Unpacker for NES ROMs included in Memorial Series games

unpack_rom.py

import sys
import os

if len(sys.argv) < 2:
    exit()

# Dump all files specified in the command line
for arg in sys.argv[1:]:
    decrypted = []

    GAME_HEADER_SIZE = 0x18 # Header specific to the Memorial Series

    with open(arg, 'rb') as f:
        f.seek(0x800)

        def toSigned16(n):
                n = n & 0xffff
                return (n ^ 0x8000) - 0x8000

        def readByte(file):
            return int.from_bytes(file.read(1), 'little')

        try:
            while True: # Read until EOF, or until control termination
                control = readByte(f)
                typeMaybe = control >> 6
                if typeMaybe == 1:
                    count = (control & 0x3F) + 1
                    byte = readByte(f)
                    while count != -1:
                        decrypted.append(byte)
                        count -= 1
                elif typeMaybe == 2:
                    count = (control & 0x3F) + 2
                    offsetHigh = readByte(f)
                    offsetLow = readByte(f)
                    offset = toSigned16((offsetHigh << 8) | offsetLow)
                    index = count + offset + 1
                    while count != -1:
                        decrypted.append(decrypted[-index])
                        count -= 1
                elif typeMaybe == 3:
                    break # Control termination
                elif typeMaybe == 0:
                    count = control & 0x3F
                    while count != -1:
                        decrypted.append(readByte(f))
                        count -= 1

        except EOFError:
            pass

    # Verify header, then strip it
    if ''.join([chr(i) for i in decrypted[:4]]) == 'CODE':
        decrypted = decrypted[GAME_HEADER_SIZE:]

    filename = os.path.splitext(os.path.basename(arg))[0]
    with open(filename + '.bin', 'wb') as f:
        f.write(bytes(decrypted))