoCore GPU and loads the third stage bootloader - start.elf. (Historically, yet another bootloader called loader.bin was loaded at this stage, but has since been phased out)
The third stage bootloader - start.elf - is where all the action happens. It starts by reading config.txt, a text file containing configuration parameters for both the VideoCore (Video/HDMI modes, memory, console frame buffers etc) and loading of the Linux Kernel (load addresses, device tree, uart/console baud rates etc).
Details of parameters contained within this file can be found at
(Prior to the introduction of the 512MB Raspberry Pi, multiple start.elf files were provided (arm128_start.elf, arm192_start.elf, arm224_start.elf & arm240_start.elf). These files would partition the memory used between the ARM processor (Linux) and the VideoCore GPU. This has now been superseded with a gpu_mem parameter in the config.txt file specifying the memory to be allocated to the GPU.)
Once the config.txt file has been loaded and parsed, the third stage bootloader will also load cmdline.txt and kernel.img. cmdline.txt is a file containing the kernel command line parameters to be passed to the kernel at boot. It will load kernel.img into the shared memory allocated to the ARM processor, and release the ARM processor from reset. Your kernel should now start booting.
The third stage bootloader - start.elf, also passes extra parameters to the kernel than contained within the cmdline.txt. For example, my Linux Kernel receives the following parameters, specifying DMA channels, GPU parameters, MAC addresses, eMMC clock speeds and allowable Kernel Memory size.
dma.dmachans=0x7f35 bcm2708_fb.fbwidth=1280 bcm2708_fb.fbheight=1024 bcm2708.boardrev=0xe bcm2708.serial=0xd9b35572 smsc95xx.macaddr=B8:27:EB:B3:55:72 sdhci-bcm2708.emmc_clock_freq=250000000 vc_mem.mem_base=0xec00000 vc_mem.mem_size=0x10000000 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 rootwait
Mainline Linux Kernels may not parse these extra parameters.