Expansions bigger than 512K extend that above standard. The LSBs of the 64K bank number is kept in D5-D3 data bits (as above), the MSBs of the 64K bank number are in A10-A8 address bits. Typically in inverted form (so the first 512K block is accessed via Port 7Fxxh, the next via 7Exxh, etc.
OUT [01111aaaxxxxxxxx],11bbbccc11bbbscc
aaa = upper bits of 64K bank selection (512K-step) (A8,A9,A10 bits)
bbb = lower bits of 64K bank selection (64K-step) (D3,D4,D5 bits)
s = Special function bit (1 = normal function) ccc (D2 bit) cc = mapping of 16K fragments (as in CPC6128) (D0,D1,D2 bits)
xxxxxxxx = should be set so that it doesn't conflict with other ports (*)