The SPL is a small binary, generated from U-Boot sources, whose goal is to fit in the SRAM and to load the main U-Boot into system RAM. Indeed, the on-chip ROM loads a binary into an SRAM, which can be very tiny (around 4 KiB or less). However, it is not enabled in the SPL mode (Secondary Program Loader).Ī lot of boot sources are not directly memory mapped. The command line is configured by the CONFIG_CMDLINE environment variable and is enabled by default. An old “simple” one, and the “hush” shell which is much more powerful (2). Operating System Initialization: The OS initializes itself, configures the hardware and starts the user interface or application.Ī command-line is also available for U-Boot. Operating System Load: U-Boot reads the OS image from non-volatile storage into memory and passes control to the OS.Bootloader Initialization: U-Boot initializes itself and the hardware, setting up the environment for loading the OS.Bootloader Load: The Boot ROM code reads the bootloader (U-Boot) from a non-volatile storage device, such as a NAND flash, into the system memory.This code is responsible for initializing the hardware and loading the bootloader. Boot ROM Code: The CPU starts executing boot code stored in the read-only memory (ROM).CPU Startup: After applying power to the system, the CPU begins executing its reset vector, which initializes the CPU as well as the peripheral components.To provide some general context on U-Boot, here are the steps followed when loading the Operating System (OS) of an embedded device: It can also implement several kinds of file systems, including FAT32, ext2, ext3 or ext4. All supported devices may be used as the ROM: an SD card, a SATA drive, a NOR flash, a NAND flash… U-Boot comes with a lot of functionalities like network support, USB protocol stack support, loading ram disk, etc. Das U-boot, the Universal Boot Loader, or U-Boot, is a bootloader for embedded boards based on PowerPC, ARM, MIPS, and several other processors, which can be installed in a boot ROM and used to initialize and test the hardware or to download and run application code (1).
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