:Bryce, my interpretation in Note 2 is EXACTLY the same as what you are saying. It maps to ALL four addresses.
:Thus, the decoding of the "lower" two addressbits (A2,A3) is total nonsense, really. --[[User:Nocash|Nocash]] 19:15, 10 May 2010 (UTC)
Yes, it maps all four addresses as you state correctly, just the bit about calling it non-sense is what I disagree with - If any of the four addresses are selected, the output of the 74LS20 activates the Chip Select on the STI and then in combination with the connections to pins 32-35 on the STI allows that each register can be individually addressed by the amstrad. How else would you achieve this without the configuration shown in the schematic? What signal should activate the STI CS pin, if not this one? How would you do it?
Bryce.