Anyway, to get access to RAM before it is clobbered by other software, you need to run your code really really early. By the time the BIOS has decided to let you run code from a USB stick, it could be way too late.
RAID cards have option ROM that can run before a bootloader has started, that is one way to get something running earlier.
RAID cards have option ROM that can run before a bootloader has started, that is one way to get something running earlier.