To do what you purpose, write a bootloader based on u-boot, would require you to read the sources.
Once you read the sources, you will be able to answer all of your own questions.
Hint:
Since u-boot may be configured for many different machines, what I did was configure the u-boot source for the target machine, compile it, then just read those sources that where included in the load map.
I even posted the step-by-step here to do that.
(In the "debricking a DX" thread)
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