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Mark Moxon 2022-09-01 10:14:30 +01:00
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#!/usr/bin/env python
#
# ******************************************************************************
#
# COMMODORE 64 ELITE FLICKER-FREE MODIFICATION SCRIPT
#
# Written by Mark Moxon
#
# This script applies flicker-free ship-drawing to Commodore 64 Elite, as
# described here:
#
# https://www.bbcelite.com/deep_dives/flicker-free_ship_drawing.html
#
# It does the following:
#
# * Decrypt the gma6 file
# * Modify the gma6 file to draw flicker-free ships
# * Encrypt the gma6 file
# * Modify the gma1 file to remove disk protection
#
# Run this script by changing directory to the folder containing the disk files
# and running the script with "python elite-modify.py"
#
# This modification script works with the elite[firebird_1986](pal)(v040486).g64
# disk image
#
# ******************************************************************************
from __future__ import print_function
import os
import sys
# Convert a C64 address into the corresponding offset within the gma6 file
def get_offset(addr):
return addr - load_address
# Insert a binary file into the game code, overwriting what's there
def insert_binary_file(data_block, addr, filename):
file = open(filename, "rb")
file_size = os.path.getsize(filename)
insert_from = get_offset(addr)
insert_to = insert_from + file_size
data_block[insert_from:insert_to] = file.read()
file.close()
print("[ Modify ] insert file {} at 0x{:02X}".format(filename, addr))
# Insert an array of bytes into the game code, overwriting what's there
def insert_bytes(data_block, addr, insert):
insert_from = get_offset(addr)
insert_to = insert_from + len(insert)
data_block[insert_from:insert_to] = insert
print("[ Modify ] insert {} bytes at 0x{:02X}".format(len(insert), addr))
# Insert a block of NOPs into the game code, overwriting what's there
def insert_nops(data_block, addr, count):
insert = [0xEA] * count
insert_bytes(data_block, addr, insert)
print("[ Modify ] insert {} NOPs at 0x{:02X}".format(count, addr))
# Fetch the platform (NTSC or PAL) from the command line arguments
if len(sys.argv) >= 2:
platform = sys.argv[1]
else:
platform = "pal"
# Print a progess message
print()
print("Modifying Commodore 64 Elite")
print("Platform: {}".format(platform.upper()))
# Configuration variables
load_address = 0x6A00 - 2
seed = 0x49
scramble_from = 0x6A00
scramble_to = 0x6A00 + 0x62D6
# Set up an array to hold the game binary, so we can modify it
data_block = bytearray()
# Load the main code file into data_block
elite_file = open("gma6", "rb")
data_block.extend(elite_file.read())
elite_file.close()
print()
print("[ Read ] gma6")
# Decrypt the main code file
updated_seed = seed
for n in range(scramble_to, scramble_from - 1, -1):
new = (data_block[n - load_address] - updated_seed) % 256
data_block[n - load_address] = new
updated_seed = new
print("[ Decrypt ] gma6")
# Write an output file containing the decrypted but unmodified game code, which
# we can use for debugging
output_file = open("gma6.decrypted", "wb")
output_file.write(data_block)
output_file.close()
print("[ Save ] gma6.decrypted")
# Set the addresses for the extra routines (LLX30, PATCH1, PATCH2) that we will
# append to the end of the main game code (where there is a bit of free space)
llx30 = 0xCCE0
patch1 = 0xCD1E
patch2 = 0xCD35
# We now modify the code to implement flicker-free ship drawing. The code
# changes are described here, which can be read alongside the following:
#
# https://www.bbcelite.com/deep_dives/backporting_the_flicker-free_algorithm.html
#
# The addresses in the following are from when the game binary is loaded into
# memory. They were calculated by analysing a memory dump of the running game,
# searching for patterns in the bytes to match them with the corrsponding code
# from the BBC Micro version (which is very similar, if you ignore any different
# addresses).
# SHPPT
#
# We start with the new version of SHPPT, which we have already assembled in
# BeebAsm and saved as the binary file shppt.bin, so we simply drop this over
# the top of the existing routine (which is slightly longer, so there is room).
insert_binary_file(data_block, 0x9932, "shppt.bin")
# LL9 (Part 1)
#
# This is the modification just after LL9. We insert the extra code with a call
# to the new PATCH1 routine, which implements the original instructions before
# moving on to the new code.
#
# From: LDA #31
# STA XX4
#
# To: JSR PATCH1
# NOP
insert_bytes(data_block, 0x9A8A, [
0x20, patch1 % 256, patch1 // 256 # JSR PATCH1
])
insert_nops(data_block, 0x9A8D, 1)
# LL9 (Part 9)
#
# This is the modification at EE31.
#
# From: LDA #%00001000
# BIT XX1+31
# BEQ LL74
# JSR LL155
#
# To: LDY #9
# LDA (XX0),Y
# STA XX20
# NOP
# NOP
# NOP
insert_bytes(data_block, 0x9F2A, [
0xA0, 0x09, # LDY #9
0xB1, 0x57, # LDA (XX0),Y
0x85, 0xAE # STA XX20
])
insert_nops(data_block, 0x9F30, 3)
# LL9 (Part 9)
#
# This is the modification just after LL74.
#
# From: LDY #9
# LDA (XX0),Y
# STA XX20
# LDY #0
# STY U
# STY XX17
# INC U
#
# To: LDY #0
# STY XX17
# NOP x10
insert_bytes(data_block, 0x9F39, [
0xA0, 0x00, # LDY #0
0x84, 0x9F # STY XX17
])
insert_nops(data_block, 0x9F3D, 10)
# LL9 (Part 9)
#
# This is the modification at the end of the routine.
#
# From: LDA XX15
# STA (XX19),Y
# INY
# LDA XX15+1
# STA (XX19),Y
# INY :
# LDA XX15+2
# STA (XX19),Y
# INY
# LDA XX15+3
# STA (XX19),Y
# INY
# STY U
#
# To: JSR LLX30
# NOP x21
insert_bytes(data_block, 0x9F87, [
0x20, llx30 % 256, llx30 // 256 # JSR LLX30
])
insert_nops(data_block, 0x9F8A, 21)
# LL9 (Part 10)
#
# This is the modification around LL75.
#
# From: STA T1
# LDY XX17
#
# To: STA CNT
# LDY #0
insert_bytes(data_block, 0x9FB4, [
0x85, 0x30, # STA CNT
0xA0, 0x00 # LDY #0
])
# LL9 (Part 10)
#
# This is the second INY after LL75.
#
# From: INY
#
# To: NOP
insert_nops(data_block, 0x9FC1, 1)
# LL9 (Part 10)
#
# This are the two modifications at LL79.
#
# From: LDA (V),Y
# TAX
# INY
# LDA (V),Y
# STA Q
# ... four lots of unchanged LDA/STA, 5 bytes each ...
# LDX Q
#
# To: INY
# LDA (V),Y
# TAX
# ... shuffle the LDA/STA block down by 4 bytes ...
# INY
# LDA (V),Y
# TAX
# NOP
# NOP
insert_bytes(data_block, 0x9FD9, [
0xC8, # INY
0xB1, 0x5B, # LDA (V),Y
0xAA # TAX
])
lda_sta_block = get_offset(0x9FDD)
for n in range(lda_sta_block, lda_sta_block + 4 * 5):
data_block[n] = data_block[n + 4]
insert_bytes(data_block, 0x9FF1, [
0xC8, # INY
0xB1, 0x5B, # LDA (V),Y
0xAA # TAX
])
insert_nops(data_block, 0x9FF5, 2)
# LL9 (Part 10)
#
# This is the modification at the end of the routine. The C64 version has an
# extra JMP LL80 instruction at this point that we can modify to jump to a
# new routine PATCH2, which lets us insert the extra JSR LLX30 without taking
# up any more bytes.
#
# From: JMP LL80
#
# To: JMP PATCH2
insert_bytes(data_block, 0xA010, [
0x4C, patch2 % 256, patch2 // 256 # JMP PATCH2
])
# LL9 (Part 11)
#
# This is the modification at LL80.
#
# We blank out the .LL80 section with 28 NOPs
insert_nops(data_block, 0xA13F, 28)
# LL9 (Part 11)
#
# We have already assembled the modified part 11 in BeebAsm and saved it as
# the binary file ll78.bin, so now we drop this over the top of the existing
# routine (which is exactly the same size).
insert_binary_file(data_block, 0xA15B, "ll78.bin")
# LL9 (Part 12)
#
# We have already assembled the modified part 11 in BeebAsm and saved it as
# the binary file ll115.bin, so now we drop this over the top of the existing
# routine (which is slightly longer, so there is room).
insert_binary_file(data_block, 0xA178, "ll155.bin")
# We now append the three extra routines required by the modifications to the
# end of the main binary (where there is enough free space for them):
#
# LLX30
# PATCH1
# PATCH2
#
# We have already assembled these in BeebAsm and saved them as the binary file
# extra.bin, so we simply append this file to the end.
elite_file = open("extra.bin", "rb")
data_block.extend(elite_file.read())
elite_file.close()
print("[ Modify ] append file extra.bin")
# All the modifications are done, so write the output file for gma6.modified,
# which we can use for debugging
output_file = open("gma6.modified", "wb")
output_file.write(data_block)
output_file.close()
print("[ Save ] gma6.modified")
# Encrypt the main code file
for n in range(scramble_from, scramble_to):
data_block[n - load_address] = (data_block[n - load_address] + data_block[n + 1 - load_address]) % 256
data_block[scramble_to - load_address] = (data_block[scramble_to - load_address] + seed) % 256
print("[ Encrypt ] gma6.modified")
# Write the output file for gma6.encrypted, which contains our modified game
# binary with the flicker-free code
output_file = open("gma6.encrypted", "wb")
output_file.write(data_block)
output_file.close()
print("[ Save ] gma6.encrypted")
# Finally, we need to remove the disk protection from gma1, as described here:
# https://www.lemon64.com/forum/viewtopic.php?t=67762&start=90
data_block = bytearray()
elite_file = open("gma1", "rb")
data_block.extend(elite_file.read())
elite_file.close()
print("[ Read ] gma1")
if platform == "pal":
# For elite[firebird_1986](pal)(v040486).g64
data_block[0x25] = 0xEA
data_block[0x26] = 0xEA
data_block[0x27] = 0xEA
data_block[0x2C] = 0xD0
else:
# For elite[firebird_1986](ntsc)(v060186)(!).g64
data_block[0x14] = 0xEA
data_block[0x16] = 0xEA
data_block[0x15] = 0xEA
print("[ Modify ] gma1")
output_file = open("gma1.modified", "wb")
output_file.write(data_block)
output_file.close()
print("[ Save ] gma1.modified")
print()