Extended MMC1 board.
Max. 512K PRG ROM.
8K CHR RAM.
8K of WRAM which is battery-backable.
This is an "extended" MMC1 cart. It uses the CHR A16 line from the MMC1 to operate
the upper address line to switch all 512K of ROM. There is 8K of CHR RAM on the board
so the mapper CHR lines wouldn't normally be used.