how does an ARM boot from a sd card?

Question

I'm a bit lost regarding the boot concept of a modern system from a serial flash device. Having programmed quite a number of simple micros ranging from 8 bit PICs to 32 bit Power Architecture down on the bare metal (always by reprogramming the normal bus-addressable flash)I wonder how modern SoCs boot from serial devices. I didn't find too much on the net either as every system seems to rely on a combination of sd-card programming tool and a secondary bootloader, both receiving little to no attention.

Answer 1

The 'how' is a mask-PROM based primary-boot loader baked into the SoC. It doesn't need to do much besides initialise the SD card interface and possibly some SDRAM with enormously conservative timings (although some devices have embedded SRAM to use at this point).

It then enumerates the card interface, reads the FAT from the first card it finds, and from whence, copies the secondary boot-loader into SRAM or SDRAM and executes it.

Often there are restrictions such as the secondary bootloader being the first file on the card and the allocated locks being contiguous.

Many systems then load yet another boot-loader at this point, which is the one which boots the operating system.