212. Ines file and constants

define iNES layout constants in

Lesson 212 of 356 · tests/chapter_02_rom_loading/test_212_ines_file_and_constants.py

define iNES layout constants in emulator/cartridge/ines.py.

Why this step exists

The iNES format belongs under cartridge, not generic memory, because these values describe a cartridge file container. Defining the shared sizes first gives the following parser lessons one consistent description of that layout.

Suggested implementation

INES_MAGIC = b"NES"
INES_HEADER_SIZE = 16
TRAINER_SIZE = 512
PRG_ROM_BANK_SIZE = 16 * 1024
CHR_ROM_BANK_SIZE = 8 * 1024

Invariants: magic is exactly four bytes; headers and trainers are fixed-size; header bytes 4 and 5 count 16 KiB PRG and 8 KiB CHR banks, respectively. Do not mistake bank counts for byte lengths.

Out of scope for this step

1. Lesson 213 adds `INesHeader`.
2. Lesson 214 adds `parse_ines_header`.
3. Lessons 215-216 add `INesRom` and `parse_ines_rom`.

Run this lesson

uv run pytest tests/chapter_02_rom_loading/test_212_ines_file_and_constants.py -v

import importlib from pathlib import Path def test_cartridge_folder_and_ines_file_exist(): """ Objective: place the iNES format constants in the cartridge package. """ assert Path("emulator/cartridge").exists() assert Path("emulator/cartridge/ines.py").exists() def test_ines_module_defines_file_format_constants(): """ Objective: Define the top-level iNES constants used by the parser. These constants describe the binary layout of a .nes file. """ ines = importlib.import_module("emulator.cartridge.ines") assert ines.INES_MAGIC == b"NES\x1A" assert ines.INES_HEADER_SIZE == 16 assert ines.TRAINER_SIZE == 512 assert ines.PRG_ROM_BANK_SIZE == 16 * 1024 assert ines.CHR_ROM_BANK_SIZE == 8 * 1024