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Added patched version for Commodore Plus/4

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Mark Moxon 2022-09-06 10:25:23 +01:00
commit f5a52fcab5
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src/elite-flicker-free-plus4.asm Normal file
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\ ******************************************************************************
\
\ BBC MASTER ELITE GAME SOURCE (FLICKER-FREE ROUTINES)
\
\ BBC Master Elite was written by Ian Bell and David Braben and is copyright
\ Acornsoft 1986
\
\ The code on this site has been reconstructed from a disassembly of the version
\ released on Ian Bell's personal website at http://www.elitehomepage.org/
\
\ The commentary is copyright Mark Moxon, and any misunderstandings or mistakes
\ in the documentation are entirely my fault
\
\ The terminology and notations used in this commentary are explained at
\ https://www.bbcelite.com/about_site/terminology_used_in_this_commentary.html
\
\ The deep dive articles referred to in this commentary can be found at
\ https://www.bbcelite.com/deep_dives
\
\ ******************************************************************************
GUARD &CE00 \ Guard against assembling over memory used by game
\ ******************************************************************************
\
\ Configuration variables
\
\ 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).
\
\ XX14 is an unused variable in BBC Micro Elite, and the corresponding address
\ in Commodore 64 Elite is used by something else. Luckily locations $FB to $FE
\ are unused by Elite and the OS, so we can sneak XX14 in there instead.
\
\ Also, the Y variable contains the height of the space view in pixels, divided
\ by 2. This is 96 on the BBC Micro, but the space view is 144 pixels on the
\ Commodore 64, so Y needs to be set to 72 instead.
\
\ ******************************************************************************
XX1 = $0009
INWK = $0009
XX19 = $002A
CNT = $0030
K3 = $0035
K4 = $0043
XX0 = $0057
V = $005B
XX15 = $006B
X1 = $006B
Y1 = $006C
X2 = $006D
Y2 = $006E
XX12 = $0071
XX17 = $009F
XX4 = $00AD
XX20 = $00AE
XX14 = $00FB
PROJ = $860F \ Note, this is not equal to $7D1F + $0900
LL75 = $9FB8 + $0900
LL30 = $B49D \ Note, this is not equal to $AB91 + $0900
Y = 72
\ ******************************************************************************
\
\ Name: SHPPT
\ Type: Subroutine
\ Category: Drawing ships
\ Summary: Draw a distant ship as a point rather than a full wireframe
\
\ ******************************************************************************
ORG $9932 + $0900
.SHPPT
JSR PROJ \ Project the ship onto the screen, returning:
\
\ * K3(1 0) = the screen x-coordinate
\ * K4(1 0) = the screen y-coordinate
\ * A = K4+1
ORA K3+1 \ If either of the high bytes of the screen coordinates
BNE nono \ are non-zero, jump to nono as the ship is off-screen
LDA K4 \ Set A = the y-coordinate of the dot
CMP #Y*2-2 \ If the y-coordinate is bigger than the y-coordinate of
BCS nono \ the bottom of the screen, jump to nono as the ship's
\ dot is off the bottom of the space view
JSR Shpt \ Call Shpt to draw a horizontal 4-pixel dash for the
\ first row of the dot (i.e. a four-pixel dash)
LDA K4 \ Set A = y-coordinate of dot + 1 (so this is the second
CLC \ row of the two-pixel-high dot)
ADC #1
JSR Shpt \ Call Shpt to draw a horizontal 4-pixel dash for the
\ first row of the dot (i.e. a four-pixel dash)
LDA #%00001000 \ Set bit 3 of the ship's byte #31 to record that we
ORA XX1+31 \ have now drawn something on-screen for this ship
STA XX1+31
JMP LL155 \ Jump to LL155 to draw any remaining lines that are
\ still in the ship line heap and return from the
\ subroutine using a tail call
.nono
LDA #%11110111 \ Clear bit 3 of the ship's byte #31 to record that
AND XX1+31 \ nothing is being drawn on-screen for this ship
STA XX1+31
JMP LL155 \ Jump to LL155 to draw any remaining lines that are
\ still in the ship line heap and return from the
\ subroutine using a tail call
.Shpt
\ This routine draws a horizontal 4-pixel dash, for
\ either the top or the bottom of the ship's dot
STA Y1 \ Store A in both y-coordinates, as this is a horizontal
STA Y2 \ dash at y-coordinate A
LDA K3 \ Set A = screen x-coordinate of the ship dot
STA X1 \ Store the x-coordinate of the ship dot in X1, as this
\ is where the dash starts
CLC \ Set A = screen x-coordinate of the ship dot + 3
ADC #3
BCC P%+4 \ If the addition overflowed, set A = 255, the
LDA #255 \ x-coordinate of the right edge of the screen
STA X2 \ Store the x-coordinate of the ship dot in X1, as this
\ is where the dash starts
JMP LLX30 \ Draw this edge using flicker-free animation, by first
\ drawing the ship's new line and then erasing the
\ corresponding old line from the screen, and return
\ from the subroutine using a tail call
SAVE "shppt-plus4.bin", SHPPT, P%
\ ******************************************************************************
\
\ Name: LL9 (Part 11 of 12)
\ Type: Subroutine
\ Category: Drawing ships
\ Summary: Draw ship: Loop back for the next edge
\ Deep dive: Drawing ships
\
\ ******************************************************************************
ORG $A15B + $0900
.LL78
LDA XX14 \ If XX14 >= CNT, skip to LL81 so we don't loop back for
CMP CNT \ the next edge (CNT was set to the maximum heap size
BCS LL81 \ for this ship in part 10, so this checks whether we
\ have just run out of space in the ship line heap, and
\ stops drawing edges if we have)
LDA V \ Increment V by 4 so V(1 0) points to the data for the
CLC \ next edge
ADC #4
STA V
BCC ll81 \ If the above addition didn't overflow, jump to ll81
INC V+1 \ Otherwise increment the high byte of V(1 0), as we
\ just moved the V(1 0) pointer past a page boundary
.ll81
INC XX17 \ Increment the edge counter to point to the next edge
LDY XX17 \ If Y >= XX20, which contains the number of edges in
CPY XX20 \ the blueprint, skip the following
BCS P%+5
JMP LL75-2 \ Loop back to LL75-2 to process the next edge (we jump
\ to LL75-2 as we have modified the code around LL75,
\ which moves the jump point back by two bytes)
.LL81
NOP
SAVE "ll78-plus4.bin", LL78, P%
\ ******************************************************************************
\
\ Name: LL9 (Part 12 of 12)
\ Type: Subroutine
\ Category: Drawing ships
\ Summary: Draw ship: Draw all the visible edges from the ship line heap
\ Deep dive: Drawing ships
\
\ ******************************************************************************
ORG $A178 + $0900
.LL155
LDY XX14 \ Set Y to the offset in the line heap XX14
.LL27
CPY XX14+1 \ If Y >= XX14+1, jump to LLEX to return from the ship
BCS LLEX \ drawing routine, because the index in Y is greater
\ than the size of the existing ship line heap, which
\ means we have alrady erased all the old ships lines
\ when drawing the new ship
\ If we get here then Y < XX14+1, which means Y is
\ pointing to an on-screen line from the old ship that
\ we need to erase
LDA (XX19),Y \ Fetch the X1 line coordinate from the heap and store
STA XX15 \ it in XX15
INY \ Increment the heap pointer
LDA (XX19),Y \ Fetch the Y1 line coordinate from the heap and store
STA XX15+1 \ it in XX15+1
INY \ Increment the heap pointer
LDA (XX19),Y \ Fetch the X2 line coordinate from the heap and store
STA XX15+2 \ it in XX15+2
INY \ Increment the heap pointer
LDA (XX19),Y \ Fetch the Y2 line coordinate from the heap and store
STA XX15+3 \ it in XX15+3
JSR LL30 \ Draw a line from (X1, Y1) to (X2, Y2) to erase it from
\ the screen
INY \ Increment the heap pointer
JMP LL27 \ Loop back to LL27 to draw (i.e. erase) the next line
\ from the heap
.LLEX
LDA XX14 \ Store XX14 in the first byte of the ship line heap
LDY #0
STA (XX19),Y
.LL82
RTS \ Return from the subroutine
SAVE "ll155-plus4.bin", LL155, P%
ORG $7240
\ ******************************************************************************
\
\ Name: LLX30
\ Type: Subroutine
\ Category: Drawing lines
\ Summary: Draw a ship line using flicker-free animation
\
\ ******************************************************************************
.LLX30
LDY XX14 \ Set Y = XX14, to get the offset within the ship line
\ heap where we want to insert our new line
CPY XX14+1 \ Compare XX14 and XX14+1 and store the flags on the
PHP \ stack so we can retrieve them later
LDX #3 \ We now want to copy the line coordinates (X1, Y1) and
\ (X2, Y2) to XX12...XX12+3, so set a counter to copy
\ 4 bytes
.LLXL
LDA X1,X \ Copy the X-th byte of X1/Y1/X2/Y2 to the X-th byte of
STA XX12,X \ XX12
DEX \ Decrement the loop counter
BPL LLXL \ Loop back until we have copied all four bytes
JSR LL30 \ Draw a line from (X1, Y1) to (X2, Y2)
LDA (XX19),Y \ Set X1 to the Y-th coordinate on the ship line heap,
STA X1 \ i.e. one we are replacing in the heap
LDA XX12 \ Replace it with the X1 coordinate in XX12
STA (XX19),Y
INY \ Increment the index to point to the Y1 coordinate
LDA (XX19),Y \ Set Y1 to the Y-th coordinate on the ship line heap,
STA Y1 \ i.e. one we are replacing in the heap
LDA XX12+1 \ Replace it with the Y1 coordinate in XX12+1
STA (XX19),Y
INY \ Increment the index to point to the X2 coordinate
LDA (XX19),Y \ Set X1 to the Y-th coordinate on the ship line heap,
STA X2
LDA XX12+2 \ Replace it with the X2 coordinate in XX12+2
STA (XX19),Y
INY \ Increment the index to point to the Y2 coordinate
LDA (XX19),Y \ Set Y2 to the Y-th coordinate on the ship line heap,
STA Y2
LDA XX12+3 \ Replace it with the Y2 coordinate in XX12+3
STA (XX19),Y
INY \ Increment the index to point to the next coordinate
STY XX14 \ and store the updated index in XX14
PLP \ Restore the result of the comparison above, so if the
BCS LL82a \ original value of XX14 >= XX14+1, then we have already
\ redrawn all the lines from the old ship's line heap,
\ so return from the subroutine (as LL82 contains an
\ RTS)
JMP LL30 \ Otherwise there are still more lines to erase from the
\ old ship on-screen, so the coordinates in (X1, Y1) and
\ (X2, Y2) that we just pulled from the ship line heap
\ point to a line that is still on-screen, so call LL30
\ to draw this line and erase it from the screen,
\ returning from the subroutine using a tail call
.LL82a
RTS \ Return from the subroutine
\ ******************************************************************************
\
\ Name: LL9 (Part 1 of 12)
\ Type: Subroutine
\ Category: Drawing ships
\ Summary: Draw ship: Check if ship is exploding, check if ship is in front
\ Deep dive: Drawing ships
\
\ ******************************************************************************
.PATCH1
\ We replace the following two instructions in part 1 of
\ LL9 with JSR PATCH1, so we start with those two
\ instructions to ensure that they still get done
LDA #31 \ Set XX4 = 31 to store the ship's distance for later
STA XX4 \ comparison with the visibility distance. We will
\ update this value below with the actual ship's
\ distance if it turns out to be visible on-screen
\ We now set things up for flicker-free ship plotting,
\ by setting the following:
\
\ XX14 = offset to the first coordinate in the ship's
\ line heap
\
\ XX14+1 = the number of bytes in the heap for the
\ ship that's currently on-screen (or 0 if
\ there is no ship currently on-screen)
LDY #1 \ Set XX14 = 1, the offset of the first set of line
STY XX14 \ coordinates in the ship line heap
DEY \ Decrement Y to 0
LDA #%00001000 \ If bit 3 of the ship's byte #31 is set, then the ship
BIT INWK+31 \ is currently being drawn on-screen, so skip the
BNE P%+5 \ following two instructions
LDA #0 \ The ship is not being drawn on screen, so set A = 0
\ so that XX14+1 gets set to 0 below (as there are no
\ existing coordinates on the ship line heap for this
\ ship)
EQUB $2C \ Skip the next instruction by turning it into
\ $2C $B1 $BD, or BIT $BDB1 which does nothing apart
\ from affect the flags
LDA (XX19),Y \ Set XX14+1 to the first byte of the ship's line heap,
STA XX14+1 \ which contains the number of bytes in the heap
RTS
\ ******************************************************************************
\
\ Name: LL9 (Part 10 of 12)
\ Type: Subroutine
\ Category: Drawing ships
\ Summary: Draw ship: Check if ship is exploding, check if ship is in front
\ Deep dive: Drawing ships
\
\ ******************************************************************************
.PATCH2
\ We replace the JMP LL78 instruction at the end of part
\ 10 of LL9 with JSR PATCH2, so this effectively inserts
\ the call to LLX30 at the end of part 10, as required
JSR LLX30 \ Draw the laser line using flicker-free animation, by
\ first drawing the new laser line and then erasing the
\ corresponding old line from the screen
JMP LL78 \ Jump down to part 11
SAVE "extra-plus4.bin", LLX30, P%

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src/elite-flicker-free.asm
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@ -61,7 +61,6 @@ XX20 = $00AE
XX14 = $00FB
PROJ = $7D1F
LL75 = $9FB8
LL80 = $A13F
LL30 = $AB91
Y = 72

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src/elite-modify-plus4.py Normal file
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#!/usr/bin/env python
#
# ******************************************************************************
#
# COMMODORE PLUS/4 ELITE FLICKER-FREE MODIFICATION SCRIPT
#
# Written by Mark Moxon
#
# This script applies flicker-free ship-drawing to Commodore Plus/4} Elite, as
# described here:
#
# https://www.bbcelite.com/deep_dives/flicker-free_ship_drawing.html
#
# It does the following:
#
# * Modify the PRG file to draw flicker-free ships
#
# 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 following disk image, which is the
# Pigmy Plus/4 version from Ian Bell's site, with the demo and packing removed:
#
# * elite_+4_unpacked.prg
#
# ******************************************************************************
from __future__ import print_function
import os
# 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))
# Print a progess message
print()
print("Modifying Commodore Plus/4 Elite")
# Configuration variables
load_address = 0x1100 - 2
# 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("elite_+4_unpacked.prg", "rb")
data_block.extend(elite_file.read())
elite_file.close()
print()
print("[ Read ] elite_+4_unpacked.prg")
# Set the addresses for the extra routines (LLX30, PATCH1, PATCH2) that we will
# load into unused portiong of the main game code
llx30 = 0x7240
patch1 = 0x727E
patch2 = 0x7295
# 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 + 0x900, "shppt-plus4.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 + 0x900, [
0x20, patch1 % 256, patch1 // 256 # JSR PATCH1
])
insert_nops(data_block, 0x9A8D + 0x900, 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 + 0x900, [
0xA0, 0x09, # LDY #9
0xB1, 0x57, # LDA (XX0),Y
0x85, 0xAE # STA XX20
])
insert_nops(data_block, 0x9F30 + 0x900, 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 + 0x900, [
0xA0, 0x00, # LDY #0
0x84, 0x9F # STY XX17
])
insert_nops(data_block, 0x9F3D + 0x900, 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 + 0x900, [
0x20, llx30 % 256, llx30 // 256 # JSR LLX30
])
insert_nops(data_block, 0x9F8A + 0x900, 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 + 0x900, [
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 + 0x900, 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 + 0x900, [
0xC8, # INY
0xB1, 0x5B, # LDA (V),Y
0xAA # TAX
])
lda_sta_block = get_offset(0x9FDD + 0x900)
for n in range(lda_sta_block, lda_sta_block + 4 * 5):
data_block[n] = data_block[n + 4]
insert_bytes(data_block, 0x9FF1 + 0x900, [
0xC8, # INY
0xB1, 0x5B, # LDA (V),Y
0xAA # TAX
])
insert_nops(data_block, 0x9FF5 + 0x900, 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 + 0x900, [
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 + 0x900, 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 + 0x900, "ll78-plus4.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 + 0x900, "ll155-plus4.bin")
# We now load the three extra routines required by the modifications into the
# memory used by the Trumble sprites, which are not used in the Plus/4 version:
#
# LLX30
# PATCH1
# PATCH2
insert_binary_file(data_block, 0x7240, "extra-plus4.bin")
# All the modifications are done, so write the output file for the modified PRG
output_file = open("elite_+4_modified.prg", "wb")
output_file.write(data_block)
output_file.close()
print("[ Save ] elite_+4_modified.prg")