What is U-Boot and what does it do?

U-Boot (Das U-Boot) is the most widely used open-source bootloader for embedded Linux systems. Its primary job is to initialize hardware that the kernel cannot initialize itself — most critically external DRAM — and then load and launch the Linux kernel. Without a bootloader, the kernel has no memory to run in and no way to find itself on the boot medium.

U-Boot provides several essential functions: it configures clocks, PLLs, and pin muxing; initializes the DRAM controller with the correct timing parameters for the specific memory chips on the board; sets up boot-medium drivers (eMMC, NAND, SPI NOR, Ethernet for TFTP boot); loads the kernel image, device tree blob, and optional initramfs into specific DRAM addresses; and passes the device tree pointer and kernel command-line arguments to the kernel before jumping to its entry point.

During development, U-Boot's interactive console is invaluable — you can read and write memory, test peripherals, load images over TFTP or USB, and modify boot parameters without reflashing. In production, U-Boot typically runs silently with a zero-second boot delay. U-Boot also supports secure boot (verified boot) by checking cryptographic signatures on the kernel and DTB before loading them, which is critical for products that must resist firmware tampering.