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elite-loader.asm
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elite-loader.asm
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\ ******************************************************************************
\
\ ELITE LOADER SOURCE
\
\ Elite was written by Ian Bell and David Braben and is copyright Acornsoft 1984
\
\ The code on this site is identical to the source discs released on Ian Bell's
\ personal website at http://www.elitehomepage.org/ (it's just been reformatted
\ to be more readable)
\
\ 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://elite.bbcelite.com/terminology
\
\ The deep dive articles referred to in this commentary can be found at
\ https://elite.bbcelite.com/deep_dives
\
\ ------------------------------------------------------------------------------
\
\ This source file produces the following binary file:
\
\ * ELITE.unprot.bin
\
\ after reading in the following files:
\
\ * DIALS.bin
\ * P.ELITE.bin
\ * P.A-SOFT.bin
\ * P.(C)ASFT.bin
\ * WORDS9.bin
\ * PYTHON.bin
\
\ ******************************************************************************
INCLUDE "1-source-files/main-sources/elite-build-options.asm"
_SOURCE_DISC = (_VARIANT = 1)
_TEXT_SOURCES = (_VARIANT = 2)
_STH_CASSETTE = (_VARIANT = 3)
GUARD &6000 \ Guard against assembling over screen memory
\ ******************************************************************************
\
\ Configuration variables
\
\ ******************************************************************************
DISC = _DISC \ Set to TRUE to load the code above DFS and relocate
\ down, so we can load the cassette version from disc,
\ or set to FALSE to load the code as if it were from a
\ cassette
PROT = _PROT \ Set to TRUE to enable the tape protection code, or set
\ to FALSE to disable the code (for TRUE, the file must
\ be saved to tape with the block data corrupted, so you
\ probably want to leave this as FALSE)
IF DISC
CODE% = &1100 \ CODE% is set to the assembly address of the loader
\ code file that we assemble in this file ("ELITE"),
\ which is at the lowest DFS page value of &1100 for the
\ version that loads from disc
LOAD% = &1100 \ LOAD% is the load address of the main game code file
\ ("ELTcode" for loading from disc, "ELITEcode" for
\ loading from tape)
L% = &1128 \ L% points to the start of the actual game code from
\ elite-source.asm, after the &28 bytes of header code
\ that are inserted by elite-bcfs.asm
ELSE
CODE% = &0E00 \ CODE% is set to the assembly address of the loader
\ code file that we assemble in this file ("ELITE"),
\ which is at the standard &0E00 address for the version
\ that loads from cassette
LOAD% = &0F1F \ LOAD% is the load address of the main game code file
\ ("ELTcode" for loading from disc, "ELITEcode" for
\ loading from tape)
L% = &0F47 \ L% points to the start of the actual game code from
\ elite-source.asm, after the &28 bytes of header code
\ that are inserted by elite-bcfs.asm
ENDIF
LEN1 = 15 \ Size of the BEGIN% routine that gets pushed onto the
\ stack and executed there
LEN2 = 18 \ Size of the MVDL routine that gets pushed onto the
\ stack and executed there
LEN = LEN1 + LEN2 \ Total number of bytes that get pushed on the stack for
\ execution there (33)
VSCAN = 57-1 \ Defines the split position in the split-screen mode
LE% = &0B00 \ LE% is the address to which the code from UU% onwards
\ is copied in part 3. It contains:
\
\ * ENTRY2, the entry point for the second block of
\ loader code
\
\ * IRQ1, the interrupt routine for the split-screen
\ mode
\
\ * BLOCK, which by this point has already been put
\ on the stack by this point
\
\ * The variables used by the above
NETV = &0224 \ The NETV vector that we intercept as part of the copy
\ protection
IRQ1V = &0204 \ The IRQ1V vector that we intercept to implement the
\ split-screen mode
OSPRNT = &0234 \ The address for the OSPRNT vector
C% = &0F40 \ C% is set to the location that the main game code gets
\ moved to after it is loaded
S% = C% \ S% points to the entry point for the main game code
IF _SOURCE_DISC
D% = &563A \ D% is set to the address of the byte after the end of
\ the code, i.e. the byte after checksum0 at XX21
ELIF _TEXT_SOURCES
D% = &5638 \ D% is set to the address of the byte after the end of
\ the code, i.e. the byte after checksum0 at XX21
ELIF _STH_CASSETTE
D% = &563A \ D% is set to the address of the byte after the end of
\ the code, i.e. the byte after checksum0 at XX21
ENDIF
LC% = &6000 - C% \ LC% is set to the maximum size of the main game code
\ (as the code starts at C% and screen memory starts
\ at &6000)
N% = 67 \ N% is set to the number of bytes in the VDU table, so
\ we can loop through them in part 2 below
SVN = &7FFD \ SVN is where we store the "saving in progress" flag,
\ and it matches the location in elite-source.asm
VEC = &7FFE \ VEC is where we store the original value of the IRQ1
\ vector, and it matches the value in elite-source.asm
VIA = &FE00 \ Memory-mapped space for accessing internal hardware,
\ such as the video ULA, 6845 CRTC and 6522 VIAs (also
\ known as SHEILA)
OSWRCH = &FFEE \ The address for the OSWRCH routine
OSBYTE = &FFF4 \ The address for the OSBYTE routine
OSWORD = &FFF1 \ The address for the OSWORD routine
\ ******************************************************************************
\
\ Name: ZP
\ Type: Workspace
\ Address: &0004 to &0005 and &0070 to &0086
\ Category: Workspaces
\ Summary: Important variables used by the loader
\
\ ******************************************************************************
ORG &0004
.TRTB%
SKIP 2 \ Contains the address of the keyboard translation
\ table, which is used to translate internal key
\ numbers to ASCII
ORG &0070
.ZP
SKIP 2 \ Stores addresses used for moving content around
.P
SKIP 1 \ Temporary storage, used in a number of places
.Q
SKIP 1 \ Temporary storage, used in a number of places
.YY
SKIP 1 \ Temporary storage, used in a number of places
.T
SKIP 1 \ Temporary storage, used in a number of places
.SC
SKIP 1 \ Screen address (low byte)
\
\ Elite draws on-screen by poking bytes directly into
\ screen memory, and SC(1 0) is typically set to the
\ address of the character block containing the pixel
\ we want to draw (see the deep dives on "Drawing
\ monochrome pixels in mode 4" and "Drawing colour
\ pixels in mode 5" for more details)
.SCH
SKIP 1 \ Screen address (high byte)
.BLPTR
SKIP 2 \ Gets set to &03CA as part of the obfuscation code
.V219
SKIP 2 \ Gets set to &0218 as part of the obfuscation code
SKIP 4 \ These bytes appear to be unused
.K3
SKIP 1 \ Temporary storage, used in a number of places
.BLCNT
SKIP 2 \ Stores the tape loader block count as part of the copy
\ protection code in IRQ1
.BLN
SKIP 2 \ Gets set to &03C6 as part of the obfuscation code
.EXCN
SKIP 2 \ Gets set to &03C2 as part of the obfuscation code
\ ******************************************************************************
\
\ ELITE LOADER
\
\ ******************************************************************************
ORG CODE%
\ ******************************************************************************
\
\ Name: Elite loader (Part 1 of 6)
\ Type: Subroutine
\ Category: Loader
\ Summary: Include binaries for recursive tokens, Python blueprint and images
\
\ ------------------------------------------------------------------------------
\
\ The loader bundles a number of binary files in with the loader code, and moves
\ them to their correct memory locations in part 3 below.
\
\ There are two files containing code:
\
\ * WORDS9.bin contains the recursive token table, which is moved to &0400
\ before the main game is loaded
\
\ * PYTHON.bin contains the Python ship blueprint, which gets moved to &7F00
\ before the main game is loaded
\
\ and four files containing images, which are all moved into screen memory by
\ the loader:
\
\ * P.A-SOFT.bin contains the "ACORNSOFT" title across the top of the loading
\ screen, which gets moved to screen address &6100, on the second character
\ row of the monochrome mode 4 screen
\
\ * P.ELITE.bin contains the "ELITE" title across the top of the loading
\ screen, which gets moved to screen address &6300, on the fourth character
\ row of the monochrome mode 4 screen
\
\ * P.(C)ASFT.bin contains the "(C) Acornsoft 1984" title across the bottom
\ of the loading screen, which gets moved to screen address &7600, the
\ penultimate character row of the monochrome mode 4 screen, just above the
\ dashboard
\
\ * P.DIALS.bin contains the dashboard, which gets moved to screen address
\ &7800, which is the starting point of the four-colour mode 5 portion at
\ the bottom of the split screen
\
\ The routine ends with a jump to the start of the loader code at ENTRY.
\
\ ******************************************************************************
PRINT "WORDS9 = ",~P%
INCBIN "3-assembled-output/WORDS9.bin"
ALIGN 256
PRINT "P.DIALS = ",~P%
INCBIN "1-source-files/images/P.DIALS.bin"
PRINT "PYTHON = ",~P%
INCBIN "3-assembled-output/PYTHON.bin"
PRINT "P.ELITE = ",~P%
INCBIN "1-source-files/images/P.ELITE.bin"
PRINT "P.A-SOFT = ",~P%
INCBIN "1-source-files/images/P.A-SOFT.bin"
PRINT "P.(C)ASFT = ",~P%
INCBIN "1-source-files/images/P.(C)ASFT.bin"
.run
JMP ENTRY \ Jump to ENTRY to start the loading process
\ ******************************************************************************
\
\ Name: B%
\ Type: Variable
\ Category: Drawing the screen
\ Summary: VDU commands for setting the square mode 4 screen
\ Deep dive: The split-screen mode in BBC Micro Elite
\ Drawing monochrome pixels in mode 4
\
\ ------------------------------------------------------------------------------
\
\ This block contains the bytes that get written by OSWRCH to set up the screen
\ mode (this is equivalent to using the VDU statement in BASIC).
\
\ It defines the whole screen using a square, monochrome mode 4 configuration;
\ the mode 5 part for the dashboard is implemented in the IRQ1 routine.
\
\ The top part of Elite's screen mode is based on mode 4 but with the following
\ differences:
\
\ * 32 columns, 31 rows (256 x 248 pixels) rather than 40, 32
\
\ * The horizontal sync position is at character 45 rather than 49, which
\ pushes the screen to the right (which centres it as it's not as wide as
\ the normal screen modes)
\
\ * Screen memory goes from &6000 to &7EFF, which leaves another whole page
\ for code (i.e. 256 bytes) after the end of the screen. This is where the
\ Python ship blueprint slots in
\
\ * The text window is 1 row high and 13 columns wide, and is at (2, 16)
\
\ * The cursor is disabled
\
\ This almost-square mode 4 variant makes life a lot easier when drawing to the
\ screen, as there are 256 pixels on each row (or, to put it in screen memory
\ terms, there's one page of memory per row of pixels). For more details of the
\ screen mode, see the deep dive on "Drawing monochrome pixels in mode 4".
\
\ There is also an interrupt-driven routine that switches the bytes-per-pixel
\ setting from that of mode 4 to that of mode 5, when the raster reaches the
\ split between the space view and the dashboard. See the deep dive on "The
\ split-screen mode" for details.
\
\ ******************************************************************************
.B%
EQUB 22, 4 \ Switch to screen mode 4
EQUB 28 \ Define a text window as follows:
EQUB 2, 17, 15, 16 \
\ * Left = 2
\ * Right = 15
\ * Top = 16
\ * Bottom = 17
\
\ i.e. 1 row high, 13 columns wide at (2, 16)
EQUB 23, 0, 6, 31 \ Set 6845 register R6 = 31
EQUB 0, 0, 0 \
EQUB 0, 0, 0 \ This is the "vertical displayed" register, and sets
\ the number of displayed character rows to 31. For
\ comparison, this value is 32 for standard modes 4 and
\ 5, but we claw back the last row for storing code just
\ above the end of screen memory
EQUB 23, 0, 12, &0C \ Set 6845 register R12 = &0C and R13 = &00
EQUB 0, 0, 0 \
EQUB 0, 0, 0 \ This sets 6845 registers (R12 R13) = &0C00 to point
EQUB 23, 0, 13, &00 \ to the start of screen memory in terms of character
EQUB 0, 0, 0 \ rows. There are 8 pixel lines in each character row,
EQUB 0, 0, 0 \ so to get the actual address of the start of screen
\ memory, we multiply by 8:
\
\ &0C00 * 8 = &6000
\
\ So this sets the start of screen memory to &6000
EQUB 23, 0, 1, 32 \ Set 6845 register R1 = 32
EQUB 0, 0, 0 \
EQUB 0, 0, 0 \ This is the "horizontal displayed" register, which
\ defines the number of character blocks per horizontal
\ character row. For comparison, this value is 40 for
\ modes 4 and 5, but our custom screen is not as wide at
\ only 32 character blocks across
EQUB 23, 0, 2, 45 \ Set 6845 register R2 = 45
EQUB 0, 0, 0 \
EQUB 0, 0, 0 \ This is the "horizontal sync position" register, which
\ defines the position of the horizontal sync pulse on
\ the horizontal line in terms of character widths from
\ the left-hand side of the screen. For comparison this
\ is 49 for modes 4 and 5, but needs to be adjusted for
\ our custom screen's width
EQUB 23, 0, 10, 32 \ Set 6845 register R10 = %00100000 = 32
EQUB 0, 0, 0 \
EQUB 0, 0, 0 \ This is the "cursor start" register, and bits 5 and 6
\ define the "cursor display mode", as follows:
\
\ * %00 = steady, non-blinking cursor
\
\ * %01 = do not display a cursor
\
\ * %10 = fast blinking cursor (blink at 1/16 of the
\ field rate)
\
\ * %11 = slow blinking cursor (blink at 1/32 of the
\ field rate)
\
\ We can therefore turn off the cursor completely by
\ setting cursor display mode %01, with bit 6 of R10
\ clear and bit 5 of R10 set
\ ******************************************************************************
\
\ Name: E%
\ Type: Variable
\ Category: Sound
\ Summary: Sound envelope definitions
\
\ ------------------------------------------------------------------------------
\
\ This table contains the sound envelope data, which is passed to OSWORD by the
\ FNE macro to create the four sound envelopes used in-game. Refer to chapter 30
\ of the BBC Micro User Guide for details of sound envelopes and what all the
\ parameters mean.
\
\ The envelopes are as follows:
\
\ * Envelope 1 is the sound of our own laser firing
\
\ * Envelope 2 is the sound of lasers hitting us, or hyperspace
\
\ * Envelope 3 is the first sound in the two-part sound of us dying, or the
\ second sound in the two-part sound of us hitting or killing an enemy ship
\
\ * Envelope 4 is the sound of E.C.M. firing
\
\ ******************************************************************************
.E%
EQUB 1, 1, 0, 111, -8, 4, 1, 8, 8, -2, 0, -1, 112, 44
EQUB 2, 1, 14, -18, -1, 44, 32, 50, 6, 1, 0, -2, 120, 126
EQUB 3, 1, 1, -1, -3, 17, 32, 128, 1, 0, 0, -1, 1, 1
EQUB 4, 1, 4, -8, 44, 4, 6, 8, 22, 0, 0, -127, 126, 0
\ ******************************************************************************
\
\ Name: swine
\ Type: Subroutine
\ Category: Copy protection
\ Summary: Resets the machine if the copy protection detects a problem
\
\ ******************************************************************************
.swine
LDA #%01111111 \ Set 6522 System VIA interrupt enable register IER
STA &FE4E \ (SHEILA &4E) bits 0-6 (i.e. disable all hardware
\ interrupts from the System VIA)
JMP (&FFFC) \ Jump to the address in &FFFC to reset the machine
\ ******************************************************************************
\
\ Name: OSB
\ Type: Subroutine
\ Category: Utility routines
\ Summary: A convenience routine for calling OSBYTE with Y = 0
\
\ ******************************************************************************
.OSB
LDY #0 \ Call OSBYTE with Y = 0, returning from the subroutine
JMP OSBYTE \ using a tail call (so we can call OSB to call OSBYTE
\ for when we know we want Y set to 0)
\ ******************************************************************************
\
\ Name: Authors' names
\ Type: Variable
\ Category: Copy protection
\ Summary: The authors' names, buried in the code
\
\ ------------------------------------------------------------------------------
\
\ Contains the authors' names, plus an unused OS command string that would
\ *RUN the main game code, which isn't what actually happens (so presumably
\ this is to throw the crackers off the scent).
\
\ ******************************************************************************
EQUS "R.ELITEcode" \ This is short for "*RUN ELITEcode"
EQUB 13
EQUS "By D.Braben/I.Bell"
EQUB 13
EQUB &B0
\ ******************************************************************************
\
\ Name: oscliv
\ Type: Variable
\ Category: Utility routines
\ Summary: Contains the address of OSCLIV, for executing OS commands
\
\ ******************************************************************************
.oscliv
EQUW &FFF7 \ Address of OSCLIV, for executing OS commands
\ (specifically the *LOAD that loads the main game code)
\ ******************************************************************************
\
\ Name: David9
\ Type: Variable
\ Category: Copy protection
\ Summary: Address used as part of the stack-based decryption loop
\
\ ------------------------------------------------------------------------------
\
\ This address is used in the decryption loop starting at David2 in part 4, and
\ is used to jump back into the loop at David5.
\
\ ******************************************************************************
.David9
EQUW David5 \ The address of David5
CLD \ This instruction is not used
\ ******************************************************************************
\
\ Name: David23
\ Type: Variable
\ Category: Copy protection
\ Summary: Address pointer to the start of the 6502 stack
\
\ ------------------------------------------------------------------------------
\
\ This two-byte address points to the start of the 6502 stack, which descends
\ from the end of page 2, less LEN bytes, which comes out as &01DF. So when we
\ push 33 bytes onto the stack (LEN being 33), this address will point to the
\ start of those bytes, which means we can push executable code onto the stack
\ and run it by calling this address with a JMP (David23) instruction. Sneaky
\ stuff!
\
\ ******************************************************************************
.David23
EQUW (512-LEN) \ The address of LEN bytes before the start of the stack
\ ******************************************************************************
\
\ Name: doPROT1
\ Type: Subroutine
\ Category: Copy protection
\ Summary: Routine to self-modify the loader code
\
\ ------------------------------------------------------------------------------
\
\ This routine modifies various bits of code in-place as part of the copy
\ protection mechanism. It is called with A = &48 and X = 255.
\
\ ******************************************************************************
.doPROT1
LDY #&DB \ Store &EFDB in TRTB%(1 0) to point to the keyboard
STY TRTB% \ translation table for OS 0.1 (which we will overwrite
LDY #&EF \ with a call to OSBYTE later)
STY TRTB%+1
LDY #2 \ Set the high byte of V219(1 0) to 2
STY V219+1
STA PROT1-255,X \ Poke &48 into PROT1, which changes the instruction
\ there to a PHA
LDY #&18 \ Set the low byte of V219(1 0) to &18 (as X = 255), so
STY V219+1,X \ V219(1 0) now contains &0218
RTS \ Return from the subroutine
\ ******************************************************************************
\
\ Name: MHCA
\ Type: Variable
\ Category: Copy protection
\ Summary: Used to set one of the vectors in the copy protection code
\
\ ------------------------------------------------------------------------------
\
\ This value is used to set the low byte of BLPTR(1 0), when it's set in PLL1
\ as part of the copy protection.
\
\ ******************************************************************************
.MHCA
EQUB &CA \ The low byte of BLPTR(1 0)
\ ******************************************************************************
\
\ Name: David7
\ Type: Subroutine
\ Category: Copy protection
\ Summary: Part of the multi-jump obfuscation code in PROT1
\
\ ------------------------------------------------------------------------------
\
\ This instruction is part of the multi-jump obfuscation in PROT1 (see part 2 of
\ the loader), which does the following jumps:
\
\ David8 -> FRED1 -> David7 -> Ian1 -> David3
\
\ ******************************************************************************
.David7
BCC Ian1 \ This instruction is part of the multi-jump obfuscation
\ in PROT1
\ ******************************************************************************
\
\ Name: FNE
\ Type: Macro
\ Category: Sound
\ Summary: Macro definition for defining a sound envelope
\
\ ------------------------------------------------------------------------------
\
\ The following macro is used to define the four sound envelopes used in the
\ game. It uses OSWORD 8 to create an envelope using the 14 parameters in the
\ I%-th block of 14 bytes at location E%. This OSWORD call is the same as BBC
\ BASIC's ENVELOPE command.
\
\ See variable E% for more details of the envelopes themselves.
\
\ ******************************************************************************
MACRO FNE I%
LDX #LO(E%+I%*14) \ Set (Y X) to point to the I%-th set of envelope data
LDY #HI(E%+I%*14) \ in E%
LDA #8 \ Call OSWORD with A = 8 to set up sound envelope I%
JSR OSWORD
ENDMACRO
\ ******************************************************************************
\
\ Name: Elite loader (Part 2 of 6)
\ Type: Subroutine
\ Category: Loader
\ Summary: Perform a number of OS calls, set up sound, push routines on stack
\
\ ------------------------------------------------------------------------------
\
\ This part of the loader does a number of calls to OS routines, sets up the
\ sound envelopes, pushes 33 bytes onto the stack that will be used later, and
\ sends us on a wild goose chase, just for kicks.
\
\ ------------------------------------------------------------------------------
\
\ Other entry points:
\
\ Ian1 Re-entry point following the wild goose chase
\ obfuscation
\
\ ******************************************************************************
.ENTRY
SEI \ Disable all interrupts
CLD \ Clear the decimal flag, so we're not in decimal mode
IF NOT(DISC)
LDA #0 \ Call OSBYTE with A = 0 and X = 255 to fetch the
LDX #255 \ operating system version into X
JSR OSBYTE
TXA \ If X = 0 then this is OS 1.00, so jump down to OS100
BEQ OS100 \ to skip the following
LDY &FFB6 \ Otherwise this is OS 1.20, so set Y to the contents of
\ &FFB6, which contains the length of the default vector
\ table
LDA &FFB7 \ Set ZP(1 0) to the location stored in &FFB7-&FFB8,
STA ZP \ which contains the address of the default vector table
LDA &FFB8
STA ZP+1
DEY \ Decrement Y so we can use it as an index for setting
\ all the vectors to their default states
.ABCDEFG
LDA (ZP),Y \ Copy the Y-th byte from the default vector table into
STA &0200,Y \ the vector table in &0200
DEY \ Decrement the loop counter
BPL ABCDEFG \ Loop back for the next vector until we have done them
\ all
.OS100
ENDIF
LDA #%01111111 \ Set 6522 System VIA interrupt enable register IER
STA &FE4E \ (SHEILA &4E) bits 0-6 (i.e. disable all hardware
\ interrupts from the System VIA)
STA &FE6E \ Set 6522 User VIA interrupt enable register IER
\ (SHEILA &6E) bits 0-6 (i.e. disable all hardware
\ interrupts from the User VIA)
LDA &FFFC \ Fetch the low byte of the reset address in &FFFC,
\ which will reset the machine if called
STA &0200 \ Set the low bytes of USERV, BRKV, IRQ2V and EVENTV
STA &0202
STA &0206
STA &0220
LDA &FFFD \ Fetch the high byte of the reset address in &FFFD,
\ which will reset the machine if called
STA &0201 \ Set the high bytes of USERV, BRKV, IRQ2V and EVENTV
STA &0203
STA &0207
STA &0221
LDX #&2F-2 \ We now step through all the vectors from &0204 to
\ &022F and OR their high bytes with &C0, so they all
\ point into the MOS ROM space (which is from &C000 and
\ upwards), so we set a counter in X to count through
\ them
.purge
LDA &0202,X \ Set the high byte of the vector in &0202+X so it
ORA #&C0 \ points to the MOS ROM
STA &0202,X
DEX \ Increment the counter to point to the next high byte
DEX
BPL purge \ Loop back until we have done all the vectors
LDA #&60 \ Store an RTS instruction in location &0232
STA &0232
LDA #&2 \ Point the NETV vector to &0232, which we just filled
STA NETV+1 \ with an RTS
LDA #&32
STA NETV
LDA #&20 \ Set A to the op code for a JSR call with absolute
\ addressing
EQUB &2C \ Skip the next instruction by turning it into
\ &2C &D0 &66, or BIT &66D0, which does nothing apart
\ from affect the flags
.Ian1
BNE David3 \ This instruction is skipped if we came from above,
\ otherwise this is part of the multi-jump obfuscation
\ in PROT1
STA David2 \ Store &20 in location David2, which modifies the
\ instruction there (see David2 for details)
LSR A \ Set A = 16
LDX #3 \ Set the high bytes of BLPTR(1 0), BLN(1 0) and
STX BLPTR+1 \ EXCN(1 0) to &3. We will fill in the high bytes in
STX BLN+1 \ the PLL1 routine, and will then use these values in
STX EXCN+1 \ the IRQ1 handler
DEX \ Set X = 2
JSR OSBYTE \ Call OSBYTE with A = 16 and X = 2 to set the ADC to
\ sample 2 channels from the joystick
EQUB &2C \ Skip the next instruction by turning it into
\ &2C &D0 &A1, or BIT &A1D0, which does nothing apart
\ from affect the flags
.FRED1
BNE David7 \ This instruction is skipped if we came from above,
\ otherwise this is part of the multi-jump obfuscation
\ in PROT1
LDX #255 \ Call doPROT1 to change an instruction in the PROT1
LDA #&48 \ routine and set up another couple of variables
JSR doPROT1
LDA #144 \ Call OSBYTE with A = 144, X = 255 and Y = 0 to move
JSR OSB \ the screen down one line and turn screen interlace on
LDA #247 \ Call OSBYTE with A = 247 and X = Y = 0 to disable the
LDX #0 \ BREAK intercept code by poking 0 into the first value
JSR OSB
\LDA #129 \ These instructions are commented out in the original
\LDY #255 \ source, along with the comment "Damn 0.1", so
\LDX #1 \ presumably MOS version 0.1 was a bit of a pain to
\JSR OSBYTE \ support - which is probably why Elite doesn't bother
\TXA \ and only supports 1.0 and 1.2
\BPL OS01
\Damn 0.1
LDA #190 \ Call OSBYTE with A = 190, X = 8 and Y = 0 to set the
LDX #8 \ ADC conversion type to 8 bits, for the joystick
JSR OSB
EQUB &2C \ Skip the next instruction by turning it into
\ &2C &D0 &E1, or BIT &E1D0, which does nothing apart
\ from affect the flags
.David8
BNE FRED1 \ This instruction is skipped if we came from above,
\ otherwise this is part of the multi-jump obfuscation
\ in PROT1
LDA #143 \ Call OSBYTE 143 to issue a paged ROM service call of
LDX #&C \ type &C with argument &FF, which is the "NMI claim"
LDY #&FF \ service call that asks the current user of the NMI
JSR OSBYTE \ space to clear it out
LDA #13 \ Set A = 13 for the next OSBYTE call
.abrk
LDX #0 \ Call OSBYTE with A = 13, X = 0 and Y = 0 to disable
JSR OSB \ the "output buffer empty" event
LDA #225 \ Call OSBYTE with A = 225, X = 128 and Y = 0 to set
LDX #128 \ the function keys to return ASCII codes for SHIFT-fn
JSR OSB \ keys (i.e. add 128)
LDA #172 \ Call OSBYTE 172 to read the address of the MOS
LDX #0 \ keyboard translation table into (Y X)
LDY #255
JSR OSBYTE
STX TRTB% \ Store the address of the keyboard translation table in
STY TRTB%+1 \ TRTB%(1 0)
LDA #200 \ Call OSBYTE with A = 200, X = 3 and Y = 0 to disable
LDX #3 \ the ESCAPE key and clear memory if the BREAK key is
JSR OSB \ pressed
IF PROT AND NOT(DISC)
CPX #3 \ If the previous value of X from the call to OSBYTE 200
BNE abrk+1 \ was not 3 (ESCAPE disabled, clear memory), jump to
\ abrk+1, which contains a BRK instruction which will
\ reset the computer (as we set BRKV to point to the
\ reset address above)
ENDIF
LDA #13 \ Call OSBYTE with A = 13, X = 2 and Y = 0 to disable
LDX #2 \ the "character entering keyboard buffer" event
JSR OSB
.OS01
LDX #&FF \ Set the stack pointer to &01FF, which is the standard
TXS \ location for the 6502 stack, so this instruction
\ effectively resets the stack
INX \ Set X = 0, to use as a counter in the following loop
.David3
LDA BEGIN%,X \ This routine pushes 33 bytes from BEGIN% onto the
\ stack, so fetch the X-th byte from BEGIN%
.PROT1
INY \ This instruction gets changed to a PHA instruction by
\ the doPROT1 routine that's called above, so by the
\ time we get here, this instruction actually pushes the
\ X-th byte from BEGIN% onto the stack
INX \ Increment the loop counter
CPX #LEN \ If X < #LEN (which is 33), loop back for the next one.
BNE David8 \ This branch actually takes us on a wild goose chase
\ through the following locations, where each BNE is
\ prefaced by an EQUB &2C that disables the branch
\ instruction during the normal instruction flow:
\
\ David8 -> FRED1 -> David7 -> Ian1 -> David3
\
\ so in the end this just loops back to push the next
\ byte onto the stack, but in a really sneaky way
LDA #LO(B%) \ Set the low byte of ZP(1 0) to point to the VDU code
STA ZP \ table at B%
LDA #&C8 \ Poke &C8 into PROT1 to change the instruction that we
STA PROT1 \ modified back to an INY instruction, rather than a PHA
LDA #HI(B%) \ Set the high byte of ZP(1 0) to point to the VDU code
STA ZP+1 \ table at B%
LDY #0 \ We are now going to send the N% VDU bytes in the table
\ at B% to OSWRCH to set up the special mode 4 screen
\ that forms the basis for the split-screen mode
.LOOP
LDA (ZP),Y \ Pass the Y-th byte of the B% table to OSWRCH
JSR OSWRCH
INY \ Increment the loop counter
CPY #N% \ Loop back for the next byte until we have done them
BNE LOOP \ all (the number of bytes was set in N% above)
LDA #1 \ In doPROT1 above we set V219(1 0) = &0218, so this
TAX \ code sets the contents of &0219 (the high byte of
TAY \ BPUTV) to 1. We will see why this later, at the start
STA (V219),Y \ of part 4
LDA #4 \ Call OSBYTE with A = 4, X = 1 and Y = 0 to disable
JSR OSB \ cursor editing, so the cursor keys return ASCII values
\ and can therefore be used in-game
LDA #9 \ Call OSBYTE with A = 9, X = 0 and Y = 0 to disable
LDX #0 \ flashing colours
JSR OSB
LDA #&6C \ Poke &6C into crunchit after EOR'ing it first (which
EOR crunchit \ has no effect as crunchit contains a BRK instruction
STA crunchit \ with opcode 0), to change crunchit to an indirect JMP
FNE 0 \ Set up sound envelopes 0-3 using the FNE macro
FNE 1
FNE 2
FNE 3
\ ******************************************************************************
\
\ Name: Elite loader (Part 3 of 6)
\ Type: Subroutine
\ Category: Loader
\ Summary: Move and decrypt recursive tokens, Python blueprint and images
\
\ ------------------------------------------------------------------------------
\
\ Move and decrypt the following memory blocks: