321 lines
8.1 KiB
Python
321 lines
8.1 KiB
Python
#!/usr/bin/env python
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#
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# ******************************************************************************
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#
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# COMMODORE PLUS/4 ELITE FLICKER-FREE MODIFICATION SCRIPT
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#
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# Written by Mark Moxon
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#
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# This script applies flicker-free ship-drawing to Commodore Plus/4} Elite, as
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# described here:
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#
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# https://www.bbcelite.com/deep_dives/flicker-free_ship_drawing.html
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#
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# It does the following:
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#
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# * Modify the PRG file to draw flicker-free ships
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#
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# Run this script by changing directory to the folder containing the disk files
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# and running the script with "python elite-modify.py"
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#
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# This modification script works with the following disk image, which is the
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# Pigmy Plus/4 version from Ian Bell's site, with the demo and packing removed:
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#
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# * elite_+4_unpacked.prg
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#
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# ******************************************************************************
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from __future__ import print_function
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import os
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# Convert a C64 address into the corresponding offset within the gma6 file
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def get_offset(addr):
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return addr - load_address
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# Insert a binary file into the game code, overwriting what's there
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def insert_binary_file(data_block, addr, filename):
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file = open(filename, "rb")
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file_size = os.path.getsize(filename)
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insert_from = get_offset(addr)
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insert_to = insert_from + file_size
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data_block[insert_from:insert_to] = file.read()
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file.close()
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print("[ Modify ] insert file {} at 0x{:02X}".format(filename, addr))
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# Insert an array of bytes into the game code, overwriting what's there
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def insert_bytes(data_block, addr, insert):
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insert_from = get_offset(addr)
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insert_to = insert_from + len(insert)
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data_block[insert_from:insert_to] = insert
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print("[ Modify ] insert {} bytes at 0x{:02X}".format(len(insert), addr))
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# Insert a block of NOPs into the game code, overwriting what's there
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def insert_nops(data_block, addr, count):
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insert = [0xEA] * count
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insert_bytes(data_block, addr, insert)
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print("[ Modify ] insert {} NOPs at 0x{:02X}".format(count, addr))
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# Print a progess message
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print()
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print("Modifying Commodore Plus/4 Elite")
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# Configuration variables
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load_address = 0x1100 - 2
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# Set up an array to hold the game binary, so we can modify it
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data_block = bytearray()
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# Load the main code file into data_block
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elite_file = open("elite_+4_unpacked.prg", "rb")
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data_block.extend(elite_file.read())
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elite_file.close()
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print()
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print("[ Read ] elite_+4_unpacked.prg")
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# Set the addresses for the extra routines (LLX30, PATCH1, PATCH2) that we will
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# load into unused portiong of the main game code
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llx30 = 0x7240
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patch1 = 0x727E
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patch2 = 0x7295
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# We now modify the code to implement flicker-free ship drawing. The code
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# changes are described here, which can be read alongside the following:
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#
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# https://www.bbcelite.com/deep_dives/backporting_the_flicker-free_algorithm.html
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#
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# The addresses in the following are from when the game binary is loaded into
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# memory. They were calculated by analysing a memory dump of the running game,
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# searching for patterns in the bytes to match them with the corrsponding code
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# from the BBC Micro version (which is very similar, if you ignore any different
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# addresses).
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# SHPPT
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#
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# We start with the new version of SHPPT, which we have already assembled in
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# BeebAsm and saved as the binary file shppt.bin, so we simply drop this over
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# the top of the existing routine (which is slightly longer, so there is room).
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insert_binary_file(data_block, 0x9932 + 0x900, "shppt-plus4.bin")
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# LL9 (Part 1)
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#
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# This is the modification just after LL9. We insert the extra code with a call
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# to the new PATCH1 routine, which implements the original instructions before
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# moving on to the new code.
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#
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# From: LDA #31
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# STA XX4
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#
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# To: JSR PATCH1
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# NOP
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insert_bytes(data_block, 0x9A8A + 0x900, [
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0x20, patch1 % 256, patch1 // 256 # JSR PATCH1
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])
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insert_nops(data_block, 0x9A8D + 0x900, 1)
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# LL9 (Part 9)
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#
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# This is the modification at EE31.
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#
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# From: LDA #%00001000
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# BIT XX1+31
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# BEQ LL74
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# JSR LL155
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#
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# To: LDY #9
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# LDA (XX0),Y
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# STA XX20
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# NOP
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# NOP
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# NOP
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insert_bytes(data_block, 0x9F2A + 0x900, [
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0xA0, 0x09, # LDY #9
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0xB1, 0x57, # LDA (XX0),Y
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0x85, 0xAE # STA XX20
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])
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insert_nops(data_block, 0x9F30 + 0x900, 3)
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# LL9 (Part 9)
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#
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# This is the modification just after LL74.
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#
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# From: LDY #9
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# LDA (XX0),Y
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# STA XX20
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# LDY #0
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# STY U
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# STY XX17
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# INC U
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#
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# To: LDY #0
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# STY XX17
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# NOP x10
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insert_bytes(data_block, 0x9F39 + 0x900, [
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0xA0, 0x00, # LDY #0
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0x84, 0x9F # STY XX17
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])
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insert_nops(data_block, 0x9F3D + 0x900, 10)
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# LL9 (Part 9)
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#
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# This is the modification at the end of the routine.
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#
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# From: LDA XX15
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# STA (XX19),Y
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# INY
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# LDA XX15+1
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# STA (XX19),Y
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# INY :
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# LDA XX15+2
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# STA (XX19),Y
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# INY
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# LDA XX15+3
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# STA (XX19),Y
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# INY
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# STY U
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#
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# To: JSR LLX30
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# NOP x21
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insert_bytes(data_block, 0x9F87 + 0x900, [
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0x20, llx30 % 256, llx30 // 256 # JSR LLX30
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])
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insert_nops(data_block, 0x9F8A + 0x900, 21)
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# LL9 (Part 10)
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#
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# This is the modification around LL75.
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#
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# From: STA T1
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# LDY XX17
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#
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# To: STA CNT
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# LDY #0
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insert_bytes(data_block, 0x9FB4 + 0x900, [
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0x85, 0x30, # STA CNT
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0xA0, 0x00 # LDY #0
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])
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# LL9 (Part 10)
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#
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# This is the second INY after LL75.
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#
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# From: INY
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#
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# To: NOP
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insert_nops(data_block, 0x9FC1 + 0x900, 1)
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# LL9 (Part 10)
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#
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# This are the two modifications at LL79.
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#
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# From: LDA (V),Y
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# TAX
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# INY
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# LDA (V),Y
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# STA Q
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# ... four lots of unchanged LDA/STA, 5 bytes each ...
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# LDX Q
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#
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# To: INY
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# LDA (V),Y
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# TAX
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# ... shuffle the LDA/STA block down by 4 bytes ...
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# INY
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# LDA (V),Y
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# TAX
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# NOP
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# NOP
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insert_bytes(data_block, 0x9FD9 + 0x900, [
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0xC8, # INY
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0xB1, 0x5B, # LDA (V),Y
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0xAA # TAX
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])
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lda_sta_block = get_offset(0x9FDD + 0x900)
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for n in range(lda_sta_block, lda_sta_block + 4 * 5):
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data_block[n] = data_block[n + 4]
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insert_bytes(data_block, 0x9FF1 + 0x900, [
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0xC8, # INY
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0xB1, 0x5B, # LDA (V),Y
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0xAA # TAX
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])
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insert_nops(data_block, 0x9FF5 + 0x900, 2)
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# LL9 (Part 10)
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#
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# This is the modification at the end of the routine. The C64 version has an
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# extra JMP LL80 instruction at this point that we can modify to jump to a
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# new routine PATCH2, which lets us insert the extra JSR LLX30 without taking
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# up any more bytes.
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#
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# From: JMP LL80
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#
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# To: JMP PATCH2
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insert_bytes(data_block, 0xA010 + 0x900, [
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0x4C, patch2 % 256, patch2 // 256 # JMP PATCH2
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])
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# LL9 (Part 11)
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#
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# This is the modification at LL80.
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#
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# We blank out the .LL80 section with 28 NOPs
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insert_nops(data_block, 0xA13F + 0x900, 28)
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# LL9 (Part 11)
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#
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# We have already assembled the modified part 11 in BeebAsm and saved it as
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# the binary file ll78.bin, so now we drop this over the top of the existing
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# routine (which is exactly the same size).
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insert_binary_file(data_block, 0xA15B + 0x900, "ll78-plus4.bin")
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# LL9 (Part 12)
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#
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# We have already assembled the modified part 11 in BeebAsm and saved it as
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# the binary file ll115.bin, so now we drop this over the top of the existing
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# routine (which is slightly longer, so there is room).
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insert_binary_file(data_block, 0xA178 + 0x900, "ll155-plus4.bin")
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# We now load the three extra routines required by the modifications into the
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# memory used by the Trumble sprites, which are not used in the Plus/4 version:
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#
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# LLX30
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# PATCH1
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# PATCH2
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insert_binary_file(data_block, 0x7240, "extra-plus4.bin")
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# All the modifications are done, so write the output file for the modified PRG
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output_file = open("elite_+4_modified.prg", "wb")
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output_file.write(data_block)
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output_file.close()
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print("[ Save ] elite_+4_modified.prg")
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