The PPU memory /CE signal is usually controlled from an inverted A13 signal (inverted
by U9, pins 5 and 6) and passes through the cartridge. This allows a cartridge to provide
their own PPU memory and to disable the internal memory. Since we dont have any cartridge,
contacts #58 (CHR /A13) must be connected to contact #57 (ciram /CE).
The A10 signal that goes to the PPU memory does not come directly from the PPU. It
goes through the cartridge too. By connecting memory signal A10 to PPU A10 or A11, the
cartridge can choose between horizontal and vertical mirroring. Since we have 4KB of memory,
mirroring is not applicable. Just connect contact #62 (CHR A10) to contact #22 (CIRAM A10).