.AnimateTyres INC irqCounter \ Increment irqCounter, so it gets incremented every \ time the IRQ routine reaches section 4 of the custom \ screen LDA playerSpeedHi \ Set tyreTravel = tyreTravel + playerSpeedHi + 48 CLC ADC #48 ADC tyreTravel STA tyreTravel BCC tyre4 \ If the addition didn't overflow, jump to tyre4 to \ return from the subroutine LDA playerMoving \ If playerMoving = 0 then the player's car is not BEQ tyre4 \ moving so we don't need to animate the tyres, so jump \ to tyre4 to return from the subroutine \ We now flip the black-and-white pixels in the tyre \ tread at the top of the left and white tyres using an \ EOR with %10, which flips pixels between %00 (black) \ and %10 (white) \ \ The tyre tread spills across three character blocks, \ with each pixel byte covering four pixels. Taking the \ left tyre, we have the following blocks: \ \ 1. Top-left block: tyreLeft1 to tyreLeft1+2 \ \ Three rows of four pixels at the top-left of the \ tyre \ \ 2. Top-right block: tyreLeft2+3, tyreLeft2+4 \ \ Two rows of two pixels at the top-right of the \ tyre \ \ 3. Bottom-left block: tyreLeft3 to tyreLeft3+4 \ \ Five rows of pixels at the bottom-left of the \ tyre, covering 4, 4, 2, 2, 1 pixels as we work \ our way down \ \ This looks like this, where the number is the block \ number above (the left side of the following is along \ the left edge of the screen): \ \ 111122 \ 111122 \ 1111 \ 3333 \ 3333 \ 33 \ 33 \ 3 \ \ The code below works through the various blocks, \ applying an EOR to the tyre pixels as follows: \ \ 1. tyreLeft1, tyreLeft1+2 \ \ EOR with mask %11110000 (all four pixels) \ \ 2. tyreLeft2+3, tyreLeft2+4 \ \ EOR with mask %11000000 (left two pixels) \ \ 3. tyreLeft3 to tyreLeft3+4 \ \ EOR with mask from tyreTreadLeft table (a wedge \ shape from all four pixels at the top down to \ just one pixel at the bottom) \ \ A similar process is applied to the right tyre, but \ with the shape reflected, like this: \ \ 221111 \ 221111 \ 1111 \ 3333 \ 3333 \ 33 \ 33 \ 3 \ \ The right tyre uses tyreRight and tyreTreadRight to \ achieve the same effect, all in the same loop as the \ left tyre animation for maximum efficiency LDX #4 \ Set a loop counter to go from 4 to 0 .tyre1 LDA tyreLeft3,X \ Set tyreLeft3 = tyreLeft3 EOR tyreTreadLeft EOR tyreTreadLeft,X \ to flip the pixels in the bottom-left of the tyre STA tyreLeft3,X LDA tyreRight3,X \ Set tyreRight3 = tyreRight3 EOR tyreTreadRight EOR tyreTreadRight,X \ to flip the pixels in the bottom-right of the tyre STA tyreRight3,X CPX #3 \ If X >= 3, jump to tyre2 to skip the following BCS tyre2 LDA tyreLeft1,X \ Flip all four pixels at tyreLeft1+X, so that's EOR #%11110000 \ tyreLeft1+2, tyreLeft1+1 and tyreLeft1 STA tyreLeft1,X LDA tyreRight1,X \ Flip all four pixels at tyreRight1+X, so that's EOR #%11110000 \ tyreRight1+2, tyreRight1+1 and tyreRight1 STA tyreRight1,X BNE tyre3 \ Jump to tyre3 to continue the loop (this BNE is \ effectively a JMP as A is never zero, because the \ screen byte in A is a chequered pattern) .tyre2 LDA tyreLeft2,X \ Flip the two left pixels at tyreLeft2+X, so that's EOR #%11000000 \ tyreLeft2+4 and tyreLeft2+3 STA tyreLeft2,X LDA tyreRight2,X \ Flip the two right pixels at tyreRight2+X, so that's EOR #%00110000 \ tyreRight2+4 and tyreRight2+3 STA tyreRight2,X .tyre3 DEX \ Decrement the loop counter BPL tyre1 \ Loop back to animate the next tyre part .tyre4 RTS \ Return from the subroutineName: AnimateTyres [Show more] Type: Subroutine Category: Dashboard Summary: Update screen memory to animate the tyresContext: See this subroutine in context in the source code References: This subroutine is called as follows: * ScreenHandler calls AnimateTyres
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Variable irqCounter (category: Screen mode)
Counter that gets incremented every time the IRQ routine reaches section 4 of the custom screen
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Variable playerMoving in workspace Zero page
Flag to denote whether the player's car is moving
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Variable playerSpeedHi in workspace Zero page
High byte of the speed of the player's car along the track
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Label tyre1 is local to this routine
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Label tyre2 is local to this routine
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Label tyre3 is local to this routine
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Label tyre4 is local to this routine
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Configuration variable tyreLeft1 = &6E85
The tread on the left tyre in screen memory (part 1)
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Configuration variable tyreLeft2 = &6E8A
The tread on the left tyre in screen memory (part 2)
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Configuration variable tyreLeft3 = &6FC0
The tread on the left tyre in screen memory (part 3)
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Configuration variable tyreRight1 = &6FBD
The tread on the right tyre in screen memory (part 1)
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Configuration variable tyreRight2 = &6FB2
The tread on the right tyre in screen memory (part 2)
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Configuration variable tyreRight3 = &70F8
The tread on the right tyre in screen memory (part 3)
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Variable tyreTravel (category: Dashboard)
Keeps track of how far we have travelled so we know when to animate the tyres
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Variable tyreTreadLeft (category: Dashboard)
Tyre tread pattern for the left tyre
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Variable tyreTreadRight (category: Dashboard)
Tyre tread pattern for the right tyre