Zero Page shadows

Eric Ball wrote on AA:

GCC had some interesting constraints they had to work with, then build workarounds into the MARIA chip.

Okay, start with the 2600, really with the 6507 & RIOT. The RIOT only has 128 bytes of RAM, and for the stack to work it has to be mapped to $0180 - $01FF. Zero page uses the same RAM, mapped to $0080 - $00FF; easy to do, just don't connect one of the address lines. This leaves $0000 - $007F for the TIA registers with easy access.

Fast forward to the 7800, which has 4K of fast RAM mapped to $1800-$27FF. Built into the MARIA chip is an address decoder which (along with some external logic) has to handle 2600 mode addressing and 7800 modes (ROM & cart).

For compatibility & sanity, the TIA registers are still mapped to $0000 - $001F, and the MARIA registers $0020 - $003F, leaving $0040-$00FF for RAM. For simplicity, this gets shadowed to $2040-$20FF, and the stack ($0140-$01FF) to $2140-$21FF (again, only one address line different).