Arm v7 was a Wild West, but with v8, Arm tried to standardize a lot. The Arm Trusted Firmware is the reference boot firmware implementation for v8+ CPUs: https:

Arm v7 was a Wild West, but with v8, Arm tried to standardize a lot. The Arm Trusted Firmware is the reference boot firmware implementation for v8+ CPUs: https://github.com/ARM-software/arm-trusted-firmware https://github.com/ARM-software/arm-trusted-firmware. I'd think most of the referece documents can be discovered from that code base. Relatedly, from the perspective of hands-on programming, the System Programmer's guide is the manual to start with: https://developer.arm.com/documentation/den0024/a/ https://developer.arm.com/documentation/den0024/a/.