.DrawScrollFrame JSR GetScrollDivisions ; Set up the division calculations for the scroll text ; and store then in the 16 bytes at BUF and the 16 bytes ; at BUF+16 LDY #$FF ; We are about to loop through the 240 bytes in the line ; coordinate tables, so set a coordinate counter in Y ; to start from 0, so set Y = -1 so the INY instruction ; at the start of the loop sets Y to 0 for the first ; coordinate .drfr1 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 INY ; Increment the coordinate counter in Y CPY #240 ; If Y = 240 then we have worked our way through all the BEQ RTS10 ; line coordinates, so jump to RTS10 to return from the ; subroutine LDA Y1TB,Y ; Set A to the y-coordinate byte that contains the start ; and end y-coordinates in the bottom and high nibbles ; respectively BEQ drfr1 ; If both y-coordinates are zero then this entry doesn't ; contain a line, so jump to drfr1 to move on to the ; next coordinate ; We now set up the following, so we can clip the line ; to the screen before drawing it: ; ; XX15(1 0) = X1 as a 16-bit coordinate (x1_hi x1_lo) ; ; XX15(3 2) = Y1 as a 16-bit coordinate (y1_hi y1_lo) ; ; XX15(5 4) = X2 as a 16-bit coordinate (x2_hi x2_lo) ; ; XX12(1 0) = Y2 as a 16-bit coordinate (y2_hi y2_lo) ; ; We calculate these values from the X1TB, X2TB and Y1TB ; line coordinate values for the line we want to draw ; (where Y1TB contains the Y1 and Y2 y-coordinates, one ; in each nibble) AND #$0F ; Set Y1 to the low nibble of A, which contains the STA Y1 ; y-coordinate of the start of the line, i.e. Y1 TAX ; Set X to the y-coordinate of the start of the line, Y1 ASL A ; Set A = A * 8 - scrollProgress ASL A ; = Y1 * 8 - scrollProgress ASL A SEC SBC scrollProgress BCC drfr1 ; If the subtraction underflowed then this coordinate is ; not on-screen, so jump to drfr1 to move on to the next ; coordinate STY YP ; Store the loop counter in YP, so we can retrieve it at ; the end of the loop LDA BUF+16,X ; Set Q to the entry from BUF+16 for the y-coordinate in STA Q ; X (which we set to Y1 above), so: ; ; Q = X * 8 + 31 - scrollProgress ; = Y1 * 8 + 31 - scrollProgress LDA X1TB,Y ; Set A to the x-coordinate of the start of the line, X1 JSR ProjectScrollText ; Set (A X) = 128 + 256 * (A - 32) / Q ; = 128 + 256 * (X1 - 32) / Q ; ; So (A X) is the x-coordinate of the start of the line, ; projected onto the scroll text so coordinates bunch ; together horizontally the further up the scroll text ; they are STX XX15 ; Set the low byte of XX15(1 0) to X LDX Y1 ; Set X = Y1, which we set to the y-coordinate of the ; start of the line, i.e. Y1 STA XX15+1 ; Set the high byte of XX15(1 0) to A, so we have: ; ; XX15(1 0) = (A X) ; 256 * (X1 - 32) ; = 128 + ---------------------------- ; Y1 * 8 + 31 - scrollProgress LDA BUF,X ; Set the low byte of XX15(3 2) to the entry from BUF+16 STA XX15+2 ; for the y-coordinate in X (which we set to Y1 above) LDA #0 ; Set the high byte of XX15(3 2) to 0 STA XX15+3 ; ; So we now have: ; ; 37 - Y1 * 2 + scrollProgress / 4 ; XX15(3 2) = 72 + ---------------------------------- ; 2 * (31 + Y1 * 8 - scrollProgress) LDA Y1TB,Y ; Set A to the combined y-coordinates of the start and ; end of the line, Y1 and Y2, with one in each nibble LSR A ; Set the high byte of XX12(1 0) to the high nibble of LSR A ; A, which contains the y-coordinate of the end of the LSR A ; line, i.e. Y2 LSR A STA XX12+1 TAX ; Set X to the y-coordinate of the end of the line, Y2 ASL A ; Set A = XX12+1 * 8 - scrollProgress ASL A ; = Y2 * 8 - scrollProgress ASL A SEC SBC scrollProgress BCC drfr1 ; If the subtraction underflowed then this coordinate is ; not on-screen, so jump to drfr1 to move on to the next ; coordinate LDA BUF,X ; Set the low byte of XX12(1 0) to the entry from BUF+16 STA XX12 ; for the y-coordinate in X (which we set to Y2 above) LDA #0 ; Set X to XX12+1, which we set to Y2 above, so X = Y2 LDX XX12+1 STA XX12+1 ; Set the high byte of XX12(1 0) to 0 ; ; So we now have: ; ; 37 - Y2 * 2 + scrollProgress / 4 ; XX12(1 0) = 72 + ---------------------------------- ; 2 * (31 + Y2 * 8 - scrollProgress) LDA BUF+16,X ; Set Q to the entry from BUF+16 for the y-coordinate in STA Q ; X (which we set to Y2 above), so: ; ; Q = X * 8 + 31 - scrollProgress ; = Y2 * 8 + 31 - scrollProgress 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 LDA X2TB,Y ; Set A to the x-coordinate of the end of the line, X2 JSR ProjectScrollText ; Set (A X) = 128 + 256 * (A - 32) / Q ; = 128 + 256 * (X1 - 32) / Q ; ; So (A X) is the x-coordinate of the end of the line, ; projected onto the scroll text so coordinates bunch ; together horizontally the further up the scroll text ; they are STX XX15+4 ; Set XX15(5 4) = (A X) STA XX15+5 ; 256 * (X2 - 32) ; = 128 + ---------------------------- ; Y2 * 8 + 31 - scrollProgress JSR CLIP_b1 ; Clip the following line to fit on-screen: ; ; XX15(1 0) = x1 as a 16-bit coordinate (x1_hi x1_lo) ; ; XX15(3 2) = y1 as a 16-bit coordinate (y1_hi y1_lo) ; ; XX15(5 4) = x2 as a 16-bit coordinate (x2_hi x2_lo) ; ; XX12(1 0) = y2 as a 16-bit coordinate (y2_hi y2_lo) ; ; and draw the line from (X1, Y1) to (X2, Y2) once it's ; clipped LDY YP ; Set Y to the loop counter that we stored in YP above JMP drfr1 ; Loop back to drfr1 to process the next coordinateName: DrawScrollFrame [Show more] Type: Subroutine Category: Combat demo Summary: Draw one frame of the scroll text Deep dive: The NES combat demoContext: See this subroutine in context in the source code References: This subroutine is called as follows: * DrawScrollFrames calls DrawScrollFrame
This routine draws each character on the scroll text by taking the character's line coordinates from the from the X1TB, X2TB and Y1TB line coordinate tables (i.e. X1, Y1, X2 and Y2) and projecting them onto a scroll that disappears into the distance, like the scroll text at the start of Star Wars. For a character line from (X1, Y1) to (X2, Y2), the calculation gives us coordinates of the line to draw on-screen from (x1, y1) to (x2, y2), as follows: 256 * (X1 - 32) x1 = XX15(1 0) = 128 + ---------------------------- Y1 * 8 + 31 - scrollProgress 256 * (X2 - 32) x2 = XX15(5 4) = 128 + ---------------------------- Y2 * 8 + 31 - scrollProgress 37 - Y1 * 2 + scrollProgress / 4 y1 = XX15(3 2) = 72 + ---------------------------------- 2 * (31 + Y1 * 8 - scrollProgress) 37 - Y2 * 2 + scrollProgress / 4 y2 = XX12(1 0) = 72 + ---------------------------------- 2 * (31 + Y2 * 8 - scrollProgress) The line then gets clipped to the screen by the CLIP_b1 routine and drawn.
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Subroutine CLIP_b1 (category: Drawing lines)
Call the CLIP routine in ROM bank 1, drawing the clipped line if it fits on-screen
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Subroutine GetScrollDivisions (category: Combat demo)
Set up the division calculations for the scroll text
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Subroutine ProjectScrollText (category: Combat demo)
Project a scroll text coordinate onto the screen
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Entry point RTS10 in subroutine ProjectScrollText (category: Combat demo)
Contains an RTS
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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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Label drfr1 is local to this routine
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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