Considering the history of ATI with video, and how they enabled GL shaders to work with video acceleration, post processing and effects before anyone was able to dream this thing (Readeon 8500 was able to use shaders to post-process and apply effects to the video), I think their silicon layout was very convoluted and unconventional for today's standards.
Considering HDCP came later, it might have just been slapped as a side module and some of their math magic might be offloaded to GPU even. I remember having old ATI drivers having a very cinematic look when video was accelerated by the GPU itself (it was surreally beautiful), but they had to throw all this machinery out from the driver when they started to modernize their stack (and they broke video playback for months due to not supporting proper overlay formats).
Similarly, I think closed source drivers were programming some of the cards during initialization, because I remember they're talking about moving all this init step to the BIOS itself, and opening some doors on the BIOS to tweak the card during the init (clocks, fans, temp reads, etc. IIRC) for allowing OS and open drivers to play nicer and with less effort with the cards in the long run.
The details are hazy, because these transitions were made almost a decade ago if not longer. However, while NVIDIA was playing the deaf, and Intel was just starting to provide good open drivers for their IGPs, AMDs move as a prime GPU vendor brought a lot of good light to them.
If you want, I can try to dig the history and create a timeline for you, but it might not be an immediate endeavor.