Hi,
The 680EC020 can only address a 24 bit (16 MByte) address space.
This is the A1200 memory map.
AddressSize
000000 to 1FFFFF 2 MBChip RAM(or system ROM overlay)
200000 to 5FFFFF 4 MBZorro II expansion space
600000 to 9FFFFF 4 MBCredit Card memory if CC present
A00000 to A1FFFF 128 KB Credit Card Attributes
A20000 to A3FFFF 128 KB Credit Card I/O
A40000 to A5FFFF 128 KB Credit Card Bits (similar to CDTV)
A60000 to A7FFFF 128 KB PC I/O
A80000 to B7FFFF 1 MB System ROM selected
B80000 to BEFFFF4 48 KB Not used(Reserved for CDTV)
BF0000 to BFFFFF 64KB 8520CIAs
C00000 to CFFFFF 1 MBC00000 Memory
D00000 to D7FFFF 512 KB PC memory
D80000 to D8FFFF 64 KB SPARE chip select
D90000 to D9FFFF 64 KB ARCNET chip select
DA0000 to DA3FFF 16 KB IDE drive
DA4000 to DA4FFF 16 KB IDE reserved
DA8000 to DAFFFF 32 KB Credit Card and IDE configregisters
DB0000 to DBFFFF 64 KB Not used(reserved for external IDE)
DC0000 to DCFFFF 64 KB Real Time Clock(RTC)
DD0000 to DDFFFF 64 KB RESERVED for DMA controller
DE0000 to DEFFFF 64 KB Not Used
DF0000 to DFFFFF 64 KB Chip Registers
E00000 to E7FFFF 512 KB System ROM(lst half if 1MB ROM)
E80000 to EFFFFF 512 KB Configuration and I/O card space
You will see 2 x 4 MByte slots, one is Zorro II space, which your current 4 MByte SIMM occupies. The other 4 MByte slot is shared with the PCMCIA slot, unfortunately, PCMCIA and another 4 MByte are mutually exclusive.
To access more than 8 MByte of RAM, a 68030 processor is normally used, which can address 4 GByte of RAM and the Extra RAM is mapped outside of the 16 MByte space of the Amiga. I just checked BBoAH, a full 68020 is used on your board but the address decode logic will not support the 32 bit address space required to work outside the PCMCIA space, sorry but without a re-design you can not modify your card.
I hope this provides a reasonable understanding of why.
Ian
