=== 256KB RAM ===
This uses 4 banks (effectively 0-3 on 464 and 21-3 on 6128). It appears the 256KB RAM detects the 6128, possibly using /EXP but this is to be confirmed, so that it doesn't conflict with the 'extra 64KB' (i.e. c4-c7) RAM of the 6128.
The decoding looks like this: