The 800 compatible upgrade I'm talking about doing to a board, I referred too as the 48/52K, I'm talking about using all 64K (any 400/800 board that is at least 48K actually use 8x64K drams and so 64K memory is there, but Atari's and some other 48K boards just ignore it unless modified.) like the Mosiac bank switches to the same continuous 48+4K memory. Similar to how the Mosiac 64K board works, but I actually don't know if it's 100% compatible with the Mosiac board. It's one of Claus's designs. so it doesn't touch the OS locations. ($C000-$D000)?? I may be mis-reading my self-edited, abbreviated memory map...I'm just teaching myself all this stuff lately, usually I just build the upgrades, I hadn't really started learning the 6502 and Atari memory map until recently, I still have to use reference constantly. I have Mapping the Atari, but this chart in 'How to program your Atari in 6502 Machine language' (A Hofacker paperback) was the easiest to find since I read the book a couple weeks ago.
Jesus... Hofacker i have not heard that for decades... similar me reading from week to week Data Beckers Atari Intern book. I have several of same book so its always available .