234. Ppu bus basic shape

Files to create/update

emulator/bus/ppu_bus.py

Class to implement

PpuBus

Why this step exists

The NES has two different address spaces

CPU address space: $0000-$FFFF
    routed by CpuBus

PPU address space: $0000-$3FFF
    routed by PpuBus

The PPU should not write directly to raw VRAM forever. It should talk to a stable

PPU-address-space boundary

PPU -> PpuBus -> VRAM / mapper CHR / palette RAM later

This allows PPUDATA behavior to stay stable while PpuBus internals become more accurate over time.

Important constant

PPU_ADDRESS_MASK = 0x3FFF

Why

PPU addresses are 14-bit. Masking with 0x3FFF folds any address into the PPU addressable range $0000-$3FFF.

Initial storage

For now, PpuBus owns a big VRAM backing store. This is a temporary simplification that lets us build PPUADDR/PPUDATA before full nametable/palette accuracy.

Suggested implementation pseudocode

from dataclasses import dataclass, field
from typing import Optional

from emulator.memory.vram import VRAM
from emulator.cartridge.mapper_interface import MapperInterface

PPU_ADDRESS_MASK = 0x3FFF


@dataclass
class PpuBus:
    vram: VRAM = field(default_factory=VRAM)
    mapper: Optional[MapperInterface] = None