226. Ppu read register

File to update

emulator/ppu/ppu.py

Method to implement

PPU.read_register(addr: int) -> int

Why this step exists

Some PPU registers can be read by the CPU. Reads are also memory-mapped I/O, not normal RAM access. The PPU decides what value the CPU receives.

Readable registers for this simplified stage

$2002 -> status
$2007 -> data

Note about $2004/OAMDATA: OAMDATA is readable, but later it becomes a port into internal OAM sprite memory. That behavior is tested in the OAM-specific step. This early read-register test does not lock itself to the old placeholder oam_data field behavior.

Note about $2007/PPUDATA: PPUDATA is readable, but later it becomes a buffered PPU memory read port. That behavior is tested in the PPUDATA-specific step. This early read-register test does not lock itself to the old placeholder data field behavior.

Why only these

Several PPU registers are primarily write-only from the CPU side. Unsupported reads should fail loudly instead of pretending all PPU registers are normal RAM.

Important future accuracy note

Real PPUSTATUS reads have side effects. Reading $2002 clears the VBlank flag and resets the internal address/scroll latch. We intentionally do not model those side effects yet. This step only creates the basic register boundary.

Suggested implementation pseudocode

def read_register(self, addr: int) -> int:
    match addr:
        case 0x2002:
            return self.status
        case 0x2007:
            return self.data
        case _:
            raise ValueError(...)