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BBC Micro Elite

Elite A source

ELITE A FILE Produces the binary file ELTA.bin that gets loaded by elite-bcfs.asm. The main game code (ELITE A through G, plus the ship data) is loaded at &1128 and is moved down to &0F40 as part of elite-loader.asm.
CODE% = &0F40 LOAD% = &1128 ORG CODE% LOAD_A% = LOAD%
Name: S% [View individually] Type: Workspace Address: &0F40 to &0F50 Category: Workspaces Summary: Vector addresses, compass colour and configuration settings
Contains addresses that are used by the loader to set up vectors, the current compass colour, and the game's configuration settings.
.S% EQUW TT170 \ The entry point for the main game; once the main code \ has been loaded, decrypted and moved to the right \ place by elite-loader.asm, the game is started by a \ JMP (S%) instruction, which jumps to the main entry \ point at TT170 via this location EQUW TT26 \ WRCHV is set to point here by elite-loader.asm EQUW IRQ1 \ IRQ1V is set to point here by elite-loader.asm EQUW BR1 \ BRKV is set to point here by elite-loader.asm .COMC EQUB 0 \ The colour of the dot on the compass \ \ * &F0 = the object in the compass is in front of us, \ so the dot is yellow/white \ \ * &FF = the object in the compass is behind us, so \ the dot is green/cyan .DNOIZ EQUB 0 \ Sound on/off configuration setting \ \ * 0 = sound is on (default) \ \ * &10 = sound is off \ \ Toggled by pressing "S" when paused, see the DK4 \ routine for details .DAMP EQUB 0 \ Keyboard damping configuration setting \ \ * 0 = damping is enabled (default) \ \ * &FF = damping is disabled \ \ Toggled by pressing Caps Lock when paused, see the \ DKS3 routine for details .DJD EQUB 0 \ Keyboard auto-recentre configuration setting \ \ * 0 = auto-recentre is enabled (default) \ \ * &FF = auto-recentre is disabled \ \ Toggled by pressing "A" when paused, see the DKS3 \ routine for details .PATG EQUB 0 \ Configuration setting to show the author names on the \ start-up screen and enable manual hyperspace mis-jumps \ \ * 0 = no author names or manual mis-jumps (default) \ \ * &FF = show author names and allow manual mis-jumps \ \ Toggled by pressing "X" when paused, see the DKS3 \ routine for details \ \ This needs to be turned on for manual mis-jumps to be \ possible. To do a manual mis-jump, first toggle the \ author display by pausing the game (COPY) and pressing \ "X", and during the next hyperspace, hold down CTRL to \ force a mis-jump. See routine ee5 for the "AND PATG" \ instruction that implements this logic .FLH EQUB 0 \ Flashing console bars configuration setting \ \ * 0 = static bars (default) \ \ * &FF = flashing bars \ \ Toggled by pressing "F" when paused, see the DKS3 \ routine for details .JSTGY EQUB 0 \ Reverse joystick Y-channel configuration setting \ \ * 0 = standard Y-channel (default) \ \ * &FF = reversed Y-channel \ \ Toggled by pressing "Y" when paused, see the DKS3 \ routine for details .JSTE EQUB 0 \ Reverse both joystick channels configuration setting \ \ * 0 = standard channels (default) \ \ * &FF = reversed channels \ \ Toggled by pressing "J" when paused, see the DKS3 \ routine for details .JSTK EQUB 0 \ Keyboard or joystick configuration setting \ \ * 0 = keyboard (default) \ \ * &FF = joystick \ \ Toggled by pressing "K" when paused, see the DKS3 \ routine for details
Name: Main flight loop (Part 1 of 16) [View individually] Type: Subroutine Category: Main loop Summary: Seed the random number generator
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Seed the random number generator Other entry points: M% The entry point for the main flight loop
.M% LDA K% \ We want to seed the random number generator with a \ pretty random number, so fetch the contents of K%, \ which is the x_lo coordinate of the planet. This value \ will be fairly unpredictable, so it's a pretty good \ candidate STA RAND \ Store the seed in the first byte of the four-byte \ random number seed that's stored in RAND
Name: Main flight loop (Part 2 of 16) [View individually] Type: Subroutine Category: Main loop Summary: Calculate the alpha and beta angles from the current pitch and roll of our ship
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Calculate the alpha and beta angles from the current pitch and roll Here we take the current rate of pitch and roll, as set by the joystick or keyboard, and convert them into alpha and beta angles that we can use in the matrix functions to rotate space around our ship. The alpha angle covers roll, while the beta angle covers pitch (there is no yaw in this version of Elite). The angles are in radians, which allows us to use the small angle approximation when moving objects in the sky (see the MVEIT routine for more on this). Also, the signs of the two angles are stored separately, in both the sign and the flipped sign, as this makes calculations easier.
LDX JSTX \ Set X to the current rate of roll in JSTX, and JSR cntr \ apply keyboard damping twice (if enabled) so the roll JSR cntr \ rate in X creeps towards the centre by 2 \ The roll rate in JSTX increases if we press ">" (and \ the RL indicator on the dashboard goes to the right). \ This rolls our ship to the right (clockwise), but we \ actually implement this by rolling everything else \ to the left (anticlockwise), so a positive roll rate \ in JSTX translates to a negative roll angle alpha TXA \ Set A and Y to the roll rate but with the sign bit EOR #%10000000 \ flipped (i.e. set them to the sign we want for alpha) TAY AND #%10000000 \ Extract the flipped sign of the roll rate and store STA ALP2 \ in ALP2 (so ALP2 contains the sign of the roll angle \ alpha) STX JSTX \ Update JSTX with the damped value that's still in X EOR #%10000000 \ Extract the correct sign of the roll rate and store STA ALP2+1 \ in ALP2+1 (so ALP2+1 contains the flipped sign of the \ roll angle alpha) TYA \ Set A to the roll rate but with the sign bit flipped BPL P%+7 \ If the value of A is positive, skip the following \ three instructions EOR #%11111111 \ A is negative, so change the sign of A using two's CLC \ complement so that A is now positive and contains ADC #1 \ the absolute value of the roll rate, i.e. |JSTX| LSR A \ Divide the (positive) roll rate in A by 4 LSR A CMP #8 \ If A >= 8, skip the following two instructions BCS P%+4 LSR A \ A < 8, so halve A again and clear the C flag, so we CLC \ can do addition later without the C flag affecting \ the result STA ALP1 \ Store A in ALP1, so we now have: \ \ ALP1 = |JSTX| / 8 if |JSTX| < 32 \ \ ALP1 = |JSTX| / 4 if |JSTX| >= 32 \ \ This means that at lower roll rates, the roll angle is \ reduced closer to zero than at higher roll rates, \ which gives us finer control over the ship's roll at \ lower roll rates \ \ Because JSTX is in the range -127 to +127, ALP1 is \ in the range 0 to 31 ORA ALP2 \ Store A in ALPHA, but with the sign set to ALP2 (so STA ALPHA \ ALPHA has a different sign to the actual roll rate) LDX JSTY \ Set X to the current rate of pitch in JSTY, and JSR cntr \ apply keyboard damping so the pitch rate in X creeps \ towards the centre by 1 TXA \ Set A and Y to the pitch rate but with the sign bit EOR #%10000000 \ flipped TAY AND #%10000000 \ Extract the flipped sign of the pitch rate into A STX JSTY \ Update JSTY with the damped value that's still in X STA BET2+1 \ Store the flipped sign of the pitch rate in BET2+1 EOR #%10000000 \ Extract the correct sign of the pitch rate and store STA BET2 \ it in BET2 TYA \ Set A to the pitch rate but with the sign bit flipped BPL P%+4 \ If the value of A is positive, skip the following \ instruction EOR #%11111111 \ A is negative, so flip the bits ADC #4 \ Add 4 to the (positive) pitch rate, so the maximum \ value is now up to 131 (rather than 127) LSR A \ Divide the (positive) pitch rate in A by 16 LSR A LSR A LSR A CMP #3 \ If A >= 3, skip the following instruction BCS P%+3 LSR A \ A < 3, so halve A again STA BET1 \ Store A in BET1, so we now have: \ \ BET1 = |JSTY| / 32 if |JSTY| < 48 \ \ BET1 = |JSTY| / 16 if |JSTY| >= 48 \ \ This means that at lower pitch rates, the pitch angle \ is reduced closer to zero than at higher pitch rates, \ which gives us finer control over the ship's pitch at \ lower pitch rates \ \ Because JSTY is in the range -131 to +131, BET1 is in \ the range 0 to 8 ORA BET2 \ Store A in BETA, but with the sign set to BET2 (so STA BETA \ BETA has the same sign as the actual pitch rate)
Name: Main flight loop (Part 3 of 16) [View individually] Type: Subroutine Category: Main loop Summary: Scan for flight keys and process the results
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Scan for flight keys and process the results Flight keys are logged in the key logger at location KY1 onwards, with a non-zero value in the relevant location indicating a key press. See the deep dive on "The key logger" for more details. The key presses that are processed are as follows: * Space and "?" to speed up and slow down * "U", "T" and "M" to disarm, arm and fire missiles * Tab to fire an energy bomb * Escape to launch an escape pod * "J" to initiate an in-system jump * "E" to deploy E.C.M. anti-missile countermeasures * "C" to use the docking computer * "A" to fire lasers
LDA KY2 \ If Space is being pressed, keep going, otherwise jump BEQ MA17 \ down to MA17 to skip the following LDA DELTA \ The "go faster" key is being pressed, so first we CMP #40 \ fetch the current speed from DELTA into A, and if BCS MA17 \ A >= 40, we are already going at full pelt, so jump \ down to MA17 to skip the following INC DELTA \ We can go a bit faster, so increment the speed in \ location DELTA .MA17 LDA KY1 \ If "?" is being pressed, keep going, otherwise jump BEQ MA4 \ down to MA4 to skip the following DEC DELTA \ The "slow down" key is being pressed, so we decrement \ the current ship speed in DELTA BNE MA4 \ If the speed is still greater than zero, jump to MA4 INC DELTA \ Otherwise we just braked a little too hard, so bump \ the speed back up to the minimum value of 1 .MA4 LDA KY15 \ If "U" is being pressed and the number of missiles AND NOMSL \ in NOMSL is non-zero, keep going, otherwise jump down BEQ MA20 \ to MA20 to skip the following LDY #&EE \ The "disarm missiles" key is being pressed, so call JSR ABORT \ ABORT to disarm the missile and update the missile \ indicators on the dashboard to green/cyan (Y = &EE) LDA #40 \ Call the NOISE routine with A = 40 to make a low, JSR NOISE \ long beep to indicate the missile is now disarmed .MA31 LDA #0 \ Set MSAR to 0 to indicate that no missiles are STA MSAR \ currently armed .MA20 LDA MSTG \ If MSTG is positive (i.e. it does not have bit 7 set), BPL MA25 \ then it indicates we already have a missile locked on \ a target (in which case MSTG contains the ship number \ of the target), so jump to MA25 to skip targeting. Or \ to put it another way, if MSTG = &FF, which means \ there is no current target lock, keep going LDA KY14 \ If "T" is being pressed, keep going, otherwise jump BEQ MA25 \ down to MA25 to skip the following LDX NOMSL \ If the number of missiles in NOMSL is zero, jump down BEQ MA25 \ to MA25 to skip the following STA MSAR \ The "target missile" key is being pressed and we have \ at least one missile, so set MSAR = &FF to denote that \ our missile is currently armed (we know A has the \ value &FF, as we just loaded it from MSTG and checked \ that it was negative) LDY #&E0 \ Change the leftmost missile indicator to yellow/white JSR MSBAR \ on the missile bar (this call changes the leftmost \ indicator because we set X to the number of missiles \ in NOMSL above, and the indicators are numbered from \ right to left, so X is the number of the leftmost \ indicator) .MA25 LDA KY16 \ If "M" is being pressed, keep going, otherwise jump BEQ MA24 \ down to MA24 to skip the following LDA MSTG \ If MSTG = &FF then there is no target lock, so jump to BMI MA64 \ MA64 to skip the following (also skipping the checks \ for Tab, Escape, "J" and "E") JSR FRMIS \ The "fire missile" key is being pressed and we have \ a missile lock, so call the FRMIS routine to fire \ the missile .MA24 LDA KY12 \ If Tab is being pressed, keep going, otherwise jump BEQ MA76 \ jump down to MA76 to skip the following ASL BOMB \ The "energy bomb" key is being pressed, so double \ the value in BOMB. If we have an energy bomb fitted, \ BOMB will contain &7F (%01111111) before this shift \ and will contain &FE (%11111110) after the shift; if \ we don't have an energy bomb fitted, BOMB will still \ contain 0. The bomb explosion is dealt with in the \ MAL1 routine below - this just registers the fact that \ we've set the bomb ticking .MA76 LDA KY13 \ If Escape is being pressed and we have an escape pod AND ESCP \ fitted, keep going, otherwise skip the next BEQ P%+5 \ instruction JMP ESCAPE \ The "launch escape pod" button is being pressed and \ we have an escape pod fitted, so jump to ESCAPE to \ launch it, and exit the main flight loop using a tail \ call LDA KY18 \ If "J" is being pressed, keep going, otherwise skip BEQ P%+5 \ the next instruction JSR WARP \ Call the WARP routine to do an in-system jump LDA KY17 \ If "E" is being pressed and we have an E.C.M. fitted, AND ECM \ keep going, otherwise jump down to MA64 to skip the BEQ MA64 \ following LDA ECMA \ If ECMA is non-zero, that means an E.C.M. is already BNE MA64 \ operating and is counting down (this can be either \ our E.C.M. or an opponent's), so jump down to MA64 to \ skip the following (as we can't have two E.C.M. \ systems operating at the same time) DEC ECMP \ The "E.C.M." button is being pressed and nobody else \ is operating their E.C.M., so decrease the value of \ ECMP to make it non-zero, to denote that our E.C.M. \ is now on JSR ECBLB2 \ Call ECBLB2 to light up the E.C.M. indicator bulb on \ the dashboard, set the E.C.M. countdown timer to 32, \ and start making the E.C.M. sound .MA64 LDA KY19 \ If "C" is being pressed, and we have a docking AND DKCMP \ computer fitted, and we are inside the space station's AND SSPR \ safe zone, keep going, otherwise jump down to MA68 to BEQ MA68 \ skip the following LDA K%+NI%+32 \ Fetch the AI counter (byte #32) of the second ship BMI MA68 \ from the ship data workspace at K%, which is reserved \ for the sun or the space station (in this case it's \ the latter as we are in the safe zone). If byte #32 is \ negative, meaning the station is hostile, then jump \ down to MA68 to skip the following (so we can't use \ the docking computer to dock at a station that has \ turned against us) JMP GOIN \ The "docking computer" button has been pressed and \ we are allowed to dock at the station, so jump to \ GOIN to dock (or "go in"), and exit the main flight \ loop using a tail call .MA68 LDA #0 \ Set LAS = 0, to switch the laser off while we do the STA LAS \ following logic STA DELT4 \ Take the 16-bit value (DELTA 0) - i.e. a two-byte LDA DELTA \ number with DELTA as the high byte and 0 as the low LSR A \ byte - and divide it by 4, storing the 16-bit result ROR DELT4 \ in DELT4(1 0). This has the effect of storing the LSR A \ current speed * 64 in the 16-bit location DELT4(1 0) ROR DELT4 STA DELT4+1 LDA LASCT \ If LASCT is zero, keep going, otherwise the laser is BNE MA3 \ a pulse laser that is between pulses, so jump down to \ MA3 to skip the following LDA KY7 \ If "A" is being pressed, keep going, otherwise jump BEQ MA3 \ down to MA3 to skip the following LDA GNTMP \ If the laser temperature >= 242 then the laser has CMP #242 \ overheated, so jump down to MA3 to skip the following BCS MA3 LDX VIEW \ If the current space view has a laser fitted (i.e. the LDA LASER,X \ laser power for this view is greater than zero), then BEQ MA3 \ keep going, otherwise jump down to MA3 to skip the \ following \ If we get here, then the "fire" button is being \ pressed, our laser hasn't overheated and isn't already \ being fired, and we actually have a laser fitted to \ the current space view, so it's time to hit me with \ those laser beams PHA \ Store the current view's laser power on the stack AND #%01111111 \ Set LAS and LAS2 to bits 0-6 of the laser power STA LAS STA LAS2 LDA #0 \ Call the NOISE routine with A = 0 to make the sound JSR NOISE \ of our laser firing JSR LASLI \ Call LASLI to draw the laser lines PLA \ Restore the current view's laser power into A BPL ma1 \ If the laser power has bit 7 set, then it's an "always \ on" laser rather than a pulsing laser, so keep going, \ otherwise jump down to ma1 to skip the following \ instruction LDA #0 \ This is an "always on" laser (i.e. a beam laser, \ as the cassette version of Elite doesn't have military \ lasers), so set A = 0, which will be stored in LASCT \ to denote that this is not a pulsing laser .ma1 AND #%11111010 \ LASCT will be set to 0 for beam lasers, and to the STA LASCT \ laser power AND %11111010 for pulse lasers, which \ comes to 10 (as pulse lasers have a power of 15). See \ MA23 below for more on laser pulsing and LASCT
Name: Main flight loop (Part 4 of 16) [View individually] Type: Subroutine Category: Main loop Summary: For each nearby ship: Copy the ship's data block from K% to the zero-page workspace at INWK
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Start looping through all the ships in the local bubble, and for each one: * Copy the ship's data block from K% to INWK * Set XX0 to point to the ship's blueprint (if this is a ship) Other entry points: MAL1 Marks the beginning of the ship analysis loop, so we can jump back here from part 12 of the main flight loop to work our way through each ship in the local bubble. We also jump back here when a ship is removed from the bubble, so we can continue processing from the next ship
.MA3 LDX #0 \ We're about to work our way through all the ships in \ our local bubble of universe, so set a counter in X, \ starting from 0, to refer to each ship slot in turn .MAL1 STX XSAV \ Store the current slot number in XSAV LDA FRIN,X \ Fetch the contents of this slot into A. If it is 0 BNE P%+5 \ then this slot is empty and we have no more ships to JMP MA18 \ process, so jump to MA18 below, otherwise A contains \ the type of ship that's in this slot, so skip over the \ JMP MA18 instruction and keep going STA TYPE \ Store the ship type in TYPE JSR GINF \ Call GINF to fetch the address of the ship data block \ for the ship in slot X and store it in INF. The data \ block is in the K% workspace, which is where all the \ ship data blocks are stored \ Next we want to copy the ship data block from INF to \ the zero-page workspace at INWK, so we can process it \ more efficiently LDY #NI%-1 \ There are NI% bytes in each ship data block (and in \ the INWK workspace, so we set a counter in Y so we can \ loop through them .MAL2 LDA (INF),Y \ Load the Y-th byte of INF and store it in the Y-th STA INWK,Y \ byte of INWK DEY \ Decrement the loop counter BPL MAL2 \ Loop back for the next byte until we have copied the \ last byte from INF to INWK LDA TYPE \ If the ship type is negative then this indicates a BMI MA21 \ planet or sun, so jump down to MA21, as the next bit \ sets up a pointer to the ship blueprint, and then \ checks for energy bomb damage, and neither of these \ apply to planets and suns ASL A \ Set Y = ship type * 2 TAY LDA XX21-2,Y \ The ship blueprints at XX21 start with a lookup STA XX0 \ table that points to the individual ship blueprints, \ so this fetches the low byte of this particular ship \ type's blueprint and stores it in XX0 LDA XX21-1,Y \ Fetch the high byte of this particular ship type's STA XX0+1 \ blueprint and store it in XX0+1
Name: Main flight loop (Part 5 of 16) [View individually] Type: Subroutine Category: Main loop Summary: For each nearby ship: If an energy bomb has been set off, potentially kill this ship
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Continue looping through all the ships in the local bubble, and for each one: * If an energy bomb has been set off and this ship can be killed, kill it and increase the kill tally
LDA BOMB \ If we set off our energy bomb by pressing Tab (see BPL MA21 \ MA24 above), then BOMB is now negative, so this skips \ to MA21 if our energy bomb is not going off CPY #2*SST \ If the ship in Y is the space station, jump to BA21 BEQ MA21 \ as energy bombs are useless against space stations LDA INWK+31 \ If the ship we are checking has bit 5 set in its ship AND #%00100000 \ byte #31, then it is already exploding, so jump to BNE MA21 \ BA21 as ships can't explode more than once LDA INWK+31 \ The energy bomb is killing this ship, so set bit 7 of ORA #%10000000 \ the ship byte #31 to indicate that it has now been STA INWK+31 \ killed JSR EXNO2 \ Call EXNO2 to process the fact that we have killed a \ ship (so increase the kill tally, make an explosion \ sound and possibly display "RIGHT ON COMMANDER!")
Name: Main flight loop (Part 6 of 16) [View individually] Type: Subroutine Category: Main loop Summary: For each nearby ship: Move the ship in space and copy the updated INWK data block back to K% Deep dive: Program flow of the ship-moving routine
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Continue looping through all the ships in the local bubble, and for each one: * Move the ship in space * Copy the updated ship's data block from INWK back to K%
.MA21 JSR MVEIT \ Call MVEIT to move the ship we are processing in space \ Now that we are done processing this ship, we need to \ copy the ship data back from INWK to the correct place \ in the K% workspace. We already set INF in part 4 to \ point to the ship's data block in K%, so we can simply \ do the reverse of the copy we did before, this time \ copying from INWK to INF LDY #(NI%-1) \ Set a counter in Y so we can loop through the NI% \ bytes in the ship data block .MAL3 LDA INWK,Y \ Load the Y-th byte of INWK and store it in the Y-th STA (INF),Y \ byte of INF DEY \ Decrement the loop counter BPL MAL3 \ Loop back for the next byte, until we have copied the \ last byte from INWK back to INF
Name: Main flight loop (Part 7 of 16) [View individually] Type: Subroutine Category: Main loop Summary: For each nearby ship: Check whether we are docking, scooping or colliding with it
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Continue looping through all the ships in the local bubble, and for each one: * Check how close we are to this ship and work out if we are docking, scooping or colliding with it
LDA INWK+31 \ Fetch the status of this ship from bits 5 (is ship AND #%10100000 \ exploding?) and bit 7 (has ship been killed?) from \ ship byte #31 into A JSR MAS4 \ Or this value with x_hi, y_hi and z_hi BNE MA65 \ If this value is non-zero, then either the ship is \ far away (i.e. has a non-zero high byte in at least \ one of the three axes), or it is already exploding, \ or has been flagged as being killed - in which case \ jump to MA65 to skip the following, as we can't dock \ scoop or collide with it LDA INWK \ Set A = (x_lo OR y_lo OR z_lo), and if bit 7 of the ORA INWK+3 \ result is set, the ship is still a fair distance ORA INWK+6 \ away (further than 127 in at least one axis), so jump BMI MA65 \ to MA65 to skip the following, as it's too far away to \ dock, scoop or collide with LDX TYPE \ If the current ship type is negative then it's either BMI MA65 \ a planet or a sun, so jump down to MA65 to skip the \ following, as we can't dock with it or scoop it CPX #SST \ If this ship is the space station, jump to ISDK to BEQ ISDK \ check whether we are docking with it AND #%11000000 \ If bit 6 of (x_lo OR y_lo OR z_lo) is set, then the BNE MA65 \ ship is still a reasonable distance away (further than \ 63 in at least one axis), so jump to MA65 to skip the \ following, as it's too far away to dock, scoop or \ collide with CPX #MSL \ If this ship is a missile, jump down to MA65 to skip BEQ MA65 \ the following, as we can't scoop or dock with a \ missile, and it has its own dedicated collision \ checks in the TACTICS routine CPX #OIL \ If ship type >= OIL (i.e. it's a cargo canister, BCS P%+5 \ Thargon or escape pod), skip the JMP instruction and JMP MA58 \ continue on, otherwise jump to MA58 to process a \ potential collision LDA BST \ If we have fuel scoops fitted then BST will be &FF, \ otherwise it will be 0 AND INWK+5 \ Ship byte #5 contains the y_sign of this ship, so a \ negative value here means the canister is below us, \ which means the result of the AND will be negative if \ the canister is below us and we have a fuel scoop \ fitted BPL MA58 \ If the result is positive, then we either have no \ scoop or the canister is above us, and in both cases \ this means we can't scoop the item, so jump to MA58 \ to process a collision
Name: Main flight loop (Part 8 of 16) [View individually] Type: Subroutine Category: Main loop Summary: For each nearby ship: Process us potentially scooping this item
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Continue looping through all the ships in the local bubble, and for each one: * Process us potentially scooping this item
LDA #3 \ Set A to 3 to denote we may be scooping an escape pod CPX #TGL \ If ship type < Thargon, i.e. it's a canister, jump BCC oily \ to oily to randomly decide the canister's contents BNE slvy2 \ If ship type <> Thargon, i.e. it's an escape pod, \ jump to slvy2 with A set to 3, so we scoop up the \ escape pod as slaves LDA #16 \ Otherwise this is a Thargon, so jump to slvy2 with BNE slvy2 \ A set to 16, so we scoop up the Thargon as alien items \ (this BNE is effectively a JMP as A will never be \ zero) .oily JSR DORND \ Set A and X to random numbers and reduce A to a AND #7 \ random number in the range 0-7 .slvy2 \ By the time we get here, we are scooping, and A \ contains the type of item we are scooping (a random \ number 0-7 if we are scooping a cargo canister, 3 if \ we are scooping an escape pod, or 16 if we are \ scooping a Thargon). These numbers correspond to the \ relevant market items (see QQ23 for a list), so a \ cargo canister can contain anything from food to \ computers, while escape pods contain slaves, and \ Thargons become alien items when scooped STA QQ29 \ Call tnpr with the scooped cargo type stored in QQ29 LDA #1 \ and A set to 1, to work out whether we have room in JSR tnpr \ the hold for the scooped item (A is preserved by this \ call, and the C flag contains the result) LDY #78 \ This instruction has no effect, so presumably it used \ to do something, but didn't get removed BCS MA59 \ If the C flag is set then we have no room in the hold \ for the scooped item, so jump down to MA59 make a \ sound to indicate failure, before destroying the \ canister LDY QQ29 \ Scooping was successful, so set Y to the type of \ item we just scooped, which we stored in QQ29 above ADC QQ20,Y \ Add A (which we set to 1 above) to the number of items STA QQ20,Y \ of type Y in the cargo hold, as we just successfully \ scooped one canister of type Y TYA \ Print recursive token 48 + A as an in-flight token, ADC #208 \ which will be in the range 48 ("FOOD") to 64 ("ALIEN JSR MESS \ ITEMS"), so this prints the scooped item's name JMP MA60 \ We are done scooping, so jump down to MA60 to set the \ kill flag on the canister, as it no longer exists in \ the local bubble .MA65 JMP MA26 \ If we get here, then the ship we are processing was \ too far away to be scooped, docked or collided with, \ so jump to MA26 to skip over the collision routines \ and move on to missile targeting
Name: Main flight loop (Part 9 of 16) [View individually] Type: Subroutine Category: Main loop Summary: For each nearby ship: If it is a space station, check whether we are successfully docking with it Deep dive: Docking checks
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Process docking with a space station For details on the various docking checks in this routine, see the deep dive on "Docking checks". Other entry points: GOIN We jump here from part 3 of the main flight loop if the docking computer is activated by pressing "C"
.ISDK LDA K%+NI%+32 \ 1. Fetch the AI counter (byte #32) of the second ship BMI MA62 \ in the ship data workspace at K%, which is reserved \ for the sun or the space station (in this case it's \ the latter), and if it's negative, i.e. bit 7 is set, \ meaning the station is hostile, jump down to MA62 to \ fail docking (so trying to dock at a station that we \ have annoyed does not end well) LDA INWK+14 \ 2. If nosev_z_hi < 214, jump down to MA62 to fail CMP #214 \ docking, as the angle of approach is greater than 26 BCC MA62 \ degrees JSR SPS4 \ Call SPS4 to get the vector to the space station \ into XX15 LDA XX15+2 \ 3. Check the sign of the z-axis (bit 7 of XX15+2) and BMI MA62 \ if it is negative, we are facing away from the \ station, so jump to MA62 to fail docking CMP #89 \ 4. If z-axis < 89, jump to MA62 to fail docking, as BCC MA62 \ we are not in the 22.0 degree safe cone of approach LDA INWK+16 \ 5. If |roofv_x_hi| < 80, jump to MA62 to fail docking, AND #%01111111 \ as the slot is more than 36.6 degrees from horizontal CMP #80 BCC MA62 .GOIN \ If we arrive here, either the docking computer has \ been activated, or we just docked successfully LDA #0 \ Set the on-screen hyperspace counter to 0 STA QQ22+1 LDA #8 \ This instruction has no effect, so presumably it used \ to do something, and didn't get removed JSR LAUN \ Show the space station launch tunnel JSR RES4 \ Reset the shields and energy banks, stardust and INWK \ workspace JMP BAY \ Go to the docking bay (i.e. show the Status Mode \ screen) .MA62 \ If we arrive here, docking has just failed LDA DELTA \ If the ship's speed is < 5, jump to MA67 to register CMP #5 \ some damage, but not a huge amount BCC MA67 JMP DEATH \ Otherwise we have just crashed into the station, so \ process our death
Name: Main flight loop (Part 10 of 16) [View individually] Type: Subroutine Category: Main loop Summary: For each nearby ship: Remove if scooped, or process collisions
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Continue looping through all the ships in the local bubble, and for each one: * Remove scooped item after both successful and failed scoopings * Process collisions
.MA59 \ If we get here then scooping failed JSR EXNO3 \ Make the sound of the cargo canister being destroyed \ and fall through into MA60 to remove the canister \ from our local bubble .MA60 \ If we get here then scooping was successful ASL INWK+31 \ Set bit 7 of the scooped or destroyed item, to denote SEC \ that it has been killed and should be removed from ROR INWK+31 \ the local bubble .MA61 \ This label is not used but is in the original source BNE MA26 \ Jump to MA26 to skip over the collision routines and \ to move on to missile targeting (this BNE is \ effectively a JMP as A will never be zero) .MA67 \ If we get here then we have collided with something, \ but not fatally LDA #1 \ Set the speed in DELTA to 1 (i.e. a sudden stop) STA DELTA LDA #5 \ Set the amount of damage in A to 5 (a small dent) and BNE MA63 \ jump down to MA63 to process the damage (this BNE is \ effectively a JMP as A will never be zero) .MA58 \ If we get here, we have collided with something in a \ potentially fatal way ASL INWK+31 \ Set bit 7 of the ship we just collided with, to SEC \ denote that it has been killed and should be removed ROR INWK+31 \ from the local bubble LDA INWK+35 \ Load A with the energy level of the ship we just hit SEC \ Set the amount of damage in A to 128 + A / 2, so ROR A \ this is quite a big dent, and colliding with higher \ energy ships will cause more damage .MA63 JSR OOPS \ The amount of damage is in A, so call OOPS to reduce \ our shields, and if the shields are gone, there's a \ a chance of cargo loss or even death JSR EXNO3 \ Make the sound of colliding with the other ship and \ fall through into MA26 to try targeting a missile
Name: Main flight loop (Part 11 of 16) [View individually] Type: Subroutine Category: Main loop Summary: For each nearby ship: Process missile lock and firing our laser
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Continue looping through all the ships in the local bubble, and for each one: * If this is not the front space view, flip the axes of the ship's coordinates in INWK * Process missile lock * Process our laser firing
.MA26 LDA QQ11 \ If this is not a space view, jump to MA15 to skip BNE MA15 \ missile and laser locking JSR PLUT \ Call PLUT to update the geometric axes in INWK to \ match the view (front, rear, left, right) JSR HITCH \ Call HITCH to see if this ship is in the crosshairs, BCC MA8 \ in which case the C flag will be set (so if there is \ no missile or laser lock, we jump to MA8 to skip the \ following) LDA MSAR \ We have missile lock, so check whether the leftmost BEQ MA47 \ missile is currently armed, and if not, jump to MA47 \ to process laser fire, as we can't lock an unarmed \ missile JSR BEEP \ We have missile lock and an armed missile, so call \ the BEEP subroutine to make a short, high beep LDX XSAV \ Call ABORT2 to store the details of this missile LDY #&0E \ lock, with the targeted ship's slot number in X JSR ABORT2 \ (which we stored in XSAV at the start of this ship's \ loop at MAL1), and set the colour of the missile \ indicator to the colour in Y (red = &0E) .MA47 \ If we get here then the ship is in our sights, but \ we didn't lock a missile, so let's see if we're \ firing the laser LDA LAS \ If we are firing the laser then LAS will contain the BEQ MA8 \ laser power (which we set in MA68 above), so if this \ is zero, jump down to MA8 to skip the following LDX #15 \ We are firing our laser and the ship in INWK is in JSR EXNO \ the crosshairs, so call EXNO to make the sound of \ us making a laser strike on another ship LDA INWK+35 \ Fetch the hit ship's energy from byte #35 and subtract SEC \ our current laser power, and if the result is greater SBC LAS \ than zero, the other ship has survived the hit, so BCS MA14 \ jump down to MA14 LDA TYPE \ Did we just hit the space station? If so, jump to CMP #SST \ MA14+2 to make the station hostile, skipping the BEQ MA14+2 \ following as we can't destroy a space station LDA INWK+31 \ Set bit 7 of the enemy ship's byte #31, to indicate ORA #%10000000 \ that it has been killed STA INWK+31 BCS MA8 \ If the enemy ship type is >= SST (i.e. missile, \ asteroid, canister, Thargon or escape pod) then \ jump down to MA8 JSR DORND \ Fetch a random number, and jump to oh if it is BPL oh \ positive (50% chance) LDY #0 \ Fetch the first byte of the hit ship's blueprint, AND (XX0),Y \ which determines the maximum number of bits of \ debris shown when the ship is destroyed, and AND \ with the random number we just fetched STA CNT \ Store the result in CNT, so CNT contains a random \ number between 0 and the maximum number of bits of \ debris that this ship will release when destroyed .um BEQ oh \ We're going to go round a loop using CNT as a counter \ so this checks whether the counter is zero and jumps \ to oh when it gets there (which might be straight \ away) LDX #OIL \ Call SFS1 to spawn a cargo canister from the now LDA #0 \ deceased parent ship, giving the spawned canister an JSR SFS1 \ AI flag of 0 (no AI, no E.C.M., non-hostile) DEC CNT \ Decrease the loop counter BPL um \ Jump back up to um (this BPL is effectively a JMP as \ CNT will never be negative) .oh JSR EXNO2 \ Call EXNO2 to process the fact that we have killed a \ ship (so increase the kill tally, make an explosion \ sound and so on) .MA14 STA INWK+35 \ Store the hit ship's updated energy in ship byte #35 LDA TYPE \ Call ANGRY to make this ship hostile, now that we JSR ANGRY \ have hit it
Name: Main flight loop (Part 12 of 16) [View individually] Type: Subroutine Category: Main loop Summary: For each nearby ship: Draw the ship, remove if killed, loop back
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Continue looping through all the ships in the local bubble, and for each one: * Draw the ship * Process removal of killed ships * Loop back up to MAL1 to move onto the next ship in the local bubble
.MA8 JSR LL9 \ Call LL9 to draw the ship we're processing on-screen .MA15 LDY #35 \ Fetch the ship's energy from byte #35 and copy it to LDA INWK+35 \ byte #35 in INF (so the ship's data in K% gets STA (INF),Y \ updated) LDA INWK+31 \ If bit 7 of the ship's byte #31 is clear, then the BPL MAC1 \ ship hasn't been killed by energy bomb, collision or \ laser fire, so jump to MAC1 to skip the following AND #%00100000 \ If bit 5 of the ship's byte #31 is clear then the BEQ NBOUN \ ship is no longer exploding, so jump to NBOUN to skip \ the following LDA TYPE \ If the ship we just destroyed was a cop, keep going, CMP #COPS \ otherwise jump to q2 to skip the following BNE q2 LDA FIST \ We shot the sheriff, so update our FIST flag ORA #64 \ ("fugitive/innocent status") to at least 64, which STA FIST \ will instantly make us a fugitive .q2 LDA DLY \ If we already have an in-flight message on-screen (in ORA MJ \ which case DLY > 0), or we are in witchspace (in BNE KS1S \ which case MJ > 0), jump to KS1S to skip showing an \ on-screen bounty for this kill LDY #10 \ Fetch byte #10 of the ship's blueprint, which is the LDA (XX0),Y \ low byte of the bounty awarded when this ship is BEQ KS1S \ killed (in Cr * 10), and if it's zero jump to KS1S as \ there is no on-screen bounty to display TAX \ Put the low byte of the bounty into X INY \ Fetch byte #11 of the ship's blueprint, which is the LDA (XX0),Y \ high byte of the bounty awarded (in Cr * 10), and put TAY \ it into Y JSR MCASH \ Call MCASH to add (Y X) to the cash pot LDA #0 \ Print control code 0 (current cash, right-aligned to JSR MESS \ width 9, then " CR", newline) as an in-flight message .KS1S JMP KS1 \ Process the killing of this ship (which removes this \ ship from its slot and shuffles all the other ships \ down to close up the gap) .NBOUN .MAC1 LDA TYPE \ If the ship we are processing is a planet or sun, BMI MA27 \ jump to MA27 to skip the following two instructions JSR FAROF \ If the ship we are processing is a long way away (its BCC KS1S \ distance in any one direction is > 224, jump to KS1S \ to remove the ship from our local bubble, as it's just \ left the building .MA27 LDY #31 \ Fetch the ship's explosion/killed state from byte #31 LDA INWK+31 \ and copy it to byte #31 in INF (so the ship's data in STA (INF),Y \ K% gets updated) LDX XSAV \ We're done processing this ship, so fetch the ship's \ slot number, which we saved in XSAV back at the start \ of the loop INX \ Increment the slot number to move on to the next slot JMP MAL1 \ And jump back up to the beginning of the loop to get \ the next ship in the local bubble for processing
Name: Main flight loop (Part 13 of 16) [View individually] Type: Subroutine Category: Main loop Summary: Show energy bomb effect, charge shields and energy banks Deep dive: Scheduling tasks with the main loop counter
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Show energy bomb effect (if applicable) * Charge shields and energy banks (every 7 iterations of the main loop)
.MA18 LDA BOMB \ If we set off our energy bomb by pressing Tab (see BPL MA77 \ MA24 above), then BOMB is now negative, so this skips \ to MA77 if our energy bomb is not going off ASL BOMB \ We set off our energy bomb, so rotate BOMB to the \ left by one place. BOMB was rotated left once already \ during this iteration of the main loop, back at MA24, \ so if this is the first pass it will already be \ %11111110, and this will shift it to %11111100 - so \ if we set off an energy bomb, it stays activated \ (BOMB > 0) for four iterations of the main loop JSR WSCAN \ Call WSCAN to wait for the vertical sync, so the whole \ screen gets drawn and the following palette change \ won't kick in while the screen is still refreshing LDA #%00110000 \ Set the palette byte at SHEILA &21 to map logical STA VIA+&21 \ colour 0 to physical colour 7 (white), but with only \ one mapping (rather than the 7 mappings requires to \ do the mapping properly). This makes the space screen \ flash with black and white stripes. See p.382 of the \ Advanced User Guide for details of why this single \ palette change creates a special effect .MA77 LDA MCNT \ Fetch the main loop counter and calculate MCNT mod 7, AND #7 \ jumping to MA22 if it is non-zero (so the following BNE MA22 \ code only runs every 8 iterations of the main loop) LDX ENERGY \ Fetch our ship's energy levels and skip to b if bit 7 BPL b \ is not set, i.e. only charge the shields from the \ energy banks if they are at more than 50% charge LDX ASH \ Call SHD to recharge our aft shield and update the JSR SHD \ shield status in ASH STX ASH LDX FSH \ Call SHD to recharge our forward shield and update JSR SHD \ the shield status in FSH STX FSH .b SEC \ Set A = ENERGY + ENGY + 1, so our ship's energy LDA ENGY \ level goes up by 2 if we have an energy unit fitted, ADC ENERGY \ otherwise it goes up by 1 BCS P%+5 \ If the value of A did not overflow (the maximum STA ENERGY \ energy level is &FF), then store A in ENERGY
Name: Main flight loop (Part 14 of 16) [View individually] Type: Subroutine Category: Main loop Summary: Spawn a space station if we are close enough to the planet
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Spawn a space station if we are close enough to the planet (every 32 iterations of the main loop)
LDA MJ \ If we are in witchspace, jump down to MA23S to skip BNE MA23S \ the following, as there are no space stations in \ witchspace LDA MCNT \ Fetch the main loop counter and calculate MCNT mod 32, AND #31 \ jumping to MA93 if it is on-zero (so the following BNE MA93 \ code only runs every 32 iterations of the main loop LDA SSPR \ If we are inside the space station safe zone, jump to BNE MA23S \ MA23S to skip the following, as we already have a \ space station and don't need another TAY \ Set Y = A = 0 (A is 0 as we didn't branch with the \ previous BNE instruction) JSR MAS2 \ Call MAS2 to calculate the largest distance to the BNE MA23S \ planet in any of the three axes, and if it's \ non-zero, jump to MA23S to skip the following, as we \ are too far from the planet to bump into a space \ station \ We now want to spawn a space station, so first we \ need to set up a ship data block for the station in \ INWK that we can then pass to NWSPS to add a new \ station to our bubble of universe. We do this by \ copying the planet data block from K% to INWK so we \ can work on it, but we only need the first 29 bytes, \ as we don't need to worry about bytes #29 to #35 \ for planets (as they don't have rotation counters, \ AI, explosions, missiles, a ship line heap or energy \ levels) LDX #28 \ So we set a counter in X to copy 29 bytes from K%+0 \ to K%+28 .MAL4 LDA K%,X \ Load the X-th byte of K% and store in the X-th byte STA INWK,X \ of the INWK workspace DEX \ Decrement the loop counter BPL MAL4 \ Loop back for the next byte until we have copied the \ first 28 bytes of K% to INWK \ We now check the distance from our ship (at the \ origin) towards the planet's surface, by adding the \ planet's nosev vector to the planet's centre at \ (x, y, z) and checking our distance to the end \ point along the relevant axis INX \ Set X = 0 (as we ended the above loop with X as &FF) LDY #9 \ Call MAS1 with X = 0, Y = 9 to do the following: JSR MAS1 \ \ (x_sign x_hi x_lo) += (nosev_x_hi nosev_x_lo) * 2 \ \ A = |x_hi| BNE MA23S \ If A > 0, jump to MA23S to skip the following, as we \ are too far from the planet in the x-direction to \ bump into a space station LDX #3 \ Call MAS1 with X = 3, Y = 11 to do the following: LDY #11 \ JSR MAS1 \ (y_sign y_hi y_lo) += (nosev_y_hi nosev_y_lo) * 2 \ \ A = |y_hi| BNE MA23S \ If A > 0, jump to MA23S to skip the following, as we \ are too far from the planet in the y-direction to \ bump into a space station LDX #6 \ Call MAS1 with X = 6, Y = 13 to do the following: LDY #13 \ JSR MAS1 \ (z_sign z_hi z_lo) += (nosev_z_hi nosev_z_lo) * 2 \ \ A = |z_hi| BNE MA23S \ If A > 0, jump to MA23S to skip the following, as we \ are too far from the planet in the z-direction to \ bump into a space station LDA #192 \ Call FAROF2 to compare x_hi, y_hi and z_hi with 192, JSR FAROF2 \ which will set the C flag if all three are < 192, or \ clear the C flag if any of them are >= 192 BCC MA23S \ Jump to MA23S if any one of x_hi, y_hi or z_hi are \ >= 192 (i.e. they must all be < 192 for us to be near \ enough to the planet to bump into a space station) LDA QQ11 \ If the current view is a space view, call WPLS to BNE P%+5 \ remove the sun from the screen, as we can't have both JSR WPLS \ the sun and the space station at the same time JSR NWSPS \ Add a new space station to our local bubble of \ universe .MA23S JMP MA23 \ Jump to MA23 to skip the following planet and sun \ altitude checks
Name: Main flight loop (Part 15 of 16) [View individually] Type: Subroutine Category: Main loop Summary: Perform altitude checks with planet and sun, process fuel scooping
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Perform an altitude check with the planet (every 32 iterations of the main loop, on iteration 10 of each 32) * Perform an an altitude check with the sun and process fuel scooping (every 32 iterations of the main loop, on iteration 20 of each 32)
.MA22 LDA MJ \ If we are in witchspace, jump down to MA23 to skip BNE MA23 \ the following, as there are no planets or suns to \ bump into in witchspace LDA MCNT \ Fetch the main loop counter and calculate MCNT mod 32, AND #31 \ which tells us the position of this loop in each block \ of 32 iterations .MA93 CMP #10 \ If this is the tenth iteration in this block of 32, BNE MA29 \ do the following, otherwise jump to MA29 to skip the \ planet altitude check and move on to the sun distance \ check LDA #50 \ If our energy bank status in ENERGY is >= 50, skip CMP ENERGY \ printing the following message (so the message is BCC P%+6 \ only shown if our energy is low) ASL A \ Print recursive token 100 ("ENERGY LOW{beep}") as an JSR MESS \ in-flight message LDY #&FF \ Set our altitude in ALTIT to &FF, the maximum STY ALTIT INY \ Set Y = 0 JSR m \ Call m to calculate the maximum distance to the \ planet in any of the three axes, returned in A BNE MA23 \ If A > 0 then we are a fair distance away from the \ planet in at least one axis, so jump to MA23 to skip \ the rest of the altitude check JSR MAS3 \ Set A = x_hi^2 + y_hi^2 + z_hi^2, so using Pythagoras \ we now know that A now contains the square of the \ distance between our ship (at the origin) and the \ centre of the planet at (x_hi, y_hi, z_hi) BCS MA23 \ If the C flag was set by MAS3, then the result \ overflowed (was greater than &FF) and we are still a \ fair distance from the planet, so jump to MA23 as we \ haven't crashed into the planet SBC #36 \ Subtract 36 from x_hi^2 + y_hi^2 + z_hi^2. The radius \ of the planet is defined as 6 units and 6^2 = 36, so \ A now contains the high byte of our altitude above \ the planet surface, squared BCC MA28 \ If A < 0 then jump to MA28 as we have crashed into \ the planet STA R \ We are getting close to the planet, so we need to JSR LL5 \ work out how close. We know from the above that A \ contains our altitude squared, so we store A in R \ and call LL5 to calculate: \ \ Q = SQRT(R Q) = SQRT(A Q) \ \ Interestingly, Q doesn't appear to be set to 0 for \ this calculation, so presumably this doesn't make a \ difference LDA Q \ Store the result in ALTIT, our altitude STA ALTIT BNE MA23 \ If our altitude is non-zero then we haven't crashed, \ so jump to MA23 to skip to the next section .MA28 JMP DEATH \ If we get here then we just crashed into the planet \ or got too close to the sun, so call DEATH to start \ the funeral preparations .MA29 CMP #20 \ If this is the 20th iteration in this block of 32, BNE MA23 \ do the following, otherwise jump to MA23 to skip the \ sun altitude check LDA #30 \ Set CABTMP to 30, the cabin temperature in deep space STA CABTMP \ (i.e. one notch on the dashboard bar) LDA SSPR \ If we are inside the space station safe zone, jump to BNE MA23 \ MA23 to skip the following, as we can't have both the \ sun and space station at the same time, so we clearly \ can't be flying near the sun LDY #NI% \ Set Y to NI%, which is the offset in K% for the sun's \ data block, as the second block at K% is reserved for \ the sun (or space station) JSR MAS2 \ Call MAS2 to calculate the largest distance to the BNE MA23 \ sun in any of the three axes, and if it's non-zero, \ jump to MA23 to skip the following, as we are too far \ from the sun for scooping or temperature changes JSR MAS3 \ Set A = x_hi^2 + y_hi^2 + z_hi^2, so using Pythagoras \ we now know that A now contains the square of the \ distance between our ship (at the origin) and the \ heart of the sun at (x_hi, y_hi, z_hi) EOR #%11111111 \ Invert A, so A is now small if we are far from the \ sun and large if we are close to the sun, in the \ range 0 = far away to &FF = extremely close, ouch, \ hot, hot, hot! ADC #30 \ Add the minimum cabin temperature of 30, so we get \ one of the following: \ \ * If the C flag is clear, A contains the cabin \ temperature, ranging from 30 to 255, that's hotter \ the closer we are to the sun \ \ * If the C flag is set, the addition has rolled over \ and the cabin temperature is over 255 STA CABTMP \ Store the updated cabin temperature BCS MA28 \ If the C flag is set then jump to MA28 to die, as \ our temperature is off the scale CMP #&E0 \ If the cabin temperature < 224 then jump to MA23 to BCC MA23 \ to skip fuel scooping, as we aren't close enough LDA BST \ If we don't have fuel scoops fitted, jump to BA23 to BEQ MA23 \ skip fuel scooping, as we can't scoop without fuel \ scoops LDA DELT4+1 \ We are now successfully fuel scooping, so it's time LSR A \ to work out how much fuel we're scooping. Fetch the \ high byte of DELT4, which contains our current speed \ divided by 4, and halve it to get our current speed \ divided by 8 (so it's now a value between 1 and 5, as \ our speed is normally between 1 and 40). This gives \ us the amount of fuel that's being scooped in A, so \ the faster we go, the more fuel we scoop, and because \ the fuel levels are stored as 10 * the fuel in light \ years, that means we just scooped between 0.1 and 0.5 \ light years of free fuel ADC QQ14 \ Set A = A + the current fuel level * 10 (from QQ14) CMP #70 \ If A > 70 then set A = 70 (as 70 is the maximum fuel BCC P%+4 \ level, or 7.0 light years) LDA #70 STA QQ14 \ Store the updated fuel level in QQ14 LDA #160 \ Print recursive token 0 ("FUEL SCOOPS ON") as an JSR MESS \ in-flight message
Name: Main flight loop (Part 16 of 16) [View individually] Type: Subroutine Category: Main loop Summary: Process laser pulsing, E.C.M. energy drain, call stardust routine
The main flight loop covers most of the flight-specific aspects of Elite. This section covers the following: * Process laser pulsing * Process E.C.M. energy drain * Jump to the stardust routine if we are in a space view * Return from the main flight loop
.MA23 LDA LAS2 \ If the current view has no laser, jump to MA16 to skip BEQ MA16 \ the following LDA LASCT \ If LASCT >= 8, jump to MA16 to skip the following, so CMP #8 \ for a pulse laser with a LASCT between 8 and 10, the BCS MA16 \ the laser stays on, but for a LASCT of 7 or less it \ gets turned off and stays off until LASCT reaches zero \ and the next pulse can start (if the fire button is \ still being pressed) \ \ For pulse lasers, LASCT gets set to 10 in ma1 above, \ and it decrements every vertical sync (50 times a \ second), so this means it pulses five times a second, \ with the laser being on for the first 3/10 of each \ pulse and off for the rest of the pulse \ \ If this is a beam laser, LASCT is 0 so we always keep \ going here. This means the laser doesn't pulse, but it \ does get drawn and removed every cycle, in a slightly \ different place each time, so the beams still flicker \ around the screen JSR LASLI2 \ Redraw the existing laser lines, which has the effect \ of removing them from the screen LDA #0 \ Set LAS2 to 0 so if this is a pulse laser, it will STA LAS2 \ skip over the above until the next pulse (this has no \ effect if this is a beam laser) .MA16 LDA ECMP \ If our E.C.M is not on, skip to MA69, otherwise keep BEQ MA69 \ going to drain some energy JSR DENGY \ Call DENGY to deplete our energy banks by 1 BEQ MA70 \ If we have no energy left, jump to MA70 to turn our \ E.C.M. off .MA69 LDA ECMA \ If an E.C.M is going off (our's or an opponent's) then BEQ MA66 \ keep going, otherwise skip to MA66 DEC ECMA \ Decrement the E.C.M. countdown timer, and if it has BNE MA66 \ reached zero, keep going, otherwise skip to MA66 .MA70 JSR ECMOF \ If we get here then either we have either run out of \ energy, or the E.C.M. timer has run down, so switch \ off the E.C.M. .MA66 LDA QQ11 \ If this is not a space view (i.e. QQ11 is non-zero) BNE MA9 \ then jump to MA9 to return from the main flight loop \ (as MA9 is an RTS) JMP STARS \ This is a space view, so jump to the STARS routine to \ process the stardust, and return from the main flight \ loop using a tail call
Name: MAS1 [View individually] Type: Subroutine Category: Maths (Geometry) Summary: Add an orientation vector coordinate to an INWK coordinate
Add a doubled nosev vector coordinate, e.g. (nosev_y_hi nosev_y_lo) * 2, to an INWK coordinate, e.g. (x_sign x_hi x_lo), storing the result in the INWK coordinate. The axes used in each side of the addition are specified by the arguments X and Y. In the comments below, we document the routine as if we are doing the following, i.e. if X = 0 and Y = 11: (x_sign x_hi x_lo) = (x_sign x_hi x_lo) + (nosev_y_hi nosev_y_lo) * 2 as that way the variable names in the comments contain "x" and "y" to match the registers that specify the vector axis to use. Arguments: X The coordinate to add, as follows: * If X = 0, add (x_sign x_hi x_lo) * If X = 3, add (y_sign y_hi y_lo) * If X = 6, add (z_sign z_hi z_lo) Y The vector to add, as follows: * If Y = 9, add (nosev_x_hi nosev_x_lo) * If Y = 11, add (nosev_y_hi nosev_y_lo) * If Y = 13, add (nosev_z_hi nosev_z_lo) Returns: A The high byte of the result with the sign cleared (e.g. |x_hi| if X = 0, etc.) Other entry points: MA9 Contains an RTS
.MAS1 LDA INWK,Y \ Set K(2 1) = (nosev_y_hi nosev_y_lo) * 2 ASL A STA K+1 LDA INWK+1,Y ROL A STA K+2 LDA #0 \ Set K+3 bit 7 to the C flag, so the sign bit of the ROR A \ above result goes into K+3 STA K+3 JSR MVT3 \ Add (x_sign x_hi x_lo) to K(3 2 1) STA INWK+2,X \ Store the sign of the result in x_sign LDY K+1 \ Store K(2 1) in (x_hi x_lo) STY INWK,X LDY K+2 STY INWK+1,X AND #%01111111 \ Set A to the sign byte with the sign cleared .MA9 RTS \ Return from the subroutine
Name: MAS2 [View individually] Type: Subroutine Category: Maths (Geometry) Summary: Calculate a cap on the maximum distance to the planet or sun
Given a value in Y that points to the start of a ship data block as an offset from K%, calculate the following: A = A OR x_sign OR y_sign OR z_sign and clear the sign bit of the result. The K% workspace contains the ship data blocks, so the offset in Y must be 0 or a multiple of NI% (as each block in K% contains NI% bytes). The result effectively contains a maximum cap of the three values (though it might not be one of the three input values - it's just guaranteed to be larger than all of them). If Y = 0 and A = 0, then this calculates the maximum cap of the highest byte containing the distance to the planet, as K%+2 = x_sign, K%+5 = y_sign and K%+8 = z_sign (the first slot in the K% workspace represents the planet). Arguments: Y The offset from K% for the start of the ship data block to use Returns: A A OR K%+2+Y OR K%+5+Y OR K%+8+Y, with bit 7 cleared Other entry points: m Do not include A in the calculation
.m LDA #0 \ Set A = 0 and fall through into MAS2 to calculate the \ OR of the three bytes at K%+2+Y, K%+5+Y and K%+8+Y .MAS2 ORA K%+2,Y \ Set A = A OR x_sign OR y_sign OR z_sign ORA K%+5,Y ORA K%+8,Y AND #%01111111 \ Clear bit 7 in A RTS \ Return from the subroutine
Name: MAS3 [View individually] Type: Subroutine Category: Maths (Arithmetic) Summary: Calculate A = x_hi^2 + y_hi^2 + z_hi^2 in the K% block
Given a value in Y that points to the start of a ship data block as an offset from K%, calculate the following: A = x_hi^2 + y_hi^2 + z_hi^2 returning A = &FF if the calculation overflows a one-byte result. The K% workspace contains the ship data blocks, so the offset in Y must be 0 or a multiple of NI% (as each block in K% contains NI% bytes). Arguments: Y The offset from K% for the start of the ship data block to use Returns A A = x_hi^2 + y_hi^2 + z_hi^2 A = &FF if the calculation overflows a one-byte result
.MAS3 LDA K%+1,Y \ Set (A P) = x_hi * x_hi JSR SQUA2 STA R \ Store A (high byte of result) in R LDA K%+4,Y \ Set (A P) = y_hi * y_hi JSR SQUA2 ADC R \ Add A (high byte of second result) to R BCS MA30 \ If the addition of the two high bytes caused a carry \ (i.e. they overflowed), jump to MA30 to return A = &FF STA R \ Store A (sum of the two high bytes) in R LDA K%+7,Y \ Set (A P) = z_hi * z_hi JSR SQUA2 ADC R \ Add A (high byte of third result) to R, so R now \ contains the sum of x_hi^2 + y_hi^2 + z_hi^2 BCC P%+4 \ If there is no carry, skip the following instruction \ to return straight from the subroutine .MA30 LDA #&FF \ The calculation has overflowed, so set A = &FF RTS \ Return from the subroutine
Name: MVEIT (Part 1 of 9) [View individually] Type: Subroutine Category: Moving Summary: Move current ship: Tidy the orientation vectors Deep dive: Orientation vectors
This routine has multiple stages. This stage does the following: * Tidy the orientation vectors for one of the ship slots Arguments: INWK The current ship/planet/sun's data block XSAV The slot number of the current ship/planet/sun TYPE The type of the current ship/planet/sun
.MVEIT LDA INWK+31 \ If bits 5 or 7 are set, jump to MV30 as the ship is AND #%10100000 \ either exploding or has been killed, so we don't need BNE MV30 \ to tidy its orientation vectors or apply tactics LDA MCNT \ Fetch the main loop counter EOR XSAV \ Fetch the slot number of the ship we are moving, EOR AND #15 \ with the loop counter and apply mod 15 to the result. BNE MV3 \ The result will be zero when "counter mod 15" matches \ the slot number, so this makes sure we call TIDY 13 \ times every 16 main loop iteration, like this: \ \ Iteration 0, tidy the ship in slot 0 \ Iteration 1, tidy the ship in slot 1 \ Iteration 2, tidy the ship in slot 2 \ ... \ Iteration 11, tidy the ship in slot 11 \ Iteration 12, tidy the ship in slot 12 \ Iteration 13, do nothing \ Iteration 14, do nothing \ Iteration 15, do nothing \ Iteration 16, tidy the ship in slot 0 \ ... \ \ and so on JSR TIDY \ Call TIDY to tidy up the orientation vectors, to \ prevent the ship from getting elongated and out of \ shape due to the imprecise nature of trigonometry \ in assembly language
Name: MVEIT (Part 2 of 9) [View individually] Type: Subroutine Category: Moving Summary: Move current ship: Call tactics routine, remove ship from scanner
This routine has multiple stages. This stage does the following: * Apply tactics to ships with AI enabled (by calling the TACTICS routine) * Remove the ship from the scanner, so we can move it
.MV3 LDX TYPE \ If the type of the ship we are moving is positive, BPL P%+5 \ i.e. it is not a planet (types 128 and 130) or sun \ (type 129), then skip the following instruction JMP MV40 \ This item is the planet or sun, so jump to MV40 to \ move it, which ends by jumping back into this routine \ at MV45 (after all the rotation, tactics and scanner \ code, which we don't need to apply to planets or suns) LDA INWK+32 \ Fetch the ship's byte #32 (AI flag) into A BPL MV30 \ If bit 7 of the AI flag is clear, then if this is a \ ship or missile it is dumb and has no AI, and if this \ is the space station it is not hostile, so in both \ cases skip the following as it has no tactics CPX #MSL \ If the ship is a missile, skip straight to MV26 to BEQ MV26 \ call the TACTICS routine, as we do this every \ iteration of the main loop for missiles only LDA MCNT \ Fetch the main loop counter EOR XSAV \ Fetch the slot number of the ship we are moving, EOR AND #7 \ with the loop counter and apply mod 8 to the result. BNE MV30 \ The result will be zero when "counter mod 8" matches \ the slot number mod 8, so this makes sure we call \ TACTICS 13 times every 8 main loop iteration, like \ this: \ \ Iteration 0, apply tactics to slots 0 and 8 \ Iteration 1, apply tactics to slots 1 and 9 \ Iteration 2, apply tactics to slots 2 and 10 \ Iteration 3, apply tactics to slots 3 and 11 \ Iteration 4, apply tactics to slots 4 and 12 \ Iteration 5, apply tactics to slot 5 \ Iteration 6, apply tactics to slot 6 \ Iteration 7, apply tactics to slot 7 \ Iteration 8, apply tactics to slots 0 and 8 \ ... \ \ and so on .MV26 JSR TACTICS \ Call TACTICS to apply AI tactics to this ship .MV30 JSR SCAN \ Draw the ship on the scanner, which has the effect of \ removing it, as it's already at this point and hasn't \ yet moved
Name: MVEIT (Part 3 of 9) [View individually] Type: Subroutine Category: Moving Summary: Move current ship: Move ship forward according to its speed
This routine has multiple stages. This stage does the following: * Move the ship forward (along the vector pointing in the direction of travel) according to its speed: (x, y, z) += nosev_hi * speed / 64
LDA INWK+27 \ Set Q = the ship's speed byte #27 * 4 ASL A ASL A STA Q LDA INWK+10 \ Set A = |nosev_x_hi| AND #%01111111 JSR FMLTU \ Set R = A * Q / 256 STA R \ = |nosev_x_hi| * speed / 64 LDA INWK+10 \ If nosev_x_hi is positive, then: LDX #0 \ JSR MVT1-2 \ (x_sign x_hi x_lo) = (x_sign x_hi x_lo) + R \ \ If nosev_x_hi is negative, then: \ \ (x_sign x_hi x_lo) = (x_sign x_hi x_lo) - R \ \ So in effect, this does: \ \ (x_sign x_hi x_lo) += nosev_x_hi * speed / 64 LDA INWK+12 \ Set A = |nosev_y_hi| AND #%01111111 JSR FMLTU \ Set R = A * Q / 256 STA R \ = |nosev_y_hi| * speed / 64 LDA INWK+12 \ If nosev_y_hi is positive, then: LDX #3 \ JSR MVT1-2 \ (y_sign y_hi y_lo) = (y_sign y_hi y_lo) + R \ \ If nosev_y_hi is negative, then: \ \ (y_sign y_hi y_lo) = (y_sign y_hi y_lo) - R \ \ So in effect, this does: \ \ (y_sign y_hi y_lo) += nosev_y_hi * speed / 64 LDA INWK+14 \ Set A = |nosev_z_hi| AND #%01111111 JSR FMLTU \ Set R = A * Q / 256 STA R \ = |nosev_z_hi| * speed / 64 LDA INWK+14 \ If nosev_y_hi is positive, then: LDX #6 \ JSR MVT1-2 \ (z_sign z_hi z_lo) = (z_sign z_hi z_lo) + R \ \ If nosev_z_hi is negative, then: \ \ (z_sign z_hi z_lo) = (z_sign z_hi z_lo) - R \ \ So in effect, this does: \ \ (z_sign z_hi z_lo) += nosev_z_hi * speed / 64
Name: MVEIT (Part 4 of 9) [View individually] Type: Subroutine Category: Moving Summary: Move current ship: Apply acceleration to ship's speed as a one-off
This routine has multiple stages. This stage does the following: * Apply acceleration to the ship's speed (if acceleration is non-zero), and then zero the acceleration as it's a one-off change
LDA INWK+27 \ Set A = the ship's speed in byte #24 + the ship's CLC \ acceleration in byte #28 ADC INWK+28 BPL P%+4 \ If the result is positive, skip the following \ instruction LDA #0 \ Set A to 0 to stop the speed from going negative LDY #15 \ Fetch byte #15 from the ship's blueprint, which \ contains the ship's maximum speed CMP (XX0),Y \ If A < the ship's maximum speed, skip the following BCC P%+4 \ instruction LDA (XX0),Y \ Set A to the ship's maximum speed STA INWK+27 \ We have now calculated the new ship's speed after \ accelerating and keeping the speed within the ship's \ limits, so store the updated speed in byte #27 LDA #0 \ We have added the ship's acceleration, so we now set STA INWK+28 \ it back to 0 in byte #28, as it's a one-off change
Name: MVEIT (Part 5 of 9) [View individually] Type: Subroutine Category: Moving Summary: Move current ship: Rotate ship's location by our pitch and roll Deep dive: Rotating the universe
This routine has multiple stages. This stage does the following: * Rotate the ship's location in space by the amount of pitch and roll of our ship. See below for a deeper explanation of this routine
LDX ALP1 \ Fetch the magnitude of the current roll into X, so \ if the roll angle is alpha, X contains |alpha| LDA INWK \ Set P = ~x_lo (i.e. with all its bits flipped) so that EOR #%11111111 \ we can pass x_lo to MLTU2 below) STA P LDA INWK+1 \ Set A = x_hi JSR MLTU2-2 \ Set (A P+1 P) = (A ~P) * X \ = (x_hi x_lo) * alpha STA P+2 \ Store the high byte of the result in P+2, so we now \ have: \ \ P(2 1 0) = (x_hi x_lo) * alpha LDA ALP2+1 \ Fetch the flipped sign of the current roll angle alpha EOR INWK+2 \ from ALP2+1 and EOR with byte #2 (x_sign), so if the \ flipped roll angle and x_sign have the same sign, A \ will be positive, else it will be negative. So A will \ contain the sign bit of x_sign * flipped alpha sign, \ which is the opposite to the sign of the above result, \ so we now have: \ \ (A P+2 P+1) = - (x_sign x_hi x_lo) * alpha / 256 LDX #3 \ Set (A P+2 P+1) = (y_sign y_hi y_lo) + (A P+2 P+1) JSR MVT6 \ = y - x * alpha / 256 STA K2+3 \ Set K2(3) = A = the sign of the result LDA P+1 \ Set K2(1) = P+1, the low byte of the result STA K2+1 EOR #%11111111 \ Set P = ~K2+1 (i.e. with all its bits flipped) so STA P \ that we can pass K2+1 to MLTU2 below) LDA P+2 \ Set K2(2) = A = P+2 STA K2+2 \ So we now have result 1 above: \ \ K2(3 2 1) = (A P+2 P+1) \ = y - x * alpha / 256 LDX BET1 \ Fetch the magnitude of the current pitch into X, so \ if the pitch angle is beta, X contains |beta| JSR MLTU2-2 \ Set (A P+1 P) = (A ~P) * X \ = K2(2 1) * beta STA P+2 \ Store the high byte of the result in P+2, so we now \ have: \ \ P(2 1 0) = K2(2 1) * beta LDA K2+3 \ Fetch the sign of the above result in K(3 2 1) from EOR BET2 \ K2+3 and EOR with BET2, the sign of the current pitch \ rate, so if the pitch and K(3 2 1) have the same sign, \ A will be positive, else it will be negative. So A \ will contain the sign bit of K(3 2 1) * beta, which is \ the same as the sign of the above result, so we now \ have: \ \ (A P+2 P+1) = K2(3 2 1) * beta / 256 LDX #6 \ Set (A P+2 P+1) = (z_sign z_hi z_lo) + (A P+2 P+1) JSR MVT6 \ = z + K2 * beta / 256 STA INWK+8 \ Set z_sign = A = the sign of the result LDA P+1 \ Set z_lo = P+1, the low byte of the result STA INWK+6 EOR #%11111111 \ Set P = ~z_lo (i.e. with all its bits flipped) so that STA P \ we can pass z_lo to MLTU2 below) LDA P+2 \ Set z_hi = P+2 STA INWK+7 \ So we now have result 2 above: \ \ (z_sign z_hi z_lo) = (A P+2 P+1) \ = z + K2 * beta / 256 JSR MLTU2 \ MLTU2 doesn't change Q, and Q was set to beta in \ the previous call to MLTU2, so this call does: \ \ (A P+1 P) = (A ~P) * Q \ = (z_hi z_lo) * beta STA P+2 \ Set P+2 = A = the high byte of the result, so we \ now have: \ \ P(2 1 0) = (z_hi z_lo) * beta LDA K2+3 \ Set y_sign = K2+3 STA INWK+5 EOR BET2 \ EOR y_sign with BET2, the sign of the current pitch EOR INWK+8 \ rate, and z_sign. If the result is positive jump to BPL MV43 \ MV43, otherwise this means beta * z and y have \ different signs, i.e. P(2 1) and K2(3 2 1) have \ different signs, so we need to add them in order to \ calculate K2(2 1) - P(2 1) LDA P+1 \ Set (y_hi y_lo) = K2(2 1) + P(2 1) ADC K2+1 STA INWK+3 LDA P+2 ADC K2+2 STA INWK+4 JMP MV44 \ Jump to MV44 to continue the calculation .MV43 LDA K2+1 \ Reversing the logic above, we need to subtract P(2 1) SBC P+1 \ and K2(3 2 1) to calculate K2(2 1) - P(2 1), so this STA INWK+3 \ sets (y_hi y_lo) = K2(2 1) - P(2 1) LDA K2+2 SBC P+2 STA INWK+4 BCS MV44 \ If the above subtraction did not underflow, then \ jump to MV44, otherwise we need to negate the result LDA #1 \ Negate (y_sign y_hi y_lo) using two's complement, SBC INWK+3 \ first doing the low bytes: STA INWK+3 \ \ y_lo = 1 - y_lo LDA #0 \ Then the high bytes: SBC INWK+4 \ STA INWK+4 \ y_hi = 0 - y_hi LDA INWK+5 \ And finally flip the sign in y_sign EOR #%10000000 STA INWK+5 .MV44 \ So we now have result 3 above: \ \ (y_sign y_hi y_lo) = K2(2 1) - P(2 1) \ = K2 - beta * z LDX ALP1 \ Fetch the magnitude of the current roll into X, so \ if the roll angle is alpha, X contains |alpha| LDA INWK+3 \ Set P = ~y_lo (i.e. with all its bits flipped) so that EOR #&FF \ we can pass y_lo to MLTU2 below) STA P LDA INWK+4 \ Set A = y_hi JSR MLTU2-2 \ Set (A P+1 P) = (A ~P) * X \ = (y_hi y_lo) * alpha STA P+2 \ Store the high byte of the result in P+2, so we now \ have: \ \ P(2 1 0) = (y_hi y_lo) * alpha LDA ALP2 \ Fetch the correct sign of the current roll angle alpha EOR INWK+5 \ from ALP2 and EOR with byte #5 (y_sign), so if the \ correct roll angle and y_sign have the same sign, A \ will be positive, else it will be negative. So A will \ contain the sign bit of x_sign * correct alpha sign, \ which is the same as the sign of the above result, \ so we now have: \ \ (A P+2 P+1) = (y_sign y_hi y_lo) * alpha / 256 LDX #0 \ Set (A P+2 P+1) = (x_sign x_hi x_lo) + (A P+2 P+1) JSR MVT6 \ = x + y * alpha / 256 STA INWK+2 \ Set x_sign = A = the sign of the result LDA P+2 \ Set x_hi = P+2, the high byte of the result STA INWK+1 LDA P+1 \ Set x_lo = P+1, the low byte of the result STA INWK \ So we now have result 4 above: \ \ x = x + alpha * y \ \ and the rotation of (x, y, z) is done
Name: MVEIT (Part 6 of 9) [View individually] Type: Subroutine Category: Moving Summary: Move current ship: Move the ship in space according to our speed
This routine has multiple stages. This stage does the following: * Move the ship in space according to our speed (we already moved it according to its own speed in part 3). We do this by subtracting our speed (i.e. the distance we travel in this iteration of the loop) from the other ship's z-coordinate. We subtract because they appear to be "moving" in the opposite direction to us, and the whole MVEIT routine is about moving the other ships rather than us (even though we are the one doing the moving).
.MV45 LDA DELTA \ Set R to our speed in DELTA STA R LDA #%10000000 \ Set A to zeroes but with bit 7 set, so that (A R) is \ a 16-bit number containing -R, or -speed LDX #6 \ Set X to the z-axis so the call to MVT1 does this: JSR MVT1 \ \ (z_sign z_hi z_lo) = (z_sign z_hi z_lo) + (A R) \ = (z_sign z_hi z_lo) - speed LDA TYPE \ If the ship type is not the sun (129) then skip the AND #%10000001 \ next instruction, otherwise return from the subroutine CMP #129 \ as we don't need to rotate the sun around its origin. BNE P%+3 \ Having both the AND and the CMP is a little odd, as \ the sun is the only ship type with bits 0 and 7 set, \ so the AND has no effect and could be removed RTS \ Return from the subroutine, as the ship we are moving \ is the sun and doesn't need any of the following
Name: MVEIT (Part 7 of 9) [View individually] Type: Subroutine Category: Moving Summary: Move current ship: Rotate ship's orientation vectors by pitch/roll
This routine has multiple stages. This stage does the following: * Rotate the ship's orientation vectors according to our pitch and roll As with the previous step, this is all about moving the other ships rather than us (even though we are the one doing the moving). So we rotate the current ship's orientation vectors (which defines its orientation in space), by the angles we are "moving" the rest of the sky through (alpha and beta, our roll and pitch), so the ship appears to us to be stationary while we rotate.
LDY #9 \ Apply our pitch and roll rotations to the current JSR MVS4 \ ship's nosev vector LDY #15 \ Apply our pitch and roll rotations to the current JSR MVS4 \ ship's roofv vector LDY #21 \ Apply our pitch and roll rotations to the current JSR MVS4 \ ship's sidev vector
Name: MVEIT (Part 8 of 9) [View individually] Type: Subroutine Category: Moving Summary: Move current ship: Rotate ship about itself by its own pitch/roll
This routine has multiple stages. This stage does the following: * If the ship we are processing is rolling or pitching itself, rotate it and apply damping if required
LDA INWK+30 \ Fetch the ship's pitch counter and extract the sign AND #%10000000 \ into RAT2 STA RAT2 LDA INWK+30 \ Fetch the ship's pitch counter and extract the value AND #%01111111 \ without the sign bit into A BEQ MV8 \ If the pitch counter is 0, then jump to MV8 to skip \ the following, as the ship is not pitching CMP #%01111111 \ If bits 0-6 are set in the pitch counter (i.e. the \ ship's pitch is not damping down), then the C flag \ will be set by this instruction SBC #0 \ Set A = A - 0 - (1 - C), so if we are damping then we \ reduce A by 1, otherwise it is unchanged ORA RAT2 \ Change bit 7 of A to the sign we saved in RAT2, so \ the updated pitch counter in A retains its sign STA INWK+30 \ Store the updated pitch counter in byte #30 LDX #15 \ Rotate (roofv_x, nosev_x) by a small angle (pitch) LDY #9 JSR MVS5 LDX #17 \ Rotate (roofv_y, nosev_y) by a small angle (pitch) LDY #11 JSR MVS5 LDX #19 \ Rotate (roofv_z, nosev_z) by a small angle (pitch) LDY #13 JSR MVS5 .MV8 LDA INWK+29 \ Fetch the ship's roll counter and extract the sign AND #%10000000 \ into RAT2 STA RAT2 LDA INWK+29 \ Fetch the ship's roll counter and extract the value AND #%01111111 \ without the sign bit into A BEQ MV5 \ If the roll counter is 0, then jump to MV5 to skip the \ following, as the ship is not rolling CMP #%01111111 \ If bits 0-6 are set in the roll counter (i.e. the \ ship's roll is not damping down), then the C flag \ will be set by this instruction SBC #0 \ Set A = A - 0 - (1 - C), so if we are damping then we \ reduce A by 1, otherwise it is unchanged ORA RAT2 \ Change bit 7 of A to the sign we saved in RAT2, so \ the updated roll counter in A retains its sign STA INWK+29 \ Store the updated pitch counter in byte #29 LDX #15 \ Rotate (roofv_x, sidev_x) by a small angle (roll) LDY #21 JSR MVS5 LDX #17 \ Rotate (roofv_y, sidev_y) by a small angle (roll) LDY #23 JSR MVS5 LDX #19 \ Rotate (roofv_z, sidev_z) by a small angle (roll) LDY #25 JSR MVS5
Name: MVEIT (Part 9 of 9) [View individually] Type: Subroutine Category: Moving Summary: Move current ship: Redraw on scanner, if it hasn't been destroyed
This routine has multiple stages. This stage does the following: * If the ship is exploding or being removed, hide it on the scanner * Otherwise redraw the ship on the scanner, now that it's been moved
.MV5 LDA INWK+31 \ Fetch the ship's exploding/killed state from byte #31 AND #%10100000 \ If we are exploding or removing this ship then jump to BNE MVD1 \ MVD1 to remove it from the scanner permanently LDA INWK+31 \ Set bit 4 to keep the ship visible on the scanner ORA #%00010000 STA INWK+31 JMP SCAN \ Display the ship on the scanner, returning from the \ subroutine with a tail call .MVD1 LDA INWK+31 \ Clear bit 4 to hide the ship on the scanner AND #%11101111 STA INWK+31 RTS \ Return from the subroutine
Name: MVT1 [View individually] Type: Subroutine Category: Moving Summary: Calculate (x_sign x_hi x_lo) = (x_sign x_hi x_lo) + (A R)
Add the signed delta (A R) to a ship's coordinate, along the axis given in X. Mathematically speaking, this routine translates the ship along a single axis by a signed delta. Taking the example of X = 0, the x-axis, it does the following: (x_sign x_hi x_lo) = (x_sign x_hi x_lo) + (A R) (In practice, MVT1 is only ever called directly with A = 0 or 128, otherwise it is always called via MVT-2, which clears A apart from the sign bit. The routine is written to cope with a non-zero delta_hi, so it supports a full 16-bit delta, but it appears that delta_hi is only ever used to hold the sign of the delta.) The comments below assume we are adding delta to the x-axis, though the axis is determined by the value of X. Arguments: (A R) The signed delta, so A = delta_hi and R = delta_lo X Determines which coordinate axis of INWK to change: * X = 0 adds the delta to (x_lo, x_hi, x_sign) * X = 3 adds the delta to (y_lo, y_hi, y_sign) * X = 6 adds the delta to (z_lo, z_hi, z_sign) Other entry points: MVT1-2 Clear bits 0-6 of A before entering MVT1
AND #%10000000 \ Clear bits 0-6 of A .MVT1 ASL A \ Set the C flag to the sign bit of the delta, leaving \ delta_hi << 1 in A STA S \ Set S = delta_hi << 1 \ \ This also clears bit 0 of S LDA #0 \ Set T = just the sign bit of delta (in bit 7) ROR A STA T LSR S \ Set S = delta_hi >> 1 \ = |delta_hi| \ \ This also clear the C flag, as we know that bit 0 of \ S was clear before the LSR EOR INWK+2,X \ If T EOR x_sign has bit 7 set, then x_sign and delta BMI MV10 \ have different signs, so jump to MV10 \ At this point, we know x_sign and delta have the same \ sign, that sign is in T, and S contains |delta_hi|, \ so now we want to do: \ \ (x_sign x_hi x_lo) = (x_sign x_hi x_lo) + (S R) \ \ and then set the sign of the result to the same sign \ as x_sign and delta LDA R \ First we add the low bytes, so: ADC INWK,X \ STA INWK,X \ x_lo = x_lo + R LDA S \ Then we add the high bytes: ADC INWK+1,X \ STA INWK+1,X \ x_hi = x_hi + S LDA INWK+2,X \ And finally we add any carry into x_sign, and if the ADC #0 \ sign of x_sign and delta in T is negative, make sure ORA T \ the result is negative (by OR'ing with T) STA INWK+2,X RTS \ Return from the subroutine .MV10 \ If we get here, we know x_sign and delta have \ different signs, with delta's sign in T, and \ |delta_hi| in S, so now we want to do: \ \ (x_sign x_hi x_lo) = (x_sign x_hi x_lo) - (S R) \ \ and then set the sign of the result according to \ the signs of x_sign and delta LDA INWK,X \ First we subtract the low bytes, so: SEC \ SBC R \ x_lo = x_lo - R STA INWK,X LDA INWK+1,X \ Then we subtract the high bytes: SBC S \ STA INWK+1,X \ x_hi = x_hi - S LDA INWK+2,X \ And finally we subtract any borrow from bits 0-6 of AND #%01111111 \ x_sign, and give the result the opposite sign bit to T SBC #0 \ (i.e. give it the sign of the original x_sign) ORA #%10000000 EOR T STA INWK+2,X BCS MV11 \ If the C flag is set by the above SBC, then our sum \ above didn't underflow and is correct - to put it \ another way, (x_sign x_hi x_lo) >= (S R) so the result \ should indeed have the same sign as x_sign, so jump to \ MV11 to return from the subroutine \ Otherwise our subtraction underflowed because \ (x_sign x_hi x_lo) < (S R), so we now need to flip the \ subtraction around by using two's complement to this: \ \ (S R) - (x_sign x_hi x_lo) \ \ and then we need to give the result the same sign as \ (S R), the delta, as that's the dominant figure in the \ sum LDA #1 \ First we subtract the low bytes, so: SBC INWK,X \ STA INWK,X \ x_lo = 1 - x_lo LDA #0 \ Then we subtract the high bytes: SBC INWK+1,X \ STA INWK+1,X \ x_hi = 0 - x_hi LDA #0 \ And then we subtract the sign bytes: SBC INWK+2,X \ \ x_sign = 0 - x_sign AND #%01111111 \ Finally, we set the sign bit to the sign in T, the ORA T \ sign of the original delta, as the delta is the STA INWK+2,X \ dominant figure in the sum .MV11 RTS \ Return from the subroutine
Name: MVT3 [View individually] Type: Subroutine Category: Moving Summary: Calculate K(3 2 1) = (x_sign x_hi x_lo) + K(3 2 1)
Add an INWK position coordinate - i.e. x, y or z - to K(3 2 1), like this: K(3 2 1) = (x_sign x_hi x_lo) + K(3 2 1) The INWK coordinate to add to K(3 2 1) is specified by X. Arguments: X The coordinate to add to K(3 2 1), as follows: * If X = 0, add (x_sign x_hi x_lo) * If X = 3, add (y_sign y_hi y_lo) * If X = 6, add (z_sign z_hi z_lo) Returns: A Contains a copy of the high byte of the result, K+3 X X is preserved
.MVT3 LDA K+3 \ Set S = K+3 STA S AND #%10000000 \ Set T = sign bit of K(3 2 1) STA T EOR INWK+2,X \ If x_sign has a different sign to K(3 2 1), jump to BMI MV13 \ MV13 to process the addition as a subtraction LDA K+1 \ Set K(3 2 1) = K(3 2 1) + (x_sign x_hi x_lo) CLC \ starting with the low bytes ADC INWK,X STA K+1 LDA K+2 \ Then the middle bytes ADC INWK+1,X STA K+2 LDA K+3 \ And finally the high bytes ADC INWK+2,X AND #%01111111 \ Setting the sign bit of K+3 to T, the original sign ORA T \ of K(3 2 1) STA K+3 RTS \ Return from the subroutine .MV13 LDA S \ Set S = |K+3| (i.e. K+3 with the sign bit cleared) AND #%01111111 STA S LDA INWK,X \ Set K(3 2 1) = (x_sign x_hi x_lo) - K(3 2 1) SEC \ starting with the low bytes SBC K+1 STA K+1 LDA INWK+1,X \ Then the middle bytes SBC K+2 STA K+2 LDA INWK+2,X \ And finally the high bytes, doing A = |x_sign| - |K+3| AND #%01111111 \ and setting the C flag for testing below SBC S ORA #%10000000 \ Set the sign bit of K+3 to the opposite sign of T, EOR T \ i.e. the opposite sign to the original K(3 2 1) STA K+3 BCS MV14 \ If the C flag is set, i.e. |x_sign| >= |K+3|, then \ the sign of K(3 2 1). In this case, we want the \ result to have the same sign as the largest argument, \ which is (x_sign x_hi x_lo), which we know has the \ opposite sign to K(3 2 1), and that's what we just set \ the sign of K(3 2 1) to... so we can jump to MV14 to \ return from the subroutine LDA #1 \ We need to swap the sign of the result in K(3 2 1), SBC K+1 \ which we do by calculating 0 - K(3 2 1), which we can STA K+1 \ do with 1 - C - K(3 2 1), as we know the C flag is \ clear. We start with the low bytes LDA #0 \ Then the middle bytes SBC K+2 STA K+2 LDA #0 \ And finally the high bytes SBC K+3 AND #%01111111 \ Set the sign bit of K+3 to the same sign as T, ORA T \ i.e. the same sign as the original K(3 2 1), as STA K+3 \ that's the largest argument .MV14 RTS \ Return from the subroutine
Name: MVS4 [View individually] Type: Subroutine Category: Moving Summary: Apply pitch and roll to an orientation vector Deep dive: Pitching and rolling
Apply pitch and roll angles alpha and beta to the orientation vector in Y. Specifically, this routine rotates a point (x, y, z) around the origin by pitch alpha and roll beta, using the small angle approximation to make the maths easier, and incorporating the Minsky circle algorithm to make the rotation more stable (though more elliptic). If that paragraph makes sense to you, then you should probably be writing this commentary! For the rest of us, there's a detailed explanation of all this in the deep dive on "Pitching and rolling". Arguments: Y Determines which of the INWK orientation vectors to transform: * Y = 9 rotates nosev: (nosev_x, nosev_y, nosev_z) * Y = 15 rotates roofv: (roofv_x, roofv_y, roofv_z) * Y = 21 rotates sidev: (sidev_x, sidev_y, sidev_z)
.MVS4 LDA ALPHA \ Set Q = alpha (the roll angle to rotate through) STA Q LDX INWK+2,Y \ Set (S R) = nosev_y STX R LDX INWK+3,Y STX S LDX INWK,Y \ These instructions have no effect as MAD overwrites STX P \ X and P when called, but they set X = P = nosev_x_lo LDA INWK+1,Y \ Set A = -nosev_x_hi EOR #%10000000 JSR MAD \ Set (A X) = Q * A + (S R) STA INWK+3,Y \ = alpha * -nosev_x_hi + nosev_y STX INWK+2,Y \ \ and store (A X) in nosev_y, so this does: \ \ nosev_y = nosev_y - alpha * nosev_x_hi STX P \ This instruction has no effect as MAD overwrites P, \ but it sets P = nosev_y_lo LDX INWK,Y \ Set (S R) = nosev_x STX R LDX INWK+1,Y STX S LDA INWK+3,Y \ Set A = nosev_y_hi JSR MAD \ Set (A X) = Q * A + (S R) STA INWK+1,Y \ = alpha * nosev_y_hi + nosev_x STX INWK,Y \ \ and store (A X) in nosev_x, so this does: \ \ nosev_x = nosev_x + alpha * nosev_y_hi STX P \ This instruction has no effect as MAD overwrites P, \ but it sets P = nosev_x_lo LDA BETA \ Set Q = beta (the pitch angle to rotate through) STA Q LDX INWK+2,Y \ Set (S R) = nosev_y STX R LDX INWK+3,Y STX S LDX INWK+4,Y STX P \ This instruction has no effect as MAD overwrites P, \ but it sets P = nosev_y LDA INWK+5,Y \ Set A = -nosev_z_hi EOR #%10000000 JSR MAD \ Set (A X) = Q * A + (S R) STA INWK+3,Y \ = beta * -nosev_z_hi + nosev_y STX INWK+2,Y \ \ and store (A X) in nosev_y, so this does: \ \ nosev_y = nosev_y - beta * nosev_z_hi STX P \ This instruction has no effect as MAD overwrites P, \ but it sets P = nosev_y_lo LDX INWK+4,Y \ Set (S R) = nosev_z STX R LDX INWK+5,Y STX S LDA INWK+3,Y \ Set A = nosev_y_hi JSR MAD \ Set (A X) = Q * A + (S R) STA INWK+5,Y \ = beta * nosev_y_hi + nosev_z STX INWK+4,Y \ \ and store (A X) in nosev_z, so this does: \ \ nosev_z = nosev_z + beta * nosev_y_hi RTS \ Return from the subroutine
Name: MVS5 [View individually] Type: Subroutine Category: Moving Summary: Apply a 3.6 degree pitch or roll to an orientation vector Deep dive: Pitching and rolling by a fixed angle
Pitch or roll a ship by a small, fixed amount (1/16 radians, or 3.6 degrees), in a specified direction, by rotating the orientation vectors. The vectors to rotate are given in X and Y, and the direction of the rotation is given in RAT2. The calculation is as follows: * If the direction is positive: X = X * (1 - 1/512) + Y / 16 Y = Y * (1 - 1/512) - X / 16 * If the direction is negative: X = X * (1 - 1/512) - Y / 16 Y = Y * (1 - 1/512) + X / 16 So if X = 15 (roofv_x), Y = 21 (sidev_x) and RAT2 is positive, it does this: roofv_x = roofv_x * (1 - 1/512) + sidev_x / 16 sidev_x = sidev_x * (1 - 1/512) - roofv_x / 16 Arguments: X The first vector to rotate: * If X = 15, rotate roofv_x * If X = 17, rotate roofv_y * If X = 19, rotate roofv_z Y The second vector to rotate: * If Y = 9, rotate nosev_x * If Y = 11, rotate nosev_y * If Y = 13, rotate nosev_z * If Y = 21, rotate sidev_x * If Y = 23, rotate sidev_y * If Y = 25, rotate sidev_z RAT2 The direction of the pitch or roll to perform, positive or negative (i.e. the sign of the roll or pitch counter in bit 7)
.MVS5 LDA INWK+1,X \ Fetch roofv_x_hi, clear the sign bit, divide by 2 and AND #%01111111 \ store in T, so: LSR A \ STA T \ T = |roofv_x_hi| / 2 \ = |roofv_x| / 512 \ \ The above is true because: \ \ |roofv_x| = |roofv_x_hi| * 256 + roofv_x_lo \ \ so: \ \ |roofv_x| / 512 = |roofv_x_hi| * 256 / 512 \ + roofv_x_lo / 512 \ = |roofv_x_hi| / 2 LDA INWK,X \ Now we do the following subtraction: SEC \ SBC T \ (S R) = (roofv_x_hi roofv_x_lo) - |roofv_x| / 512 STA R \ = (1 - 1/512) * roofv_x \ \ by doing the low bytes first LDA INWK+1,X \ And then the high bytes (the high byte of the right SBC #0 \ side of the subtraction being 0) STA S LDA INWK,Y \ Set P = nosev_x_lo STA P LDA INWK+1,Y \ Fetch the sign of nosev_x_hi (bit 7) and store in T AND #%10000000 STA T LDA INWK+1,Y \ Fetch nosev_x_hi into A and clear the sign bit, so AND #%01111111 \ A = |nosev_x_hi| LSR A \ Set (A P) = (A P) / 16 ROR P \ = |nosev_x_hi nosev_x_lo| / 16 LSR A \ = |nosev_x| / 16 ROR P LSR A ROR P LSR A ROR P ORA T \ Set the sign of A to the sign in T (i.e. the sign of \ the original nosev_x), so now: \ \ (A P) = nosev_x / 16 EOR RAT2 \ Give it the sign as if we multiplied by the direction \ by the pitch or roll direction STX Q \ Store the value of X so it can be restored after the \ call to ADD JSR ADD \ (A X) = (A P) + (S R) \ = +/-nosev_x / 16 + (1 - 1/512) * roofv_x STA K+1 \ Set K(1 0) = (1 - 1/512) * roofv_x +/- nosev_x / 16 STX K LDX Q \ Restore the value of X from before the call to ADD LDA INWK+1,Y \ Fetch nosev_x_hi, clear the sign bit, divide by 2 and AND #%01111111 \ store in T, so: LSR A \ STA T \ T = |nosev_x_hi| / 2 \ = |nosev_x| / 512 LDA INWK,Y \ Now we do the following subtraction: SEC \ SBC T \ (S R) = (nosev_x_hi nosev_x_lo) - |nosev_x| / 512 STA R \ = (1 - 1/512) * nosev_x \ \ by doing the low bytes first LDA INWK+1,Y \ And then the high bytes (the high byte of the right SBC #0 \ side of the subtraction being 0) STA S LDA INWK,X \ Set P = roofv_x_lo STA P LDA INWK+1,X \ Fetch the sign of roofv_x_hi (bit 7) and store in T AND #%10000000 STA T LDA INWK+1,X \ Fetch roofv_x_hi into A and clear the sign bit, so AND #%01111111 \ A = |roofv_x_hi| LSR A \ Set (A P) = (A P) / 16 ROR P \ = |roofv_x_hi roofv_x_lo| / 16 LSR A \ = |roofv_x| / 16 ROR P LSR A ROR P LSR A ROR P ORA T \ Set the sign of A to the opposite sign to T (i.e. the EOR #%10000000 \ sign of the original -roofv_x), so now: \ \ (A P) = -roofv_x / 16 EOR RAT2 \ Give it the sign as if we multiplied by the direction \ by the pitch or roll direction STX Q \ Store the value of X so it can be restored after the \ call to ADD JSR ADD \ (A X) = (A P) + (S R) \ = -/+roofv_x / 16 + (1 - 1/512) * nosev_x STA INWK+1,Y \ Set nosev_x = (1-1/512) * nosev_x -/+ roofv_x / 16 STX INWK,Y LDX Q \ Restore the value of X from before the call to ADD LDA K \ Set roofv_x = K(1 0) STA INWK,X \ = (1-1/512) * roofv_x +/- nosev_x / 16 LDA K+1 STA INWK+1,X RTS \ Return from the subroutine
Name: MVT6 [View individually] Type: Subroutine Category: Moving Summary: Calculate (A P+2 P+1) = (x_sign x_hi x_lo) + (A P+2 P+1)
Do the following calculation, for the coordinate given by X (so this is what it does for the x-coordinate): (A P+2 P+1) = (x_sign x_hi x_lo) + (A P+2 P+1) A is a sign bit and is not included in the calculation, but bits 0-6 of A are preserved. Bit 7 is set to the sign of the result. Arguments: A The sign of P(2 1) in bit 7 P(2 1) The 16-bit value we want to add the coordinate to X The coordinate to add, as follows: * If X = 0, add to (x_sign x_hi x_lo) * If X = 3, add to (y_sign y_hi y_lo) * If X = 6, add to (z_sign z_hi z_lo) Returns: A The sign of the result (in bit 7)
.MVT6 TAY \ Store argument A into Y, for later use EOR INWK+2,X \ Set A = A EOR x_sign BMI MV50 \ If the sign is negative, i.e. A and x_sign have \ different signs, jump to MV50 \ The signs are the same, so we can add the two \ arguments and keep the sign to get the result LDA P+1 \ First we add the low bytes: CLC \ ADC INWK,X \ P+1 = P+1 + x_lo STA P+1 LDA P+2 \ And then the high bytes: ADC INWK+1,X \ STA P+2 \ P+2 = P+2 + x_hi TYA \ Restore the original A argument that we stored earlier \ so that we keep the original sign RTS \ Return from the subroutine .MV50 LDA INWK,X \ First we subtract the low bytes: SEC \ SBC P+1 \ P+1 = x_lo - P+1 STA P+1 LDA INWK+1,X \ And then the high bytes: SBC P+2 \ STA P+2 \ P+2 = x_hi - P+2 BCC MV51 \ If the last subtraction underflowed, then the C flag \ will be clear and x_hi < P+2, so jump to MV51 to \ negate the result TYA \ Restore the original A argument that we stored earlier EOR #%10000000 \ but flip bit 7, which flips the sign. We do this \ because x_hi >= P+2 so we want the result to have the \ same sign as x_hi (as it's the dominant side in this \ calculation). The sign of x_hi is x_sign, and x_sign \ has the opposite sign to A, so we flip the sign in A \ to return the correct result RTS \ Return from the subroutine .MV51 LDA #1 \ Our subtraction underflowed, so we negate the result SBC P+1 \ using two's complement, first with the low byte: STA P+1 \ \ P+1 = 1 - P+1 LDA #0 \ And then the high byte: SBC P+2 \ STA P+2 \ P+2 = 0 - P+2 TYA \ Restore the original A argument that we stored earlier \ as this is the correct sign for the result. This is \ because x_hi < P+2, so we want to return the same sign \ as P+2, the dominant side RTS \ Return from the subroutine
Name: MV40 [View individually] Type: Subroutine Category: Moving Summary: Rotate the planet or sun by our ship's pitch and roll
Rotate the planet or sun's location in space by the amount of pitch and roll of our ship. We implement this using the same equations as in part 5 of MVEIT, where we rotated the current ship's location by our pitch and roll. Specifically, the calculation is as follows: 1. K2 = y - alpha * x 2. z = z + beta * K2 3. y = K2 - beta * z 4. x = x + alpha * y See the deep dive on "Rotating the universe" for more details on the above.
.MV40 LDA ALPHA \ Set Q = -ALPHA, so Q contains the angle we want to EOR #%10000000 \ roll the planet through (i.e. in the opposite STA Q \ direction to our ship's roll angle alpha) LDA INWK \ Set P(1 0) = (x_hi x_lo) STA P LDA INWK+1 STA P+1 LDA INWK+2 \ Set A = x_sign JSR MULT3 \ Set K(3 2 1 0) = (A P+1 P) * Q \ \ which also means: \ \ K(3 2 1) = (A P+1 P) * Q / 256 \ = x * -alpha / 256 \ = - alpha * x / 256 LDX #3 \ Set K(3 2 1) = (y_sign y_hi y_lo) + K(3 2 1) JSR MVT3 \ = y - alpha * x / 256 LDA K+1 \ Set K2(2 1) = P(1 0) = K(2 1) STA K2+1 STA P LDA K+2 \ Set K2+2 = K+2 STA K2+2 STA P+1 \ Set P+1 = K+2 LDA BETA \ Set Q = beta, the pitch angle of our ship STA Q LDA K+3 \ Set K+3 to K2+3, so now we have result 1 above: STA K2+3 \ \ K2(3 2 1) = K(3 2 1) \ = y - alpha * x / 256 \ We also have: \ \ A = K+3 \ \ P(1 0) = K(2 1) \ \ so combined, these mean: \ \ (A P+1 P) = K(3 2 1) \ = K2(3 2 1) JSR MULT3 \ Set K(3 2 1 0) = (A P+1 P) * Q \ \ which also means: \ \ K(3 2 1) = (A P+1 P) * Q / 256 \ = K2(3 2 1) * beta / 256 \ = beta * K2 / 256 LDX #6 \ K(3 2 1) = (z_sign z_hi z_lo) + K(3 2 1) JSR MVT3 \ = z + beta * K2 / 256 LDA K+1 \ Set P = K+1 STA P STA INWK+6 \ Set z_lo = K+1 LDA K+2 \ Set P+1 = K+2 STA P+1 STA INWK+7 \ Set z_hi = K+2 LDA K+3 \ Set A = z_sign = K+3, so now we have: STA INWK+8 \ \ (z_sign z_hi z_lo) = K(3 2 1) \ = z + beta * K2 / 256 \ So we now have result 2 above: \ \ z = z + beta * K2 EOR #%10000000 \ Flip the sign bit of A to give A = -z_sign JSR MULT3 \ Set K(3 2 1 0) = (A P+1 P) * Q \ = (-z_sign z_hi z_lo) * beta \ = -z * beta LDA K+3 \ Set T to the sign bit of K(3 2 1 0), i.e. to the sign AND #%10000000 \ bit of -z * beta STA T EOR K2+3 \ If K2(3 2 1 0) has a different sign to K(3 2 1 0), BMI MV1 \ then EOR'ing them will produce a 1 in bit 7, so jump \ to MV1 to take this into account \ If we get here, K and K2 have the same sign, so we can \ add them together to get the result we're after, and \ then set the sign afterwards LDA K \ We now do the following sum: \CLC \ ADC K2 \ (A y_hi y_lo -) = K(3 2 1 0) + K2(3 2 1 0) \ \ starting with the low bytes (which we don't keep) \ \ The CLC instruction is commented out in the original \ source. It isn't needed because MULT3 clears the C \ flag, so this is an example of the authors finding \ one more precious byte to save LDA K+1 \ We then do the middle bytes, which go into y_lo ADC K2+1 STA INWK+3 LDA K+2 \ And then the high bytes, which go into y_hi ADC K2+2 STA INWK+4 LDA K+3 \ And then the sign bytes into A, so overall we have the ADC K2+3 \ following, if we drop the low bytes from the result: \ \ (A y_hi y_lo) = (K + K2) / 256 JMP MV2 \ Jump to MV2 to skip the calculation for when K and K2 \ have different signs .MV1 LDA K \ If we get here then K2 and K have different signs, so SEC \ instead of adding, we need to subtract to get the SBC K2 \ result we want, like this: \ \ (A y_hi y_lo -) = K(3 2 1 0) - K2(3 2 1 0) \ \ starting with the low bytes (which we don't keep) LDA K+1 \ We then do the middle bytes, which go into y_lo SBC K2+1 STA INWK+3 LDA K+2 \ And then the high bytes, which go into y_hi SBC K2+2 STA INWK+4 LDA K2+3 \ Now for the sign bytes, so first we extract the sign AND #%01111111 \ byte from K2 without the sign bit, so P = |K2+3| STA P LDA K+3 \ And then we extract the sign byte from K without the AND #%01111111 \ sign bit, so A = |K+3| SBC P \ And finally we subtract the sign bytes, so P = A - P STA P \ By now we have the following, if we drop the low bytes \ from the result: \ \ (A y_hi y_lo) = (K - K2) / 256 \ \ so now we just need to make sure the sign of the \ result is correct BCS MV2 \ If the C flag is set, then the last subtraction above \ didn't underflow and the result is correct, so jump to \ MV2 as we are done with this particular stage LDA #1 \ Otherwise the subtraction above underflowed, as K2 is SBC INWK+3 \ the dominant part of the subtraction, so we need to STA INWK+3 \ negate the result using two's complement, starting \ with the low bytes: \ \ y_lo = 1 - y_lo LDA #0 \ And then the high bytes: SBC INWK+4 \ STA INWK+4 \ y_hi = 0 - y_hi LDA #0 \ And finally the sign bytes: SBC P \ \ A = 0 - P ORA #%10000000 \ We now force the sign bit to be negative, so that the \ final result below gets the opposite sign to K, which \ we want as K2 is the dominant part of the sum .MV2 EOR T \ T contains the sign bit of K, so if K is negative, \ this flips the sign of A STA INWK+5 \ Store A in y_sign \ So we now have result 3 above: \ \ y = K2 + K \ = K2 - beta * z LDA ALPHA \ Set A = alpha STA Q LDA INWK+3 \ Set P(1 0) = (y_hi y_lo) STA P LDA INWK+4 STA P+1 LDA INWK+5 \ Set A = y_sign JSR MULT3 \ Set K(3 2 1 0) = (A P+1 P) * Q \ = (y_sign y_hi y_lo) * alpha \ = y * alpha LDX #0 \ Set K(3 2 1) = (x_sign x_hi x_lo) + K(3 2 1) JSR MVT3 \ = x + y * alpha / 256 LDA K+1 \ Set (x_sign x_hi x_lo) = K(3 2 1) STA INWK \ = x + y * alpha / 256 LDA K+2 STA INWK+1 LDA K+3 STA INWK+2 \ So we now have result 4 above: \ \ x = x + y * alpha JMP MV45 \ We have now finished rotating the planet or sun by \ our pitch and roll, so jump back into the MVEIT \ routine at MV45 to apply all the other movements
Save output/ELTA.bin
PRINT "ELITE A" PRINT "Assembled at ", ~CODE% PRINT "Ends at ", ~P% PRINT "Code size is ", ~(P% - CODE%) PRINT "Execute at ", ~LOAD% PRINT "Reload at ", ~LOAD_A% PRINT "S.ELTA ", ~CODE%, " ", ~P%, " ", ~LOAD%, " ", ~LOAD_A% SAVE "output/ELTA.bin", CODE%, P%, LOAD%