@Mathy According to atarimuseum.org up to 3 1090 style 64K boards could be switched to "1064 mode" where it would replace the system RAM, or "banked" to add 192K to the base 64K -- Incompatible with PortB, but uses the same address space (16384-32767) banking 4 banks by poking 53758 (D1FE) with 0-3 for the first card, 16-19 for the 2nd card - manual doesn't say anything about a 3rd though.   @atari-passion This page has links to schematic and manual for the board: http://www.atari