Patrick of Coreboot has a blog post on UEFI!
UEFI memory mapping
Recently I got into UEFI (TianoCore) development. One of UEFI’s properties is that a part of it survives the OS load and remains resident to provide a limited set of firmware services to the OS.[…]
https://patrick.georgi.family/2018/04/18/uefi-memory-mapping/
See-also:
https://blogs.coreboot.org/blog/author/patrickgeorgi/