What does a bootloader do?
In an embedded Linux system, the bootloader has two main jobs: basic system initialization and the loading of the kernel. In fact, the first job is somewhat subsidiary to the second in that it is only necessary to get as much of the system working as is needed to load the kernel.
When the first lines of bootloader code are executed, following power-on or a reset, the system is in a very minimal state. The DRAM controller will not have been set up so main memory is not accessible, likewise other interfaces will not have been configured so storage accessed via NAND flash controllers, MMC controllers, and so on, are also not usable. Typically, the only resources operational at the beginning are a single CPU core and some on-chip static memory. As a result, system bootstrap consists of several phases of code, each bringing more of the system into operation.
The early boot phase stops once the interfaces required to load a kernel are working. That includes main memory...