65 lines
2.5 KiB
Python
65 lines
2.5 KiB
Python
"""Build an iNES (.nes) NROM cartridge: 6502 PPU-init viewer + image data + CHR.
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Layout: 16-byte iNES header, 16K PRG-ROM (viewer at $C000, 32-byte palette at
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$F000, 1024-byte nametable+attributes at $F020, vectors at $FFFA), then 8K
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CHR-ROM (256 background tiles in pattern table 0).
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"""
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from __future__ import annotations
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import os
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import shutil
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import subprocess
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import tempfile
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VIEWER_DIR = os.path.dirname(os.path.abspath(__file__))
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PRG_BASE = 0xC000
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PRG_SIZE = 0x4000 # 16K
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PAL_ADDR = 0xF000 # 32-byte palette
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NT_ADDR = 0xF020 # 1024-byte nametable + attribute
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def have_xa() -> bool:
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return shutil.which("xa") is not None
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def _assemble() -> bytes:
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if not have_xa():
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raise RuntimeError("The 'xa' (xa65) assembler was not found (apt install xa65).")
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with tempfile.TemporaryDirectory() as td:
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out = os.path.join(td, "v.bin")
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proc = subprocess.run(["xa", "-o", out, "viewer.s"],
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capture_output=True, text=True, cwd=VIEWER_DIR)
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if proc.returncode != 0:
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raise RuntimeError(f"xa failed:\n{proc.stdout}{proc.stderr}")
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with open(out, "rb") as f:
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return f.read()
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def build_rom(palette: bytes, nametable: bytes, chr_rom: bytes) -> bytes:
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"""palette: 32 bytes; nametable: 1024 (960 names + 64 attr); chr_rom: 8192."""
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if len(palette) != 32 or len(nametable) != 1024 or len(chr_rom) != 8192:
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raise ValueError("bad NES data block sizes")
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code = _assemble() # 6502 viewer, org $C000
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if len(code) > PAL_ADDR - PRG_BASE:
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raise RuntimeError("viewer code overruns the $F000 data area")
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prg = bytearray(PRG_SIZE)
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prg[0:len(code)] = code # start == $C000
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prg[PAL_ADDR - PRG_BASE:PAL_ADDR - PRG_BASE + 32] = palette
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prg[NT_ADDR - PRG_BASE:NT_ADDR - PRG_BASE + 1024] = nametable
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# CPU vectors $FFFA/$FFFC/$FFFE all point at start ($C000)
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for off in (0x3FFA, 0x3FFC, 0x3FFE):
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prg[off] = PRG_BASE & 0xFF
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prg[off + 1] = PRG_BASE >> 8
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header = bytes([0x4E, 0x45, 0x53, 0x1A, # "NES\x1A"
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1, # 16K PRG units
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1, # 8K CHR units
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0x00, 0x00] + [0] * 8) # NROM (mapper 0)
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return header + bytes(prg) + bytes(chr_rom)
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def write_nes(rom: bytes, path: str) -> str:
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with open(path, "wb") as f:
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f.write(rom)
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return path
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