However, as indicated U-Boot provides many useful tools for developing and debugging embedded systems such as loading a newly compiled kernel via TFTP over the network for testing. Hence, the Linux kernel can be booted without the need of U-Boot. Once complete, the third stage bootloader will release the ARM processor from reset. Both are loaded into shared memory allocated to the ARM processor. Once the config.txt file has been parsed, the third stage bootloader will load cmdline.txt – a file containing the kernel command line parameters to be passed to the kernel and kernel.img – the Linux kernel. 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 the Linux Kernel (load addresses, device tree, UART/console baud rates etc). The third stage bootloader – start.elf – is where all the action happens. Both these bootloaders are closed firmware, available as binary blobs from Broadcom. The second stage bootloader – bootcode.bin – is executed on the VideoCore GPU and loads the third stage bootloader – start.elf. This extremely simple first stage bootloader is designed to load the second stage bootloader from a FAT32 or FAT16 filesystem located on the SD Card. The VideoCore GPU loads the first stage bootloader from a ROM embedded within the SoC. The VideoCore GPU is responsible for booting the Broadcom BCM283x system on a chip (SoC), contained on the Raspberry Pi. The SoC will boot up with its main ARM processor held in reset. This also offers convenience – in some environments – as SD cards are not only cheap, but easy to swap out and upgrade. Rather, a SD/MMC card slot is provided for this purpose. To reduce cost, the Raspberry Pi ( with the exception of the compute module) omits any on-board non-volatile memory. The on-board flash may also include an environment variable space, the Linux kernel and filesystem. In these systems, U-Boot is normally flashed into non-volatile memory at the reset vector. Upon reset, the CPU will start loading instructions beginning at the reset vector. More traditional embedded systems contain on-board flash memory mapped at the reset vector. Understanding the Raspberry Pi Boot Process It also includes an environment variable engine allowing for easy configuration. This allows the developer to load new images and file-systems from a variety of sources for testing and/or reflashing. ![]() This includes support for many common file-systems including FAT, ext3, ext4, NFS etc and interfaces such as USB, Ethernet (IP), MMC and even Asynchronous Serial (Kermit/xmodem/ymodem). While its primary purpose is to boot an operating system, such as Linux, it also provides many useful tools for developing and debugging embedded systems. It is licenced under the GNU General Public Licence version 2. U-Boot (The Universal Bootloader) is a popular, feature rich, open source bootloader for embedded systems.
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