I stumbled upon it when I got lost in the interwebz and I think it has not yet been posted here, so:
http://bread80.com/2021/06/03/understanding-the-amstrad-cpc-video-ram-and-gate-array-subsystem/ (http://bread80.com/2021/06/03/understanding-the-amstrad-cpc-video-ram-and-gate-array-subsystem/)
PS: This crazy editor widget still creates "size 2px" crap around some of my posts ..