The second and later stage boot software loaded by the boot ROM, which initialises memory and peripherals, verifies and loads further images and hands control to the operating system or application.
Source Work product in the Firmware and software stack lifecycle
It carries the chain of trust and hardware initialisation from the fixed boot ROM to the operating system. Weak boot loaders fail to boot on some boards, let security be bypassed after the boot ROM and make field updates unsafe.
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