Gate Array, Port 7Fxxh:
7-6 Must be 3 both set to 1 (aka Gate Array Register 3)
5-3 Bank number in 64K units (for expansions bigger than 64K) (max = 512K)
2 Special feature bit. For this kind of RAM banking it must be set to 1. "0" Enables special features. 1-0 Bank mapping in 16K fragments (as on normal CPC 6128 Gate Array)
The 16K fragments are consisting of 8 banks. Banks 0-3 are the internal RAM, banks 4-7 are external RAM (taken from the currently selected 64K block).