002. Ram read write

File to update

emulator/memory/ram.py

Location

class RAM

Why this step exists

The NES contains 2 KiB of internal CPU RAM. This class owns only physical byte storage; the CPU bus will introduce address mirroring in the next lesson.

Complete example implementation

from dataclasses import dataclass, field


@dataclass
class RAM:
    _data: bytearray = field(
        default_factory=lambda: bytearray(0x800),
        init=False,
    )

    def write(self, addr: int, value: int) -> None:
        self._data[addr] = value

    def read(self, addr: int) -> int:
        return self._data[addr]

Important invariants

• storage contains exactly 0x800 bytes
• a write changes the byte read from the same physical address
• RAM does not translate or mirror CPU addresses

Common misconception

RAM does not need to understand addresses $0800, $1000, or $1800. Those are CPU bus aliases of physical RAM and belong to the mapping mechanism introduced in Test 003.

Out of scope

• a shared memory-device interface
• ROM storage
• CPU bus routing