Decode and display the boot logo in a Game Boy ROM

gb_logo.py

#!/usr/bin/python3

# Reads the Nintendo logo from a Game Boy ROM,
# pairs up 32-bit DWORDs from each half of the logo,
# pairs up nibbles from each half of the DWORD,
# then outputs the decoded logo as a readable bitmap.
#
# codeman38
# original script coded 8 June 2010
# revised 3 July 2011
# cleaned up and updated for Python 3, 20 Feb 2022

import argparse
import struct

def main():
    parser = argparse.ArgumentParser(
        description='Decodes a Game Boy boot logo in a ROM.')
    parser.add_argument('filename',
        help='ROM filename')
    parser.add_argument('offset', nargs='?',
        type=int, default=0x184,
        help='offset to read from (default: %(default)d)')
    args = parser.parse_args()
    fp = open(args.filename, 'rb')
    address = args.offset
    fp.seek(address)
    logo = fp.read(0x30)
    dwords = struct.unpack('>12I',logo)
    decoded = [decode_dword(d) for d in pair_dwords(dwords)]
    logo_dec = struct.pack('>12I', *decoded)
    print_logo(logo_dec)

def pair_dwords(dwords):
    """
    Logo is stored as all even-numbered DWORDs followed by
    all odd-numbered DWORDs. This gets them in the right order.
    """
    for x in range(len(dwords)//2):
        yield dwords[x]
        yield dwords[len(dwords)//2 + x]

def decode_dword(dword):
    """
    For input nibbles 12345678, output nibbles are 15263748.
    In other words, pair up nibbles from each half of the DWORD.
    """
    outval = 0
    for x in range(3,-1,-1):
        outval *= 16
        outval += (dword>>16+4*x)%16
        outval *= 16
        outval += (dword>>4*x)%16
    return outval

def print_logo(logo):
    lines = ['']*8
    for y in range(len(logo)):
        lines[y%8] += '{0:08b}'.format(logo[y])
    for y in lines:
        print(y.replace('0',' ').replace('1','#'))

if __name__ == '__main__':
    main()