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Typical SD Card Layout for ARM Systems
Most modern ARM boards and systems support at least one SD, Micro SD, or eMMC device. This article outlines some of the common ways partitions are laid out due to the requirements of various boot loaders. Within this arcticle, when the term SD is used it refers to SD cards, Micro-SD cards, and eMMC devices.
Booting an ARM system
Many systems use SD as the primary boot device; the SoC itself has ROM code that can load U-Boot directly from the SD. Occasionally the primary boot device is SPI or NAND flash which contains just U-Boot and other files required for very early boot, and the main OS and its filesystems are on the SD card.
When the SoC has builtin support for booting directly from SD, sometimes that support knows nothing of filesystems, and it just loads whatever is at some fixed sector number on the SD. Other SoCs have support for FAT16 or FAT32 filesystems and load a file with a fixed name from that filesystem, which typically needs to be the first partition in an MBR partition map in sector 0.