What's the boot sequence of BeagleBoard xM?
Since there’s no NAND or NOR flash on xM, after power on, it directly boots from MMC/SD card. The boot sequence is as follows:
1. Power on
2. Load file MLO from micro SD card. Default MLO is X-Loader.
3. X-Loader loads file u-boot.bin, which is U-Boot, from micro SD card.
4. U-Boot loads file uImage , which is the OS image, from micro SD card.
5. OS takes control of the system.
Why does it have to load both X-Loader and U-Boot? Why can’t it directly load U-Boot?
The SRAM of xM is only 64 KB, which is too small for U-Boot (196 KB) to fit in. X-Loader, on the other hand, is only 24 KB.