I removed the original 6507 from the Atari 2600 and
replaced with a 6502. This gives you back the other 3 address lines
from the 6502 core and increases the addressable space from 8K to 64K.
In the original Atari 2600 the A12 address line serves as
chip select for the cartridge ROM and the on board memory mapped modules
(RIOT & TIA). I cut the A12 trace to the cartridge and soldered
a jumper to the A12 pin on the cartridge plug. This allowed me to
have separate control of the cartridge ROM and onboard modules. Without
doing this, I wouldn't be able to use any of the the memory space because
the either the ROM or the modules would always be selected.
I wired up the address and data lines to a 32K SRAM.
I then added some glue logic to select the SRAM, the cart ROM and the
modules and got what you see here.
See the memory maps for a
comparison. The current (wire wrapped) version is Super Atari Cart
Boot. This means it boots from the cartridge and has the RAM
available. The future PCB version will have an 8K EEPROM for a boot
ROM for an operating system.