The CPC6128 second 64K is controlled by a PAL 16L8 chip.
The following equations were worked out by Porchy (member on CPCWiki Forum):
A15OUT = (!X2 & !X1 & A14
# !X3 & !X2 & A14
# A15);
!X1 = (!A15 & D7ANDD6 & RESET & !IOWR & D0
# !X1 & RESET & IOWR
# !X1 & !D7ANDD6 & RESET
# !X1 & A15 & RESET);
!X2 = (!A15 & D7ANDD6 & RESET & !IOWR & D1
# !X2 & RESET & IOWR
# !X2 & !D7ANDD6 & RESET
# !X2 & A15 & RESET);
!X3 = (!A15 & D7ANDD6 & RESET & !IOWR & D2
# !X3 & RESET & IOWR
# !X3 & !D7ANDD6 & RESET
# !X3 & A15 & RESET);
!CAS1 = (X3 & !X1 & A15 & A14 & !NCAS & !RAMDIS & !CPU & CAS0
# !X3 & !A15 & A14 & !NCAS & !RAMDIS & !CPU & CAS0
# X3 & !X2 & X1 & !NCAS & !RAMDIS & !CPU & CAS0
# !NCAS & CAS0 & !CAS1);
!CAS0 = (X3 & X2 & X1 & !NCAS & !RAMDIS & CAS1
# X3 & !X1 & !A15 & !NCAS & !RAMDIS & CAS1
# !X3 & A15 & !NCAS & !RAMDIS & CAS1
# !X1 & !A14 & !NCAS & !RAMDIS & CAS1
# !X3 & !A14 & !NCAS & !RAMDIS & CAS1
# !NCAS & !RAMDIS & CPU & CAS1
# !NCAS & !CAS0 & CAS1);
A14OUT = (A15 & A14
# !X1 & A14
# X3 & A14);