.GetScrollDivisions LDY #15 ; We are going to populate 16 bytes in the buffer at BUF ; and another 16 in the buffer at BUF+16, so set a loop ; counter in Y .sdiv1 SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 STY T ; Store the loop counter in T (though we don't read this ; again, so this has no effect) TYA ; Set R = Y * 2 ASL A STA R ASL A ; Set S = Y * 4 STA S ASL A ; Set the Y-th entry in BUF+16 to the following: ADC #31 ; SBC scrollProgress ; Y * 8 + 31 - scrollProgress STA BUF+16,Y ; ; We know the C flag is clear because Y is a maximum of ; 15 so the three ASL A instructions will shift zeroes ; into the C flag each time BPL sdiv4 ; If A < 128, jump to sdiv4 to set Q and A as follows, ; but with both scaled up as far as possible to make the ; calculation more accurate STA Q ; Set Q = A LDA scrollProgress ; Set A = 37 + scrollProgress / 4 - R LSR A ; = 37 + scrollProgress / 4 - Y * 2 LSR A ADC #37 SBC R .sdiv2 CMP Q ; If A >= Q, jump to sdiv3 to store 255 as the result BCS sdiv3 ; in the Y-th byte of BUF JSR LL28 ; Call LL28 to calculate: ; ; R = 256 * A / Q ; ; = 37 - Y * 2 + scrollProgress / 4 ; ------------------------------- ; 31 + Y * 8 - scrollProgress LSR R ; Set R = R / 2 ; ; = 37 - Y * 2 + scrollProgress / 4 ; --------------------------------- ; 2 * (31 + Y * 8 - scrollProgress) LDA #72 ; Set the Y-th entry in BUF to 72 + R CLC ADC R STA BUF,Y DEY ; Decrement the loop counter in Y BPL sdiv1 ; Loop back until we have calculated all 16 values RTS ; Return from the subroutine .sdiv3 LDA #255 ; Set the Y-th entry in BUF to 255 STA BUF,Y DEY ; Decrement the loop counter in Y BPL sdiv1 ; Loop back until we have calculated all 16 values RTS ; Return from the subroutine .sdiv4 ASL A ; Set A = A * 2 BPL sdiv5 ; If A < 128, jump to sdiv5 STA Q ; Set Q = A * 2 LDA scrollProgress ; Set A = 73 + scrollProgress / 2 - Y * 4 LSR A ADC #73 SBC S ; So we have: ; ; Q = A * 2 ; ; A = 73 + scrollProgress / 2 - Y * 4 ; ; So when we divide them at sdiv2 above, this is the ; same as having: ; ; Q = A ; ; A = 37 + scrollProgress / 4 - Y * 2 ; ; but with both the numerator and denominator scaled up ; by the same factor of 2 JMP sdiv2 ; Jump to sdiv2 to continue the calculation with these ; scaled up values of Q and A .sdiv5 ASL A ; Set Q = A * 4 STA Q LDA scrollProgress ; Set A = 144 + scrollProgress - Y * 2 ADC #144 SBC S SBC S ; So we have: ; ; Q = A * 4 ; ; A = 144 + scrollProgress - Y * 2 ; ; So when we divide them at sdiv2 above, this is the ; same as having: ; ; Q = A ; ; A = 37 + scrollProgress / 4 - Y * 2 ; ; but with both the numerator and denominator scaled up ; by the same factor of 4 JMP sdiv2 ; Jump to sdiv2 to continue the calculation with these ; scaled up values of Q and AName: GetScrollDivisions [Show more] Type: Subroutine Category: Combat demo Summary: Set up the division calculations for the scroll textContext: See this subroutine in context in the source code References: This subroutine is called as follows: * DrawScrollFrame calls GetScrollDivisions
This routine sets up a division table to use in the calculations for drawing the scroll text in DrawScrollFrame.
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Subroutine LL28 (category: Maths (Arithmetic))
Calculate R = 256 * A / Q
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Macro SETUP_PPU_FOR_ICON_BAR (category: PPU)
If the PPU has started drawing the icon bar, configure the PPU to use nametable 0 and pattern table 0
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Variable scrollProgress in workspace WP
Keeps track of the progress of the demo scroll text, starting from zero and increasing as the text scrolls up the screen
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Label sdiv1 is local to this routine
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Label sdiv2 is local to this routine
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Label sdiv3 is local to this routine
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Label sdiv4 is local to this routine
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Label sdiv5 is local to this routine