Because U-Boot's commands are fairly low-level, it takes several steps to boot a kernel, but this also makes U-Boot more flexible than other bootloaders, since the same commands can be used for more general tasks. Unlike PC bootloaders which obscure or automatically choose the memory locations of the kernel and other boot data, U-Boot requires its boot commands to explicitly specify the physical memory addresses as destinations for copying data (kernel, ramdisk, device tree, etc.) and for jumping to the kernel and as arguments for the kernel. There are also commands to read device information, read and write flash memory, download files (kernels, boot images, etc.) from the serial port or network, manipulate device trees, and work with environment variables (which can be written to persistent storage, and are used to control U-Boot behavior such as the default boot command and timeout before auto-booting, as well as hardware data such as the Ethernet MAC address).
Using the CLI, users can load and boot a kernel, possibly changing parameters from the default. U-Boot runs a command-line interface on a console or a serial port. UEFI binaries like GRUB or the Linux kernel can be booted via the boot manager or from the command-line interface. U-Boot implements a subset of the UEFI specification as defined in the Embedded Base Boot Requirements (EBBR) specification. Regardless of whether the SPL is used, U-Boot performs both first-stage (e.g., configuring memory controllers and SDRAM) and second-stage booting (performing multiple steps to load a modern operating system from a variety of devices that must be configured, presenting a menu for users to interact with and control the boot process, etc.). DRAM initialization using CPU cache as RAM) and load the larger, fully featured version of U-Boot. If there are size constraints, U-Boot may be split into two stages: the platform would load a small SPL (Secondary Program Loader), which is a stripped-down version of U-Boot, and the SPL would do some initial hardware configuration (e.g. onchip ROM of the ARM CPU) from a supported boot device, such as an SD card, SATA drive, NOR flash (e.g. U-Boot is both a first-stage and second-stage bootloader. It is available for a number of computer architectures, including 68k, ARM, Blackfin, MicroBlaze, MIPS, Nios, SuperH, PPC, RISC-V and x86. Das U-Boot (subtitled "the Universal Boot Loader" and often shortened to U-Boot see History for more about the name) is an open-source, primary boot loader used in embedded devices to package the instructions to boot the device's operating system kernel.
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