Starting around 2013, ARM started to release “ARM Trusted Firmware” as a BSD-licensed Github-hosted open source project. ARM Trusted Firmware is the trusted execution environment that runs behinds the scenes of the OS on AArch64 platforms. It works in conjunction with UEFI, including Secure Boot.
In upcoming blog posts, I’ll be writing some articles with more details about this project. For now, I’ll suggest reading their Firmware Design Guide and watching the below Youtube-hosted Linaro intro video.