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elite-loader3.asm
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elite-loader3.asm
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\ ******************************************************************************
\
\ DISC ELITE LOADER (PART 3) SOURCE
\
\ Elite was written by Ian Bell and David Braben and is copyright Acornsoft 1984
\
\ 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://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:
\
\ * ELITE4.bin
\
\ ******************************************************************************
INCLUDE "1-source-files/main-sources/elite-build-options.asm"
_IB_DISC = (_VARIANT = 1)
_STH_DISC = (_VARIANT = 2)
_SRAM_DISC = (_VARIANT = 3)
GUARD &6000 \ Guard against assembling over screen memory
\ ******************************************************************************
\
\ Configuration variables
\
\ ******************************************************************************
CODE% = &1900 \ The address where the code will be run
LOAD% = &1900 \ The address where the code will be loaded
Q% = _MAX_COMMANDER \ Set Q% to TRUE to max out the default commander, FALSE
\ for the standard default commander
N% = 67 \ N% is set to the number of bytes in the VDU table, so
\ we can loop through them below
VSCAN = 57 \ Defines the split position in the split-screen mode
POW = 15 \ Pulse laser power
VEC = &7FFE \ VEC is where we store the original value of the IRQ1
\ vector, matching the address in the elite-missile.asm
\ source
BRKV = &0202 \ The break vector that we intercept to enable us to
\ handle and display system errors
IRQ1V = &0204 \ The IRQ1V vector that we intercept to implement the
\ split-screen mode
WRCHV = &020E \ The WRCHV vector that we intercept with our custom
\ text printing routine
NETV = &0224 \ The NETV vector that we intercept as part of the copy
\ protection
LASCT = &0346 \ The laser pulse count for the current laser, matching
\ the address in the main game code
HFX = &0348 \ A flag that toggles the hyperspace colour effect,
\ matching the address in the main game code
ESCP = &0386 \ The flag that determines whether we have an escape pod
\ fitted, matching the address in the main game code
S% = &11E3 \ The address of the main entry point workspace in the
\ main game code
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
OSCLI = &FFF7 \ The address for the OSCLI vector
\ ******************************************************************************
\
\ Name: ZP
\ Type: Workspace
\ Address: &0070 to &008B
\ Category: Workspaces
\ Summary: Important variables used by the loader
\
\ ******************************************************************************
IF _SRAM_DISC
ORG &0004
.TRTB%
SKIP 2 \ Contains the address of the keyboard translation
\ table, which is used to translate internal key
\ numbers to ASCII
ENDIF
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)
.CHKSM
SKIP 2 \ Used in the copy protection code
ORG &008B
.DL
SKIP 1 \ Vertical sync flag
\
\ DL gets set to 30 every time we reach vertical sync on
\ the video system, which happens 50 times a second
\ (50Hz). The WSCAN routine uses this to pause until the
\ vertical sync, by setting DL to 0 and then monitoring
\ its value until it changes to 30
\ ******************************************************************************
\
\ ELITE LOADER
\
\ ******************************************************************************
ORG CODE%
\ ******************************************************************************
\
\ 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, 126, 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: 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 1 of 3)
\ Type: Subroutine
\ Category: Loader
\ Summary: Set up the split screen mode, move code around, set up the sound
\ envelopes and configure the system
\
\ ******************************************************************************
.ENTRY
IF _STH_DISC OR _IB_DISC
JSR PROT1 \ Call PROT1 to calculate checksums into CHKSM
ELIF _SRAM_DISC
JSR PROT4 \ Fetch the address of the keyboard translation table
\ before calling PROT1 to calculate checksums into CHKSM
ENDIF
LDA #144 \ Call OSBYTE with A = 144, X = 255 and Y = 0 to move
LDX #255 \ the screen down one line and turn screen interlace on
JSR OSB
LDA #LO(B%) \ Set the low byte of ZP(1 0) to point to the VDU code
STA ZP \ table at B%
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
.loop1
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 loop1 \ all (the number of bytes was set in N% above)
JSR PLL1 \ Call PLL1 to draw Saturn
LDA #16 \ Call OSBYTE with A = 16 and X = 3 to set the ADC to
LDX #3 \ sample 3 channels from the joystick/Bitstik
JSR OSBYTE
LDA #&60 \ Store an RTS instruction in location &0232
STA &0232
LDA #&02 \ Point the NETV vector to &0232, which we just filled
STA NETV+1 \ with an RTS
LDA #&32
STA NETV
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
IF _STH_DISC OR _IB_DISC
LDA #200 \ Call OSBYTE with A = 200, X = 0 and Y = 0 to enable
LDX #0 \ the ESCAPE key and disable memory clearing if the
JSR OSB \ BREAK key is pressed
ELIF _SRAM_DISC
LDA #219 \ Store 219 in location &9F. This gets checked by the
STA &9F \ TITLE routine in the main docked code as part of the
\ copy protection (the game hangs if it doesn't match)
\
\ This is normally done in the OSBmod routine, but the
\ sideways RAM variant doesn't call OSBmod as that part
\ of the copy protection is disabled, so we set the
\ value of location &BF here instead
NOP \ Pad out the code so it takes up the same amount of
NOP \ space as in the original version
NOP
ENDIF
LDA #13 \ Call OSBYTE with A = 13, X = 0 and Y = 0 to disable
LDX #0 \ the "output buffer empty" event
JSR OSB
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 #12 \ Set A = 12 and X = 0 to pretend that this is an
LDX #0 \ innocent call to OSBYTE to reset the keyboard delay
\ and auto-repeat rate to the default, when in reality
\ the OSB address in the next instruction gets modified
\ to point to OSBmod
.OSBjsr
IF _STH_DISC OR _IB_DISC
JSR OSB \ This JSR gets modified by code inserted into PLL1 so
\ that it points to OSBmod instead of OSB, so this
\ actually calls OSBmod to calculate some checksums
ELIF _SRAM_DISC
NOP \ The sideways RAM variant has this part of the copy
NOP \ protection disabled, so pad out the code so it takes
NOP \ up the same amount of space as in the original version
ENDIF
LDA #13 \ Call OSBYTE with A = 13, X = 2 and Y = 0 to disable
LDX #2 \ the "character entering buffer" event
JSR OSB
LDA #4 \ Call OSBYTE with A = 4, X = 1 and Y = 0 to disable
LDX #1 \ cursor editing, so the cursor keys return ASCII values
JSR OSB \ 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
JSR PROT3 \ Call PROT3 to do more checks on the CHKSM checksum
LDA #&00 \ Set the following:
STA ZP \
LDA #&11 \ ZP(1 0) = &1100
STA ZP+1 \ P(1 0) = TVT1code
LDA #LO(TVT1code)
STA P
LDA #HI(TVT1code)
STA P+1
JSR MVPG \ Call MVPG to move and decrypt a page of memory from
\ TVT1code to &1100-&11FF
LDA #&00 \ Set the following:
STA ZP \
LDA #&78 \ ZP(1 0) = &7800
STA ZP+1 \ P(1 0) = DIALS
LDA #LO(DIALS) \ X = 8
STA P
LDA #HI(DIALS)
STA P+1
LDX #8
JSR MVBL \ Call MVBL to move and decrypt 8 pages of memory from
\ DIALS to &7800-&7FFF
SEI \ Disable interrupts while we set up our interrupt
\ handler to support the split-screen mode
LDA VIA+&44 \ Read the 6522 System VIA T1C-L timer 1 low-order
STA &0001 \ counter (SHEILA &44), which decrements one million
\ times a second and will therefore be pretty random,
\ and store it in location &0001, which is among the
\ main game code's random seeds (so this seeds the
\ random number generator for the main game)
LDA #%00111001 \ Set 6522 System VIA interrupt enable register IER
STA VIA+&4E \ (SHEILA &4E) bits 0 and 3-5 (i.e. disable the Timer1,
\ CB1, CB2 and CA2 interrupts from the System VIA)
LDA #%01111111 \ Set 6522 User VIA interrupt enable register IER
STA VIA+&6E \ (SHEILA &6E) bits 0-7 (i.e. disable all hardware
\ interrupts from the User VIA)
LDA IRQ1V \ Copy the current IRQ1V vector address into VEC(1 0)
STA VEC
LDA IRQ1V+1
STA VEC+1
LDA #LO(IRQ1) \ Set the IRQ1V vector to IRQ1, so IRQ1 is now the
STA IRQ1V \ interrupt handler
LDA #HI(IRQ1)
STA IRQ1V+1
LDA #VSCAN \ Set 6522 System VIA T1C-L timer 1 high-order counter
STA VIA+&45 \ (SHEILA &45) to VSCAN (57) to start the T1 counter
\ counting down from 14622 at a rate of 1 MHz
CLI \ Re-enable interrupts
LDA #&00 \ Set the following:
STA ZP \
LDA #&61 \ ZP(1 0) = &6100
STA ZP+1 \ P(1 0) = ASOFT
LDA #LO(ASOFT)
STA P
LDA #HI(ASOFT)
STA P+1
JSR MVPG \ Call MVPG to move and decrypt a page of memory from
\ ASOFT to &6100-&61FF
LDA #&63 \ Set the following:
STA ZP+1 \
LDA #LO(ELITE) \ ZP(1 0) = &6300
STA P \ P(1 0) = ELITE
LDA #HI(ELITE)
STA P+1
JSR MVPG \ Call MVPG to move and decrypt a page of memory from
\ ELITE to &6300-&63FF
LDA #&76 \ Set the following:
STA ZP+1 \
LDA #LO(CpASOFT) \ ZP(1 0) = &7600
STA P \ P(1 0) = CpASOFT
LDA #HI(CpASOFT)
STA P+1
JSR MVPG \ Call MVPG to move and decrypt a page of memory from
\ CpASOFT to &7600-&76FF
LDA #&00 \ Set the following:
STA ZP \
LDA #&04 \ ZP(1 0) = &0400
STA ZP+1 \ P(1 0) = WORDS
LDA #LO(WORDS) \ X = 4
STA P
LDA #HI(WORDS)
STA P+1
LDX #4
JSR MVBL \ Call MVBL to move and decrypt 4 pages of memory from
\ WORDS to &0400-&07FF
LDX #35 \ We now want to copy the disc catalogue routine from
\ CATDcode to CATD, so set a counter in X for the 36
\ bytes to copy
.loop2
LDA CATDcode,X \ Copy the X-th byte of CATDcode to the X-th byte of
STA CATD,X \ CATD
DEX \ Decrement the loop counter
BPL loop2 \ Loop back to copy the next byte until they are all
\ done
LDA &76 \ Set the drive number in the CATD routine to the
STA CATBLOCK \ contents of &76, which gets set in ELITE3
FNE 0 \ Set up sound envelopes 0-3 using the FNE macro
FNE 1
FNE 2
FNE 3
LDX #LO(MESS1) \ Set (Y X) to point to MESS1 ("DIR E")
LDY #HI(MESS1)
JSR OSCLI \ Call OSCLI to run the OS command in MESS1, which
\ changes the disc directory to E
LDA #LO(LOAD) \ Set the following:
STA ZP \
LDA #HI(LOAD) \ ZP(1 0) = LOAD
STA ZP+1 \ P(1 0) = LOADcode
LDA #LO(LOADcode)
STA P
LDA #HI(LOADcode)
STA P+1
LDY #0 \ We now want to move and decrypt one page of memory
\ from LOADcode to LOAD, so set Y as a byte counter
.loop3
LDA (P),Y \ Fetch the Y-th byte of the P(1 0) memory block
IF _STH_DISC OR _IB_DISC
EOR #&18 \ Decrypt it by EOR'ing with &18
ELIF _SRAM_DISC
EOR CHKSM \ Decrypt it by EOR'ing with the checksum value
ENDIF
STA (ZP),Y \ Store the decrypted result in the Y-th byte of the
\ ZP(1 0) memory block
DEY \ Decrement the byte counter
BNE loop3 \ Loop back to copy the next byte until we have done a
\ whole page of 256 bytes
JMP LOAD \ Jump to the start of the routine we just decrypted
\ ******************************************************************************
\
\ Name: CHECK
\ Type: Subroutine
\ Category: Copy protection
\ Summary: Calculate a checksum from two 256-byte portions of the loader code
\
\ ******************************************************************************
.CHECK
CLC \ Clear the C flag for the addition below
LDY #0 \ We are going to loop through 256 bytes, so set a byte
\ counter in Y
.p2
ADC PLL1,Y \ Set A = A + Y-th byte of PLL1
EOR ENTRY,Y \ Set A = A EOR Y-th byte of ENTRY
DEY \ Decrement the byte counter
BNE p2 \ Loop back to checksum the next byte
RTS \ Return from the subroutine
\ ******************************************************************************
\
\ Name: LOADcode
\ Type: Subroutine
\ Category: Loader
\ Summary: Encrypted LOAD routine, bundled up in the loader so it can be
\ moved to &0B00 to be run
\
\ ------------------------------------------------------------------------------
\
\ This section is encrypted by EOR'ing with &18. The encryption is done by the
\ elite-checksum.py script, and decryption is done in part 1 above, at the same
\ time as it is moved to &0B00.
\
\ ******************************************************************************
.LOADcode
ORG &0B00
\ ******************************************************************************
\
\ Name: LOAD
\ Type: Subroutine
\ Category: Loader
\ Summary: Load the main docked code, set up various vectors, run a checksum
\ and start the game
\
\ ------------------------------------------------------------------------------
\
\ This routine also contains a hidden message from the authors for potential
\ crackers to enjoy:
\
\ Does your mother know you do this?
\
\ I bet this made quite a few people smile back in the day...
\
\ ******************************************************************************
.LOAD
LDX #LO(LTLI) \ Set (Y X) to point to LTLI ("L.T.CODE")
LDY #HI(LTLI)
JSR OSCLI \ Call OSCLI to run the OS command in LTLI, which loads
\ the T.CODE binary (the main docked code) to its load
\ address of &11E3
LDA #LO(S%+11) \ Point BRKV to the fifth entry in the main docked
STA BRKV \ code's S% workspace, which contains JMP BRBR1
LDA #HI(S%+11)
STA BRKV+1
LDA #LO(S%+6) \ Point WRCHV to the third entry in the main docked
STA WRCHV \ code's S% workspace, which contains JMP CHPR
LDA #HI(S%+6)
STA WRCHV+1
SEC \ Set the C flag so the checksum we calculate in A
\ starts with an initial value of 18 (17 plus carry)
LDY #0 \ Set Y = 0 to act as a byte pointer
STY ZP \ Set the low byte of ZP(1 0) to 0, so ZP(1 0) always
\ points to the start of a page
LDX #&11 \ Set X = &11, so ZP(1 0) will point to &1100 when we
\ stick X in ZP+1 below
TXA \ Set A = &11 = 17, to set the initial value of the
\ checksum to 18 (17 plus carry)
.l1
STX ZP+1 \ Set the high byte of ZP(1 0) to the page number in X
ADC (ZP),Y \ Set A = A + the Y-th byte of ZP(1 0)
DEY \ Decrement the byte pointer
BNE l1 \ Loop back to add the next byte until we have added the
\ whole page
INX \ Increment the page number in X
CPX #&54 \ Loop back to checksum the next page until we have
BCC l1 \ checked up to (but not including) page &54
CMP &55FF \ Compare the checksum with the value in &55FF, which is
\ in the docked file we just loaded, in the byte before
\ the ship hangar blueprints at XX21
IF _REMOVE_CHECKSUMS
NOP \ If we have disabled checksums, then ignore the result
NOP \ of the checksum comparison
ELSE
IF _STH_DISC OR _IB_DISC
BNE P% \ If the checksums don't match then enter an infinite
\ loop, which hangs the computer
ELIF _SRAM_DISC
NOP \ The sideways RAM variant ignores the result of the
NOP \ checksum comparison
ENDIF
ENDIF
JMP S%+3 \ Jump to the second entry in the main docked code's S%
\ workspace to start a new game
.LTLI
EQUS "L.T.CODE" \ This is short for "*LOAD T.CODE"
EQUB 13
EQUS "Does your mother know you do this?"
COPYBLOCK LOAD, P%, LOADcode
ORG LOADcode + P% - LOAD
\ ******************************************************************************
\
\ Name: CATDcode
\ Type: Subroutine
\ Category: Save and load
\ Summary: The CATD routine, bundled up in the loader so it can be moved to
\ &0D7A to be run
\
\ ******************************************************************************
.CATDcode
ORG &0D7A
\ ******************************************************************************
\
\ Name: CATD
\ Type: Subroutine
\ Category: Save and load
\ Summary: Load disc sectors 0 and 1 to &0E00 and &0F00 respectively
\
\ ------------------------------------------------------------------------------
\
\ This routine is copied to &0D7A in part 1 above. It is called by both the main
\ docked code and the main flight code, just before the docked code, flight code
\ or ship blueprint files are loaded.
\
\ ******************************************************************************
.CATD
DEC CATBLOCK+8 \ Decrement sector number from 1 to 0
DEC CATBLOCK+2 \ Decrement load address from &0F00 to &0E00
JSR CATL \ Call CATL to load disc sector 1 to &0E00
INC CATBLOCK+8 \ Increment sector number back to 1
INC CATBLOCK+2 \ Increment load address back to &0F00
.CATL
LDA #127 \ Call OSWORD with A = 127 and (Y X) = CATBLOCK to
LDX #LO(CATBLOCK) \ load disc sector 1 to &0F00
LDY #HI(CATBLOCK)
JMP OSWORD
.CATBLOCK
EQUB 0 \ 0 = Drive = 0
EQUD &00000F00 \ 1 = Data address = &0F00
EQUB 3 \ 5 = Number of parameters = 3
EQUB &53 \ 6 = Command = &53 (read data)
EQUB 0 \ 7 = Track = 0
EQUB 1 \ 8 = Sector = 1
EQUB %00100001 \ 9 = Load 1 sector of 256 bytes
EQUB 0 \ 10 = The result of the OSWORD call is returned here
COPYBLOCK CATD, P%, CATDcode
ORG CATDcode + P% - CATD
\ ******************************************************************************
\
\ Name: PROT1
\ Type: Subroutine
\ Category: Copy protection
\ Summary: Part of the CHKSM copy protection checksum calculation
\
\ ******************************************************************************
.PROT1
LDA #85 \ We start by calculating a checksum in A, with an
\ initial value of 85
LDX #64 \ The checksum calculation in CHECK gets run 65 times,
\ so set a counter in X to count them
.p1a
JSR CHECK \ Call CHECK to calculate the checksum and add it to A
DEX \ Decrement the loop counter
BPL p1a \ Loop back until we have run the checksum 65 times
STA RAND+2 \ Store the checksum result in the random number seeds
\ used to generate the Saturn
ORA #0 \ If bit 7 of the checksum is clear, skip to p1b
BPL p1b
LSR CHKSM \ Bit 7 of the checksum is set, so shift the C flag that
\ was returned by CHECK into bit 7 of CHKSM
.p1b
JMP PROT2 \ Jump to PROT2 for more checksums, returning from the
\ subroutine using a tail call
EQUB &AC \ This byte appears to be unused
\ ******************************************************************************
\
\ Name: PLL1 (Part 1 of 3)
\ Type: Subroutine
\ Category: Drawing planets
\ Summary: Draw Saturn on the loading screen (draw the planet)
\ Deep dive: Drawing Saturn on the loading screen
\
\ ******************************************************************************
.PLL1
\ The following loop iterates CNT(1 0) times, i.e. &300
\ or 768 times, and draws the planet part of the
\ loading screen's Saturn
LDA VIA+&44 \ Read the 6522 System VIA T1C-L timer 1 low-order
STA RAND+1 \ counter (SHEILA &44), which decrements one million
\ times a second and will therefore be pretty random,
\ and store it in location RAND+1, which is among the
\ main game code's random seeds in RAND (so this seeds
\ the random number generator)
JSR DORND \ Set A and X to random numbers, say A = r1
JSR SQUA2 \ Set (A P) = A * A
\ = r1^2
STA ZP+1 \ Set ZP(1 0) = (A P)
LDA P \ = r1^2
STA ZP
LDA #LO(OSBmod) \ As part of the copy protection, the JSR OSB
STA OSBjsr+1 \ instruction at OSBjsr gets modified to point to OSBmod
\ instead of OSB, and this is where we modify the low
\ byte of the destination address
JSR DORND \ Set A and X to random numbers, say A = r2
STA YY \ Set YY = A
\ = r2
JSR SQUA2 \ Set (A P) = A * A
\ = r2^2
TAX \ Set (X P) = (A P)
\ = r2^2
LDA P \ Set (A ZP) = (X P) + ZP(1 0)
ADC ZP \
STA ZP \ first adding the low bytes
TXA \ And then adding the high bytes
ADC ZP+1
BCS PLC1 \ If the addition overflowed, jump down to PLC1 to skip
\ to the next pixel
STA ZP+1 \ Set ZP(1 0) = (A ZP)
\ = r1^2 + r2^2
LDA #1 \ Set ZP(1 0) = &4001 - ZP(1 0) - (1 - C)
SBC ZP \ = 128^2 - ZP(1 0)
STA ZP \
\ (as the C flag is clear), first subtracting the low
\ bytes
LDA #&40 \ And then subtracting the high bytes
SBC ZP+1
STA ZP+1
BCC PLC1 \ If the subtraction underflowed, jump down to PLC1 to
\ skip to the next pixel
\ If we get here, then both calculations fitted into
\ 16 bits, and we have:
\
\ ZP(1 0) = 128^2 - (r1^2 + r2^2)
\
\ where ZP(1 0) >= 0
JSR ROOT \ Set ZP = SQRT(ZP(1 0))
LDA ZP \ Set X = ZP >> 1
LSR A \ = SQRT(128^2 - (a^2 + b^2)) / 2
TAX
LDA YY \ Set A = YY
\ = r2