CPU Test Mode

Pin 30 of the 2A03 can be activated to enable a special test mode for the CPU. This activates a few registers in the range of $4018-401F.

Test mode activates registers for testing the APU at $4018-401A.

Visual inspection of the CPU indicates there was planned IRQ functionality from $401C-401F, but this is left unfinished and unusable.

• Forum: Breaking NES apart (WARNING: traffic)
• Forum: Memory map and 2A03 register map
• See: File:Apu address.jpg
• See: CPU pin out and signal description