LDA objectStatus,X \ If bit 7 of the driver's car object status byte is ASL A \ set, then the car is not visible, so jump to mar20 to BCS mcar20 \ move on to the next car BMI mcar17 \ If bit 6 of the driver's car object status byte is \ set, then the car has finished racing, so jump to \ mcar17 to steer the car LDA carSteering,X \ If bit 6 of the car's carSteering is clear, jump to AND #%01000000 \ mcar17 to steer the car BEQ mcar17 LDA carRacingLine,X \ If bit 7 of the car's racing line and carSteering are EOR carSteering,X \ the same (so the car is steering into the opposite BPL mcar17 \ half of the track), jump to mcar17 to steer the car LDA carRacingLine,X \ If the car's racing line < 128, then the car is in the BPL mcar15 \ right half of the track, so jump to mcar15 to check \ how close it is to the right verge CMP #236 \ If the car's racing line < 236, jump to mcar14 BCC mcar14 \ If we get here then the car's racing line >= 236, \ which is very close to the left verge, so we steer a \ little to the right DEC carRacingLine,X \ Steer the car a little to the right BCS mcar20 \ Jump to mcar20 (this BCS is effectively a JMP as we \ just passed through a BCC) .mcar14 CMP #226 \ If the car's racing line < 226, jump to mcar17 to BCC mcar17 \ steer the car \ If we get here, then the car's racing line >= 226, \ which is quite close to the left verge BCS mcar20 \ Jump to mcar20 (this BCS is effectively a JMP as we \ just passed through a BCC) .mcar15 CMP #20 \ If the car's racing line >= 20, jump to mcar16 BCS mcar16 \ If we get here then the car's racing line < 20, which \ is very close to the right verge, so we steer a little \ to the left INC carRacingLine,X \ Steer the car a little to the left BCC mcar20 \ Jump to mcar20 (this BCC is effectively a JMP as we \ just passed through a BCS) .mcar16 CMP #30 \ If the car's racing line < 30, jump to mcar20 to move BCC mcar20 \ on to the next driver .mcar17 \ If we get here, then we need to apply the steering \ given in carSteering \ \ carSteering is a sign-magnitude number, where bit 7 is \ the sign and bits 0-5 contain the magnitude, so before \ we can apply the steering, we need to convert it into \ a signed number LDA carSteering,X \ Set A to the car's carSteering AND #%10111111 \ Clear bit 6 CLC \ Clear the C flag in preparation for the addition below BPL mcar18 \ If bit 7 of A is clear, then the amount of steering is \ positive and the number is already correct, so jump to \ mcar18 to add it to the racing line \ If we get here then the sign-magnitude number in A is \ negative, so we need to convert this into a signed \ number \ \ We do this by first setting bit 6 (which we just \ cleared above) so bits 6 and 7 are both set, and we \ then need to flip bits 0-5, to convert the positive \ magnitude into its negative equivalent \ \ We can do this in one EOR instruction, as follows EOR #%01111111 \ Set bit 6 of A and flip bits 0-5, so A is now a \ negative number that we can add to the racing line in \ order to subtract the correct amount of steering ADC carRacingLine,X \ Steer the car right by the amount in A BCS mcar19 \ If the subtraction didn't underflow then we are still \ on the track, so jump to mcar19 to update the racing \ line with the new figure BCC mcar20 \ Otherwise the subtraction just underflowed, which \ means we just steered the car off the track, so jump \ to mcar20 so we don't update the car's racing line \ (this BCC is effectively a JMP as we just passed \ through a BCS) .mcar18 ADC carRacingLine,X \ Steer the car left by the amount in A BCS mcar20 \ If the addition just overflowed then this would steer \ the car off the track, so jump to mcar20 so we don't \ update the car's racing line (so we don't actually do \ the steer) .mcar19 STA carRacingLine,X \ Update the car's racing line with the updated figure, \ to steer the car across the track .mcar20 DEX \ Decrement the loop counter to point to the next driver BMI mcar21 \ If we have worked our way through all 20 drivers, jump \ to mcar21 to return from the subroutine CPX currentPlayer \ If driver X is the current player, jump up to mcar20 BEQ mcar20 \ to move on to the next driver JMP mcar1 \ Jump up to mcar1 to process the next driver .mcar21 RTS \ Return from the subroutineName: MoveCars (Part 2 of 2) [Show more] Type: Subroutine Category: Car geometry Summary: Move the cars forward around the track, and apply steering Deep dive: Placing cars on the track Tactics of the non-player driversContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
This part moves the car round the track by the speed we just calculated, and applies steering if required. The steering algorithm works as follows: * Only apply steering if the car is visible. * If any of the following are true, apply the amount of steering given in carSteering: * Bit 6 of the car's objectStatus is set * Bit 6 of the car's carSteering is clear * Bit 7 of the car's racing line = bit 7 of carSteering, so: * Car is in the right half and is steering left (when bit 7 is clear) * Car is in the left half and is steering right (when bit 7 is set) * The car is not within 30 of either verge, 30 <= carRacingLine < 226 * Otherwise, if the car's racing line is within 20 of either verge, steer away from the verge by one.
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Variable carRacingLine in workspace Stack variables
Each car's position on the track in the x-axis
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Variable carSteering in workspace Stack variables
Contains the steering to apply to each car
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Variable currentPlayer in workspace Zero page
The number of the current player
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Label mcar1 in subroutine MoveCars (Part 1 of 2)
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Label mcar14 is local to this routine
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Label mcar15 is local to this routine
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Label mcar16 is local to this routine
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Label mcar17 is local to this routine
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Label mcar18 is local to this routine
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Label mcar19 is local to this routine
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Label mcar20 is local to this routine
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Label mcar21 is local to this routine
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Variable objectStatus in workspace Stack variables
Various status flags for each object