NES TDD

007. Cpu bus rom

Map FakeROM into the CPU program-ROM address range.

Lesson 7 of 356 · tests/chapter_01_cpu/test_007_cpu_bus_rom.py

File to update

emulator/bus/cpu_bus.py

Location

class CpuBus

Reference

https://en.wikibooks.org/wiki/NES_Programming/Memory_Map

Why this step exists

CPU addresses $8000-$FFFF must fetch program bytes from local FakeROM offsets $0000-$7FFF. The bus owns this translation; the memory device sees only local offsets.

Complete example implementation

from dataclasses import dataclass, field
from typing import Optional

from emulator.memory.memory_device import MemoryDevice
from emulator.memory.ram import RAM


@dataclass
class CpuBus:
    program_rom: Optional[MemoryDevice] = None
    ram: RAM = field(default_factory=RAM)

    def read(self, addr: int) -> int:
        if 0x0000 <= addr <= 0x1FFF:
            return self.ram.read(addr & 0x07FF)

        if 0x8000 <= addr <= 0xFFFF:
            if self.program_rom is None:
                raise ValueError("No program ROM attached")
            return self.program_rom.read(addr - 0x8000)

        raise ValueError(f"Unsupported CPU bus read: {addr:04X}")

    def write(self, addr: int, value: int) -> None:
        if 0x0000 <= addr <= 0x1FFF:
            self.ram.write(addr & 0x07FF, value)
            return

        raise ValueError(f"Unsupported CPU bus write: {addr:04X}")

Important invariant

rom_offset = cpu_address - 0x8000

Common misconception

The reset-vector CPU address $FFFC is not FakeROM index $FFFC. After bus translation, it is local index $7FFC.

Out of scope

  • CPU reset
  • real cartridge parsing and mappers
  • writes to cartridge space

Run this lesson

uv run pytest tests/chapter_01_cpu/test_007_cpu_bus_rom.py -v